$45.7 “"713 73 September 1993 H132 "" Li8§e=~< APR 0 5 ‘AS AaM um; Dissipation, Movement, and Environmental Impact of Herbicides on Texas e Rangelands A E5-Year Summary The Texas Agricultural Experiment Station, Edward A. Hiler, Director, The Texas A&M University System, College Station, Texas [Blank Pige in orgm Bulletin] % r)" s, Dissipation, Movement, and Environmental Impact of Herbicides on Texas Rangelands — A 25-Year Summary LISQARY APR 0 5 1994 T," r». ,, . £2,143 a/"iéigif? Rodney W. Bovey‘ Keywords: herbicide residues, herbicide degradation, herbicide leaching, herbicide phytotoxicity, herbicide photodecomposition, herbicide spray drift, herbicide volatility ‘Research Agronomist, USDA-ARS Weed Science Laboratory, Beltsville, Maryland, and the Texas Agricultural Experiment Station (Department of Rangeland Ecology and Management), College Station, Texas “ [Blank Page m Origiafl mum; A ( .,‘ gt n Q‘ f - ' I , , ; .-. ~ 4 _ H r . ~= I 14;. \ __ I y s \“’\ »\ \ \ \ , *9» Contents Summary ................................................................................................................................................................. ..1 Introduction ............................................................................................................................................................. .. 1 Picloram .............................................................................................................................................................. ..3 Persistence in soil ........................................................................................................................................... ..3 Subhumid rangeland sites ......................................................................................................................... ..3 Fallowed areas and high rates .................................................................................................................. ..3 Texas studies ............................................................................................................................................ ..3 Puerto Rican studies ................................................................................................................................. ..4 Semiarid sites ............................................................................................................................................ ..4 Widely diverse locations ............................................................................................................................ ..4 Factors affecting soil persistence .............................................................................................................. ..4 Soil texture ............................................................................................................................................. ..4 Environment ........................................................................................................................................... ..4 Effect of repeated treatment .................................................................................................................. ..4 Effect of picloram soil residues on plant growth ........................................................................................ ..5 Persistence in plants ...................................................................................................................................... ..5 Herbaceous plants .................................................................................. ........................................... ..5 Semiarid areas ....................................................................................................................................... ..5 Humid areas ........................................................................................................................................... ..5 Woody plants ........................................................................................................................................... ..6 Semiarid areas ...................................................... ............................................................................... ..6 Humid areas ........................................................................................................................................... ..6 Effect on plants ......................................................................................................................................... ..6 Factors affecting degradation .................................................................................................................... ..6 Persistence and movement in water sources ................................................................................................. ..6 Movement of picloram in surface water .................................................................................................... ..6 Movement of picloram in subsurface water ............................................................................................... ..7 Dissipation of picloram from impounded water sources ............................................................................ ..7 Clopyralid ............................................................................................................................................................ ..8 Persistence in soil and movement in water sources ....................................................................................... ..8 Persistence in plants ...................................................................................................................................... ..8 Phenoxys ............................................................................................................................................................ ..8 Persistenceirtsoil....- ...... ............................................................................................................................... ..8 Influence of high rates ......................................................................................................................... ..8 Effect of repeated treatment ..... ..... .............. ........................................................................................ ..8 Modes ofbreakdown in in plants .' ...... ..... .. ....... ..... ....... ............................................................................. ..9 Persistence and movement in water lrtpounded water 1 0 (iroundwater Mode of breakdown in 11 Spray drift 11 Dicamba ............................................................................................................................................................ .. 1 1 Persistence in soil ......................................................................................................................................... .. 11 Modes of breakdown in soil ..................................................................................................................... ..12 Persistence in plants .................................................................................................................................... .. 12 Herbaceous plants .................................................................................................................................. ..12 Effect on forage grasses and cotton .................................................................................... ................. ..12 Modes of breakdown in plants ................................................................................................................ ..12 Persistence and movement in water sources ............................................................................................... ..12 Impounded water .................................................................................................................................... ..13 Influence of dicamba in irrigation water on seedling crops ..................................................................... .. 13 Triclopyr ............................................................................................................................................................ .. 13 Persistence in soil ......................................................................................................................................... .. 13 Mode of breakdown in soils ..................................................................................................................... .. 13 Persistence in plants .................................................................................................................................... ..13 Effect on plants ....................................................................................................................................... .. 13 Factors affecting degradation .................................................................................................................. .. 14 Persistence and movement in water sources ............................................................................................... .. 14 Surface runoff water ................................................................................................................................ .. 14 Groundwater ........................................................................................................................................... ..14 Impounded water .................................................................................................................................... ..14 Mode of breakdown in water ................................................................................................................... ..15 Tebuthiuron ....................................................................................................................................................... ..15 Persistence in soil ......................................................................................................................................... .. 15 Factors affecting dissipation and leaching .............................................................................................. .. 15 Distribution and dissolution of pellets ...................................................................................................... .. 15 Effect on plant growth ............................................................................................................................. ..16 ’ Greenhouse ......................................................................................................................................... ..16 Field ..................................................................................................................................................... ..16 Natural areas ....................................................................................................................................... .. 16 Reseeding on treated areas .................................................................................................................... ..16 Persistence in plants .................................................................................................................................... ..16 Herbaceous plants .................................................................................................................................. ..16 Woody plants .......................................................................................................................................... .. 17 Persistence and movement in water sources ............................................................................................... ..17 Surface runoff water ................................................................................................................................ .. 17 Hydrologic effects .................................................................................................................................... ..17 Hexazinone ....................................................................................................................................................... .. 17 Persistence in soil ......................................................................................................................................... ..17 Persistence in plants .................................................................................................................................... .. 18 Effect on plants ....................................................................................................................................... .. 18 Persistence and movement in water sources ............................................................................................... ..18 Aquatic environment ............................................................................................................................... ..18 Karbutilate ......................................................................................................................................................... .. 18 Glyphosate ........................................................................................................................................................ .. 19 Literature Cited ...................................................................................................................................................... ..19 Summary Herbicides are necessary to control weeds and brush on rangelands. Safe and effective use of herbicides requires that their properties, behavior, and impact on the environment be thoroughly understood. Herbicides considered in this review are picloram, clopyralid, the phenoxys, dicamba, triclopyr, tebuthiuron, hexazinone, karbutilate, and glyphosate. Picloram degrades within 3 to 6 months in Texas soils. Half-life of this herbicide varies widely depending upon rainfall and soil temperature. Picloram tends to leach to lower soil depths, but most remains in the upper meter of soil. Picloram may move in surface runoff water, but its removal from watersheds is usually less than 5% of the total amount applied. Picloram is de- graded slowly by soil microorganisms and plant metabo- lism but is degraded rapidly by sunlight. Picloram is phytotoxic to a wide range of plants especially broadleaf plants, so care must be taken to limit its movement. Clopyralid is chemically similar to picloram but has a shorter half-life in soil than does picloram and is subject to degradation by soil microbes. lt resists degradation by sunlight. Clopyralid moves in water sources as does picloram but is not as phytotoxic to most plants as is picloram. The phenoxy herbicides such as 2,4-D and 2,4,5-T are short lived in the southwestern U.S. environment and have limited mobility, so movement into groundwa- ter is unlikely. Phenoxys are rapidly decomposed by soil microbes, sunlight, and plant metabolism. Phenoxy herbicides are generally less phytotoxic to broadleaf plants than is picloram. However, preventing spray drift or vapors to susceptible plants such as cotton is essen- tial. Dicamba, in moist, warm soil has a half-life of <14 days as a result of microbial degradation. Half-life in native grasses and litter is 3 to 4 weeks. Under simu- lated rainfall conditions, a maximum of 5.5% of dicamba applied to a watershed was removed in runoff water. Dicamba levels found in streams after application to large watersheds were several orders of magnitude below threshold response levels for fish and mammals. In Texas, triclopyr persisted in soil for 3 months after summer application. Mode of breakdown in soils is by leaching, photodegradation, and microbial activity. Mo- bility in runoff water is similar to 2,4-D. ln semiarid and humid regions, tebuthiuron may per- sist for long periods. Tebuthiuron is readily adsorbed in soil having high organic matter or high clay content but may leach in soils low in organic matter or low clay content. Tebuthiuron content, however, in forage plants is typically well below legal residue limits when applied as pellets. Concentrations of tebuthiuron in runoff water decreased rapidly and was <0.05 ppm after 3 months from a watershed in central Texas. Tebuthiuron resists photodecomposition and volatilization, and breakdown by microbial activity is slow. Hexazinone is mobile in runoff water and readily leaches in some soils. Spot-gun application to brush species indicated limited movement and transport from treated watersheds in stream discharge. Half-life varied from 1 to 6 months in soil depending upon location. Glyphosate is rapidly inactivated in soil by adsorption and microbial activity. Chance of environmental con- tamination is remote. Introduction Weeds and brush represent one of the most serious barriers to profitable livestock production on U.S. range- lands. Most of the 43 million hectares (ha) of grassland in Texas are infested with weeds and brush. ln some areas, only one or two species are troublesome, but in others, several species are a problem. Weeds and brush reduce forage quantity and quality; deplete soil water and nutrients; harbor insects vectors and predators; and may be poisonous, unpalatable, or mechanically injuri- ous to livestock. Dense stands of weeds increase diffi- culty of handling livestock and reduce land values. Loss of grass cover to brush and weeds encourages soil erosion. Herbicides are an important means of selectively managing weeds and brush on rangelands and may be the only alternative where steep, rocky terrain limits mechanical methods or where inadequate fuel limits prescribed burning. Herbicides are also important in combination with fire, mechanical, or biological control measures in integrated management systems. A variety of herbicides, herbicide formulations, and herbicide combinations are available commercially. Prop erties, behavior, and impact of herbicides on the envi- ronment must be thoroughly understood so they can be used safely and effectively. This report summarizes and highlights data obtained on the behavior and fate of important herbicides used on Texas rangelands and includes an assessment of their effect on environmental quality. These herbicides are picloram, clopyralid, the phenoxys, dicamba, triclopyr, tebuthiuron, hexazinone, karbutilate, and glyphosate. See Table 1 fordetails on chemistry, formulations used, mode of entry, major uses and rates, water solubility, .00__00cm0 c000 0>00 2-0.0.0 0c0 x0>_.0 .0 0000 __< 0 .0_0m_.0>m >__m.20EEo0 0oz 0 .2022: + 0-1m 0cm .00Em0.0 + 0-0.0 E2220 + 002m .._>020.:+ E2220 0020.00.21 0._2>020 000.02 _2.co0 000.0 0c0 0003 c. .2000 02000E 0Eow .000.0.0._00 .050 00.3 00._000E c. 0_00_.0>0 >__m.20EEo0 00.0 2m 00.0.. 000.2200 >cm_2 0 0020 020coc 0c0 _00c0_0mcm._ .0022 .02 20.00 _>_._.0>x900 0.0m 2.00m _>xo.._>c.0.c>0 0m 0K 00... 0. 2 00.0 00 0208 0020 000 000; 000 000.0“. 000 =00 00.20.2000» -0-20_62-0.0.0.. 0.000.; 00cm.0mcm._ 0.0m 2.00m 0mm 000 0mm m o. 0.0 c0 _2.co0 000.0 0cm 000>> 002.00 0.0.00 0c0 0200 0000m> .>.8c000oc0_00.=.0_¢.m. 00.0.0.0 .2600 m20_>£0E.0-.Z.Z 02.0.00? .06. 00020000.. 0.2.00 -=>.m-_o~m.00.£-v.m; .00 30 000.0 0... 2 0.0 00 0208 0020 2a 002s .80 000 .0028 002.0; -._>20_>20e_0. 2. 0-0.2 02220002 0020 02ococ 0c0 00020002 .02 0.00 0..>xo0._000c.0..>0-m .00. 000.0 00¢ 0 o. 3.0 co _2.co0 000.0 0c0 000>> 0cm 002.00 0200 0c.Em .0 E0.00m.00 -20_00.._.-0.0.m-0c.Em-v E2220 0.080200 ._>20_>20e_0. 2. 0 0020 00._0coc 0cm .0.cm_0 _>c000 _0c.Em =>co0cm0 .00 000.0 000 00 2 0 0.000.010.0000 000; 0.0000 .000 20%.. 02500 002.0; .0=_e0_>..._e.0...-0 00.020200. 000.030.0010 -00.~02-0.0. 2.0.202 000.0 .022 .0020 020coc .0__00 .0 0.02. -l0c.Em_>£0E.0. 0mm 000; 00 2.00 m_. o. m c0 _2.co0 000.0 0cm 000>> .000. 0_0.00.E ..00>>00 0.00.00 -0._>x00o_0>0.0 0c02~mx0I _2.co0 000; 00.2.0 0m 000.0 000.00 m o. 0.0 0>..00_00coc .E0._.0000 002m 002.00. =00 0c.Em_>00._000_ ._>0..0Eoc0..00o.._0...a 0.002.026 .0820 >000; 200mm 20:00 c0 0.0 00200 .. 000. I00. 0 0.00.0 >000; 0.0m 2.000 .00 000.. 2 000 000 2 2 0 2 00.0 0.0000 .0000; 0000020 00200 20.00 000 200 000=0> .0a00000_0_..0_0-<.0. 0-0.0 .02 0.00 0.0~c00>x0...0E .00 000.0 000.0 Z o. 0.0 0020 0cm 0000>> 0c0 002.00 =00 02Em_>£0E.o 0.20.00.00.00 m0Em0.o 0..0000E >000... c0 02.0000 .02 0.0m 0._>xo0cm0 00m 000.0 000.. 0.0 2 00.0 .0020 000 0000; 0000020 000 000:0“. =00 00.200.00.008: 00_0§0.0.0.0_6_0-0.0 0._2>00_0 00.0000 825530.. E00. .050... 020m 0000 20.0.2 >320 0000.020. 0E0: .00.E000 00.0.0.0: 30h 2500.00 . .0 000E _0.0._0EEo0 2.0.08. c0=0EE02 20.03 mfiUCfl-QOCG; CO _O.3CO0 £295 ECU U00; 00w U005 000.0300... n w 03Gb. ‘\ and mammalian toxicity. Most herbicides used on range- land are of moderate to low toxicity. Some, however, are persistent and water soluble enough to move in soils and water. Foliar-applied herbicides such as clopyralid, 2,4- D, dicamba, picloram, and triclopyr may drift when sprayed. Such materials must be properly applied to prevent damage to nontarget susceptible vegetation. The potential of each herbicide to persistent in soil, vegetation, or water and to move to off-target areas in surface runoff and to groundwater will be discussed. Environmental problems, if any, will be indicated. This report is based on research conducted on Texas grazing lands. Data from outside Texas are included only to illustrate a point or to fill information gaps. Picloram The fate of picloram in grassland ecosystems was summarized (38) in 1971, but considerable data have been generated since. Picloram can control many broad- leaf weed and brush species on grasslands (38). lt has a very low order of toxicity to warm-blooded animals but is relatively persistent in the environment. lts persis- tence results in its effectiveness as an herbicide. lf not photodecomposed, some picloram may move laterally on the soil surface or vertically through the soil profile to a limited degree. Biological breakdown of picloram by microbes or higher plants is slow, but dilution by runoff and dissipation in impounded water are important modes of dissipation. Environmental problems with picloram is related to susceptible plant life where contaminated runoff or irrigation water could result in damage or where spray drift injures vegetation. Persistence in Soil Subhumid Rangeland Sites Bioassays 1 year aftertreatment have shown that <1 ppb of picloram was detected in soil at five Texas rangeland sites when applied at 1.1 to 4.5 kg/ha (38). However, the soils were sampled only once to a depth of 61 or 91 cm; thus the sampling did not account for possible leaching of herbicide beyond 91 cm. In other studies, picloram was applied at 2.2 and 9 kg/ ha on the Gulf Coast Prairie at Victoria on a Katy gravelly sandy loam (fine-loamy, siliceous, thermic Aquic Paleudalfs) and on the Post Oak Savannah at College Station on an Axtell fine sandy loam (fine, montrnorillo- nitic thermic family of Udertic Paleustalfs) (72). Annual rainfall at both sites was 81 cm. Frequent sampling at Victoria and College Station has shown that 2.2 kg/ha of piclo ram disappeared from the top 61 cm of soil by 6 and 12 weeks after treatment, respectively. Picloram was not detected after 1 year, regardless of herbicide rate or sampling depth. Bioassays using field beans confirmed chemical analysis, indicating absence of detectable residues. Additional studies at College Station were conducted to determine picloram residue levels to a depth of 2.4 m for 2 years after application of 1.12 kg/ha (8). Annual rainfall was 62 and 48 cm for year 1 and 2, respectively. After 30 days, residues were 93 ppb in the top 15 cm of the Axtell fine sandy loam soil and <5 ppb at a depth of 46 to 122 cm. After 6 months, residues were between 5 and 10 ppb to a depth of 183 cm and <5 ppb between 198 and 244 cm deep. Residues to depths of 244 cm were <5 ppb 1 year aftertreatment. Retreatment after 1 year did not cause picloram accumulation, and dissipation of picloram was similar for spray and granular formula- tions. Although picloram leached tothe lower soil profile, concentrations detected were extremely low and would not likely contaminate the soil or groundwater. Fallowed Areas and High Rates Much of the picloram spray is intercepted by vegeta- tion and plant litter. To investigate the magnitude of this effect, residues and leaching of picloram applied to bare soil was determined (25). Picloram was applied at 1.1, 3.4, and 10.1 kg/ha to a Erving clay loam and Lakeland sand at College Station and a Nipe clay, Fraternidad clay, and Catano sand near Mayagiiez, Puerto Rico. Annual rainfall in Texas was 71 and 74 cm at the sand and clay sites, respectively, and 81, 175, and 196 cm, respectively, on the Fraternidad clay, Nipe clay, and Catano sand sites in Puerto Rico. Texas Studies Picloram was applied to dry soil that received <1 cm of rainfall during the first 6 weeks after treatment. Loss of picloram was rapid during this period (25), presum- ably by photodeoomposition (71) because picloram was exposed to sunlight on bare soil. After 3 months, piclo- ram applied at 1.1 kg/ha disappeared from sandy soil. Picloram concentrations in the sand and clay soils treated at all rates were considerably reduced; these soils received 23 and 30 cm of rainfall, respectively. Six months aftertreatment at 1 .1 , 3.4, and 10.1 kg/ha, picloram was present in the upper 15, 30, and 91 cm of the clay soils, respectively (25). Picloram was detected at nearly all levels down to 122 cm in sand, but had dissipated at the 1 .1 -kg/ha rate. After 1 8 months, a small amount ot picloram (0.03 ppm) was found in only the top 15 cm of clay soil treated at 3.4 kg/ha. Plots treated with 1 0.1 kg/ha had picloram residues in the top 91 cm of clay soil at 3 and 6 months after treatment but were found only in the top 61 cm ot soil after 18 months (25). Picloram in the sandy soil was found at 122 cm deep at the 6- and 18-month sampling dates. Bioassay with ‘Black Valentine’ beans ( Phaseolus vulgaris L.) 18 months alter treatment detected no picloram residues where 1.1 kg/ha was applied on clay and sandy soils and in the sandy soil at 3.4 kg/ha. However, beans grown in clay soil from 0 to 15 or 15 to 30 cm deep treated with picloram at 3.4 kg/ha were injured. Greatest injury to beans occurred when grown in the top 60 cm of clay soil receiving 10.1 kg/ha of picloram. Some injury was also recorded from soil at depths of 91 to 122 cm. The greatest picloram injury to beans grown in sandy soil was at 91 to 122 cm deep probably because of leaching after receiving abundant rainfall on the site. Puerto Rican Studies Three months after treatment, picloram was distrib- _uted throughout the upper 51 cm of soil profile in clay soils at all rates of treatment (25). Picloram residues increased as the rate was increased. Picloram at 3.4 and 10.1 kg/ha persisted in the Fratenidad clay soil for 1 year. Disappearance of picloram was related to soil type and rainfall. Picloram was most persistent in the Fraternidad clay where rainfall was lowest. Disappear- ance of picloram from the Catano sand was rapid, and no herbicide was detected 6 months after treatment in the upper 100 cm of soil. Rainfall of 122 cm on the Catano sand may have leached much of the picloram from the soil. By 1 year after treatment in the Nipe clay, picloram was detected only where 10.1 kg/ha had been applied, but detectable concentrations were <10 ppb. Use of picloram rates >1.1 kg/ha would be uncommon. At 1.1 kg/ha, picloram residues disappeared rapidly from the sandy soil but were detected at as much as 23 ppb in clay soils for at least 6 months. Bioassay and gas-liquid chromatographic (GLC) techniques were com- pared tor Nipe and Fraternidad clay soils receiving 3.4 kg/ha of picloram (25). Both methods show similar trends in picloram concentrations at most depths of sampling, but for undetermined reasons, the bioassay consistently gave higher readings for the Fraternidad clay than did gas chromatography. Bioassay proce- dures with ‘Puerto Rico 39' cucumbers (Cucumis sativus L.) are accurate and sensitive to 5 ppb oi picloram. Other studies also showed close correlation between bioas- say and GLC techniques (94). Semiarld Sites ln the Rolling Plains of Texas, detectable picloram residues occurred in the upper 30 to 45 cm of soil after application of 0.28 kg/ha (95). Picloram was applied to sandy loam soils in June or July, and then soils were irrigated to cause leaching. After application of as much as 23 cm of irrigation water for 15 hours within 20 days after picloram treatment, residues were typically in the upper 45 cm of soil. On seven rangeland sites in the Rolling Plains, picloram at 0.28 kg/ha usually dissipated from the soil profile within 1 year after treatment. Widely Diverse Locations In studies at five diverse locations including east- central, south, and west Texas, picloram disappeared from soils by 1 year after application of 0.26 to 1.1 kg/ha regardless o1 cumulative rainfall or location (56). Factors Affecting $0" PQTSISIEIICG Soil Texture. As indicated in Texas and Puerto Rico, picloram disappeared more rapidly from sandy than from clay soils (25). Scifres et al. (99) found that picloram disappeared within 56 to 112 days after appli- cation from two watersheds on sandy soils in east- central Texas, and most picloram was restricted to the upper 15 cm of soil. No picloram was detected deeper than 60 cm. Bovey and Richardson (36) detected piclo- ram as long as 181 days after application in Houston black clay, mostly in the upper 30 cm of soil. Application rate was 0.56 kg/ha in both studies. Environment. Laboratory studies (71) indicated that dissipation of picloram was accelerated at high tempera- tures (38 °C versus 4 and 20 °C) and by leaching. Photodecomposition may be an important means of loss if the herbicide remains on the soil surface for several days. All these dissipation pathways have been docu- mented (8, 25, 36, 38, 56, 72, 95, 99). Effect of Repeated Treatment. Bovey et al. (23, 24) applied a 1 :1 mixture of 2,4,5-T plus picloram at 1.1 kg/ ha every 6 months for a total of five times on a native- grass pasture watershed and five times at 2.2 kg/ha every 6 months on an adjacent watershed. Herbicide content of the Houston black clay remained low (0 to 238 ppb) during the study. Picloram dissipated and did not accumulate at either soil site. Effect of Picloram Soil Residues on Plant Growth Picloram is widely used for weed and brush control in forage grass crops (32, 38, 39). ln Puerto Rico, ‘USDA- 34’ corn (Zea mays L.), ‘Combine Kafir-60' sorghum (Sorghum bicoIorL.), ‘Mentana' wheat ( Triticum aestivum L.), ‘Taichung Native No. 1' rice (Oryza sativa L.), and ‘Blightmaster' cotton (Gossypium hirsutum L.) could be grown without reduction in fresh weight as early as 3 months after application of 6.72 kg/ha. ‘Clark’ Soybean [Glycine max (L.) Merrill] were injured as long as 6.5 months after treatment of picloram at 6.72 kg/ha (35). Activated carbon applied at as much as 672 kg/ha in a Toa silty clay protected oats from picloram applied at 0.56 kg/ha but did not completely protect ‘Ashly' cucum- bers or ‘Black Valentine‘ beans (33). At College Station, Texas, ‘Tophand’ sorghum was grown in Wilson clay loam 12 months after application of 1.12 kg/ha picloram without reduction in plant numbers, dry matter produc- tion, flowering, or germination (34). No picloram was detected in sorghum seed harvested from plants grow- ing in picloram-treated soil as early as 6 month after application. ‘Hill’ soybean numbers per hectare and total dry matter production we re slightly depressed 14 months after picloram application (34). Ryegrass (Lolium perenne L. and multiflorum Lam.) could be grown as early as 75 days (and 16 cm rainfall) after application of picloram at 1 .1 or3.4 kg/ha as a spray or granule in a Wilson clay loam soil (5). Scifres and Halifax (97) found that picloram did not influence germination but did affect range grass seed- ling growth. Radicle elongation of sideoats grama [Bouteloua curtipendula (Michx.) Torr.], buffalograss [Buchloe dacty/oides (Nutt.) Engelm.], and switchgrass (Panicum virgatum L.) in petri dishes was reduced by 125 ppb picloram, whereas shoot elongation was not retarded by 1,000 ppb. Buffalograss, sideoats grama, and switchgrass seedlings germinated in soil contain- ing 500 ppb picloram and were generally not reduced in topgrowth production. However, topgrowth produc- tion of Arizona cottontop [Digitaria Californica (Benth.) Henr.] and vine mesquite (Panicum obtusum H.B.K.) was reduced by 125 to 250 ppb of picloram in soil. Root production and rootcshoot ratios of switchgrass seed- lings were decreased when 1,000 or 2,000 ppb of picloram were placed on the soil surface or at 7.5 cm deep (98). Sideoats grama root production decreased by applicationof 1,000 ppb of picloram placed 2.5 cm deep, but production was increased in soil with 1,000 ppb of picloram placed 15 cm deep. Rootshoot ratios in picloram-treated soil were typically no different than those in untreated soil, but root growth pattern was affected. Grain sorghum varieties varied in their response to pre- or postemergence irrigation and postemergence spray of various picloram concentrations (93). Signifi- cant increases in dry weight of Pioneer 820, RS-625, and PAG-665 occurred when treated preemergence with irrigation water containing 500, 1,000, and 2,000 ppb of picloram, whereas GA-615, RS-671, Tophand, and RS-626 were retarded by these concentrations. Dry weights of Pioneer 820 and RS-625 were increased by irrigation water containing 1 to 5 ppb of picloram applied postemergence or when treated with sprays containing 0.035 to 0.7 kg/ha. Persistence in Plants Herbaceous Plants Getzenduner et al. (51) showed that picloram resi- dues in grass collected from various U.S. locations generally degraded after 1 year, and residues were lower from granular than from liquid formulation. No bound form of picloram was found in grasses. Semiarid Areas. Picloram dissipated from grass, primarily buffalograss, at rates of 2.5 to 3% per day after applications of 0.28 kg/ha for honey mesquite control (96). Thus more than 90% of the picloram dissipated from grasses and broadleaf herbs within 30 days after application. Dissipation of picloram from grasses was not affected by irrigation to surtace runoff within 10, 20, or 30 days after application (96). Hoffman (56) found that picloram disappeared from grasses in 3 to 6 months at more than 20 locations (mainly semiarid sites) in Texas. Picloram was applied with equal rates of 2,4,5-T for honey mesquite control. Rates of piclo ram varied from 0.28 to 0.84 kg/ha. If rates of picloram are from 2.24 to 4.48 kg/ha, persistence may be as long as 2 years after treatment of redberry juniper (Juniperus pinchoti Sudw.) with picloram granules (38). Predominant grasses in the study were little bluestem and sideoats grama. Humld Areas. Dissipation of picloram in range grass- es in humid areas was relatively rapid. Zero to small concentrations were detected 6 months after treatment (15, 16, 23, 24). Even when treatments were repeated every 6 months for a total of five applications at recom- mended (0.56 + 0.56 kg/ha) and at twice recommended rates of picloram and 2,4,5-T, picloram dissipated rap- idly and did not accumulate in the grasses or environ- ment (23, 24). Woody Plants Semlarld Areas. Honey mesquite leaves contained about 25 ppm of picloram the day of application of 0.28 kg/ha (96), but contained <1 ppm 23 days later. Picloram dissipated slower from sand shinnery oak leaves (Ouercus havardii Rydb.) at the same site than from honey mesquite. Nearly 2 ppm were detected in the oak leaves after 60 days. Picloram in soil surface leaf litter dissipated after 120 days. Humid Areas. Baur et al. (15) reported that the amounts of 2,4,5-T detected in live oak at Victoria, Texas, were greater when applied with eitherthe potas- sium salt or isooctyl ester of picloram than when 2,4,5-T was applied alone. Most herbicide had dissi- pated from live oak stems 6 months aftertreatment. Less than 1 ppb of picloram was detected in yaupon (Ilex vomitoria Ait.) stems or roots 6 months aftertreatment or retreatment with 1.12 kg/ha of picloram (16). Effect on Plants Fourteen days after planting, aqueous picloram solu- tions were used to water plants growing in pots (11). Water containing 0.25 and 0.50 ppb stimulated fresh shoot weights of ‘Texas No. 30' corn, ‘ATX31 97 MS Kafir 60' sorghum, ‘Stoneville 21 3’ cotton, ‘Alabama Blackeye’ cowpea [ Vigna unguicula (L.) Walp.], and ‘Lee’ soybean 21 days after planting. lt took 100 ppb to get the same effect in ‘Milam’ wheat. Significant stimulation in dry weight production occurred in corn, sorghum, cotton, and soybean at 0.25 ppb and cowpea at 1 ppb. Fresh and dry weight decreased in corn, wheat, and sorghum at 1 ,000 ppb and in all dioot species at 100 ppb. Picloram at levels as great as 1,000 ppb had no effect on dry weights of ‘Bluebell’ rice and wheat or fresh weight of rice. Picloram caused reductions in soluble protein con- centrations in all monocot species and in ‘HA-61' sun- flower. Significant increases in soluble protein occurred at 0.25 and 1 ppb in cowpea and cotton, respectively. In the field, picloram, tebuthiuron, and 2,4-D did not reduce protein concentration in kleingrass (Panicum coloratum L.), buffelgrass (Cenchrus ciliaris L.), and Coastal bermudagrass [Cynodn dactylon (L.) Pers.] but did reduce protein concentrations in a buffel X birdwood hybrid (Cenchrus sefigerus Vahl.) (12). Glyphosate increased protein content in buffelgrass and kleingrass and sometimes reduced production of buffelgrass and buffelgrass X birdwood hybrid. Factors Affecting Degradation Concentrations as high as 70 ppm of picloram were detected on grasses 2 hours after foliar application of 0.56 kg/ha or 1.12 kg/ha each of picloram and 2,4,5-T (23, 24). Only 842 ppb of picloram at 0.56 kg/ha could be detected on grass after application of 3.8 cm simulated rainfall (24). Photodecomposition may also be important in loss of picloram from treated vegetation (13). Herbi- cide dilution and metabolism by plant growth also influ- ences picloram loss. Persistence and Movement in Water Sources Movement of Picloram in Surface Water Trichell et al. (109) determined movement of picloram in runoff water from small plots 24 hours after applica- tion. Loss of picloram was greater from sod than from fallow. The maximum loss obtained for picloram, dicamba, or 2,4,5-T was 5.5%, and the average was approximately 3%. The time interval from picloram appli- cation to the first rainfall determined the amount of picloram that moved into the soil profile and/or the amount that moved away from the point of application with surface runoff. Four months after application, piclo- ram losses were <1% of that lost during the initial 24 hours after application. Scifres et al. (95) indicated that picloram moved in surface runoff when 0.28 kg/ha was applied in the Rolling Plains of Texas for control of honey mesquite. Irrigation the first 10 days after application resulted in a concentration of 17 ppb of picloram in surface runoff. irrigation at 20 or 30 days resulted in <1 ppb of picloram residue in runoff water. No more than 1 or2 ppb picloram was detected after dilution of runoff water into ponds. Baur et al. (14) studied picloram residues from a 6.1-ha watershed treated with the potassium salt of picloram at 1.12 kg/ha near Carlos, Texas, on an Axtell fine sandy loam soil. Samples were collected directly below the treated area and in streams below the plots after each heavy rainfall. Within 4 days after treatment, picloram residues in runoff water ranged from 9 to 168 ppb after heavy rainfall. After 3 months, concentrations of 5 ppb or less of picloram were found in runoff water. After 1.5 weeks with initial treatments in April, no piclo- ram was found in streams from 0.8 to 3.2 km from the treated area. After a 6.1-cm rainfall 10 months after treatment, no picloram was detected in runoff water regardless of sampling location. \ Research shows that herbicide residues can occur in surface runoff water if heavy rainfall occurs soon after treatment. When pelleted picloram was applied at 2.24 kg/ha to a 1 .3-ha rangeland watershed, surface runoff of 1.5 cm from a 21cm rain received 2 days after treat- ment contained an average of 2.8 ppm of picloram (37). However, picloram content declined rapidly in each successive runoff event, and runoff water contained <5 ppb by 2.5 months after application. Loss of the potas- sium salt of picloram from grassland watersheds in surface mnoff water was similar whether the picloram was applied as aqueous sprays or as pellets on a Houston black clay soil. Picloram plus 2,4,5-T at 0.56 kg/ ha each were applied May 4, 1970, December 4, 1970, May 4, 1971 , and October8, 1971 (24). No runoff event occurred until July 25, 1971, 72 days alter the third herbicide treatment. Concentration of 2,4,5-T and piclo- ram averaged 7 and 12 ppb, respectively, in runoff water and <5 ppb during subsequent runoff events. Data indicated that picloram or 2,4,5-T content was typically <5 ppb in runoff if major storms occurred 1 month or longer after treatment on the Houston black clay. On sandy soils, Scifres et al. (99) found only trace amounts of picloram or 2,4,5-T, which had been applied at 0.56 kg/ha each, in surface runoff water following storms about 30 days after application. Mayeux et al. (69) found that maximum concentra- tions of picloram were 48 and 250 ppb in initial runoff from an 8-ha area treated with 1.12 kg/ha in 1978 and 1979, respectively. Herbicide concentration decreased with distance from the treated area in proportion to the size of adjacent, untreated watershed subunits that contributed runoff water to streamflow. About 6% of the applied picloram was lost from the treated area during active transport. Movement of Picloram in Subsurface Water Bovey et al. (23) conducted an investigation to deter- mine the concentration of 2,4,5-T and picloram in sub- surface water after spray applications to the surface of a seepy area watershed and lysimeter site in the Black- lands of Texas. A 1 :1 mixture of the triethylamine salts of 2,4,5-T plus picloram was sprayed at 2.24 kg/ha every 6 months onthe same areafor atotal of five applications. Seepage water was collected on 36 different dates, and 1 to 6wells inthe watershed were sampled at 10 different dates during 1971, 1972, and 1973. Concentration of 2,4,5-T and picloram in seepage and well waterfrom the treated area was extremely low (<1 ppb) during the 3- year study. No 2,4,5-T was detected from 122 drainage samples from a field lysimeter at another site sampled fort year aftertreatment with 1.12 kg/ha of a 1 :1 mixture of the triethylamine salt of 2,4,5-T plus picloram. Piclo- ram at levels of 1 to 4 ppb was detected in lysimeter water from 2 to 9 months after treatment. Supplemental irrigation in addition to a total of 85.5 cm natural rainfall was used to leach picloram into the subsoil. In another study, Bovey and Richardson (36) found ~ that picloram and clopyralid remained in the uppermost 30 cm of a Houston black clay soil. The herbicides were sprayed at 0.56 kg/ha each on the same area in 1988 - and 1989 on a seepy site overlying a shallow, perched water table. No herbicide was detected in subsurface water from the area in 1988, but concentrations of <6 ppb of both herbicides were detected in subsurface water collected 11 days and from 41 to 48 days after treatment in 1989. Dissipation of Picloram from Impounded Water Sources Research conducted in semiarid and subhumid envi- ronments have shown that most picloram was dissi- pated from impounded, natural water sources within a month to 6 weeks after introduction (53). However, concentrations of picloram from 1 to 2 ppb were detect- able a year after application of 1.1 kg/ha to these ponds. In no case did treated areas adjacent to domestic water wells that were 9 to 46 m deep result in picloram residues in wells. Once picloram moved into water catchments in the Rolling Plains, residues were detected for at least a year after treatment (95). Dilution is important in the dissipation of picloram from impounded water. Photodecomposition is also important in reducing picloram concentration in water. In the photolysis of picloram, certain levels of light energy are necessary for degradation of each molecule. As- suming light energy is randomly dispersed, then inter- ception of photons by picloram molecules would be a random occurrence. In such a system, degradation of picloram would be expected to occur rapidly at first, then decrease as fewer molecules were available for light interception. Such dissipation curves were reported by Haas et al. (53), who found that most rapid dissipation occurred within the first 3 to 4 weeks after application of picloram to impounded water. In such a concentration- dependent system, more energy must be applied for degradation of the remaining herbicide molecules than required at higher picloram concentrations (13). Clopyralid Clopyralid is included next because it is chemically closely related to picloram but reacts differently to cer- tain weed species and is less persistent in the environ- ment (3). Clopyralid is less effective than picloram on most broadleaf species but is highly effective against certain broadleaf weeds such as those in the Polygonaceae, Compositae, and Leguminosae fami- lies. It has little or no activity against grasses or crucifers. Mixtures of clopyralid with other growth regulator-type herbicides extends the spectrum ot species controlled. Differing from picloram, clopyralid is resistant to photo- decomposition but is more susceptible to degradation by microbes (3). In a wide range ot soils across the United States, clopyralid degrades at a medium to fast rate. lt has a halt-life ranging from of 12 to 70 days (3). Persistence in Soil and Movement in Water Sources A 1:1 mixture of the rnonoethanolamine salts of clopyralid and the tri-isopropanolamine salt of picloram was applied at 0.56 kg/ha each in May 1988 and June 1989 to the same area (36). Approximately 90 days after treatment, >99% of the clopyralid as compared with 92% of the picloram had dissipated. Most herbicide was detected in the upper 3O cm of soil. Neither herbicide was detected after 1 year. Neither herbicide was detected in subsurface water from the treated area in 1988, but concentrations of <6 ppb of clopyralid or picloram were detected in subsur- tace water collected 1 1 days and from 41 to 48 days after treatment in 1989. The study represents a worst case scenario because the herbicides were applied twice to bare soil and were disked into the soil to prevent loss from photodegradation. Under normal practices, the herbicides may be applied once every 5 to 20 years to weeds and brush and are not protected from photo- degradation by disking. Persistence in Plants See triclopyr section on persistence in plants. Phenoxys Persistence in Soil For more than 40 years, many investigators have recognized that 2,4-D was rapidly inactivated in moist soil (40). Warm, moist soil accelerates degradation ot phenoxy herbicides by stimulating microbial activity. After application in three Oklahoma soil types, Altom and Stritzke (2) found that the average half-life of the diethanolamine salts of 2,4-D, dichlorprop [(i)-2-(2,4- dichlorophenoxy) propanoic acid], silvex [(2,4,5- trichlorophenoxy) propanoic acid], and" 2,4,5-T were 4, 10, 17, and 40 days, respectively. ln Texas, Bovey and Baur (20) applied the propylene glycol butyl ether esters of 2,4,5-T at 0.56 and 1.12 kg/ ha to soils at five locations. Alter 6 weeks, 2,4,5-T had disappeared from all locations. At three different loca- tions on sandy soils in central Texas, Scitres et al. (99) found that 2,4,5-T was reduced to trace levels of <10 ppb in 7, 28, and 56 days. Residues of 2,4,5-T were not detected below 15 cm and generally remained in the upper 2.5 cm ot soil. Influence of High Rates Early work by Crafts (45) and others indicated that 2,4-D typically did not persist from one growing season to another even at high rates, largely because of micro- bial breakdown. Work by Bovey et al. (35) in Puerto Rico indicated that corn, sorghum, wheat, rice, soybean, and cotton could be grown in soils 3 months after applica- tion of a 1:1 mixture of the n-butyl esters ot 2,4-D plus 2,4,5-T at 26.9 kg/ha without reduction in fresh weight. Except tor soybean, which was sensitive to picloram residues, similar results were obtained with these crops for a 2:211 mixture of 2,4-D plus 2,4,5-T plus picloram at 16.8 kg/ha. Young et al. (117) reported that 2,4-D and 2,4,5-T were applied at massive doses to three areas at Eglin Air Force Base in Florida in the 1960's. Chemical analysis of soil cores collected in 1970 from the treated area indicated that the herbicides had degraded. Effect of Repeated Treatment Gas chromatographic analysis of Canadian soils indi- cated no residual amounts of 2,4-D and MCPA after 40 and 34 annual treatments, respectively (105). Under laboratory conditions, the breakdown of 2 kg/ha of (“C)2,4-D or (“C)MCPA [(4-chloro-2-methlyphenoxy) acetic acid] was slightly faster in soils that had received continuous applications with the appropriate herbicide, which suggests that soil microbial populations adapted in response to repeated long-term use. In two separate studies in Texas, Bovey et al. (23, 24) tound that 2,4,5-T did not accumulate in soils when applied five times at 0.56 or 1.12 kg/ha every 6 months on the same area. In plots receiving 0.56 and 1.12 kg/ha of 2,4,5-T, average concentration did not exceed 95 and '\ 144 ppb, respectively, and most herbicide was confined to the upper 15 cm of soil and generally disappeared by the time of retreatment. Modes of Breakdown in Soil As indicated, soil microorganisms contribute greatly to the detoxification of phenoxy herbicides (40). Other means of degradation include chemical decomposition, thermal loss and volatilization, absorption in soils, and photodegradation. Temperature of the soil surface may easily reach 6O °C in the summer. Baur et al. (13) found 55% loss of 2,4,5-T as the free acid exposed to 60 °C but no loss at 30 °C after 2 days. The K* salt of 2,4,5-T adjusted to pH 7 showed 30% loss at both 30 and 60 °C after 7-day exposure. Baur and Bovey (9) exposed dry preparations of 2,4-D to 60 °C, which resulted in 75% loss of 2,4-D within 1 day after treatment. Herbicides 2,4-D and 2,4,5-T were also subject to breakdown by long-wave ultraviolet (356 nm) irradiation (9, 13). There- fore, 2,4-D or 2,4,5-T on soil and plant surfaces would be subject to loses by ultraviolet, thermal, and volatility in the field. Persistence in Plants Over a 3-year period, Morton et al. (76) studied the disappearance of 2,4-D, 2,4,5-T, and dicamba from pastures containing silver beardgrass (Andropogon saccharoides Swartz), little bluestem (A. scoparius Michx.), dallisgrass (Paspalum dilatatum Poir.), and sideoats grama. No important differences were found in persistence of different herbicides. The half-life of 2,4-D, 2,4,5-T, and dicamba in green tissue was from 2 to 3 weeks after application. Half-life in grass litter was slightly longer (3 to 4 weeks) than in green tissue. Shorter half-life of herbicides in green tissues was attributed to dilution by growth. Rainfall was important in hastening herbicide disappearance. Baur et al. (15) applied 2.24 kg/ha of the 2-ethylhexyl ester of 2,4,5-T alone and with 0.56, 1.12, and 2.24 kg/ ha of the potassium salt or isooctyl ester of picloram to pastures supporting infestations of live oak (Ouercus virginiana Mill.). Grass species indigenous to the site were little bluestem, brownseed paspalum (Paspalum plicatulum Michx.), and indiangrass (Sorghastrum spp.). Recovery of 2,4,5-T acid and ester from woody and grass tissues was greatest when applied with piclo- ram. Herbicide recovery in all treatments, however, were generally <10 and 0.1 ppm, 1 and 6 months, respectively, after application. Bovey and Baur (20) analyzed forage grasses from five locations in Texas comprising different grass spe- cies, soils, and climate that had been treated with the propylene glycol butyl ether esters of 2,4,5-T at 0.56 and 1.12 kg/ha. Six weeks after treatment, an overall aver- age of 98% of the 2,4,5-T had been lost from all treated areas. After 26 weeks, the herbicide levels in grass were very low, ranging from 0 to 51 ppb. In two separate studies, Bovey et al. (23, 24) applied a 1:1 mixture of the triethylamine salts of 2,4,5-T and picloram at a total of 1.12 and 2.24 kg/ha to the first and second experiment, respectively, on pasture land in central Texas. Repeat treatments were made every 6 months to the same area for a total of five applications. Herbicide content on native grass was high (28 to 113 ppm) immediately after spraying but degraded rapidly after each treatment and disappeared before new appli- cations were made. There was no accumulation of 2,4,5-T in soils or vegetation. Baur et al. (15) found that most of the 2,4,5-T applied at 2.24 kg/ha as the 2-ethylhexyl ester to live oak disappeared in 6 months. However, they detected small amounts of both the acid (93 ppb) and ester (233 ppb)‘of 2,4,5-T. At 1 and 6 months, more 2,4,5-T was found ‘in live oak tissue at the top of the plant than at the middle and lower stem because the top portion intercepts more spray than do lower regions. ln live oak, more 2,4,5-T was found when combined with picloram than was 2,4,5-T alone at equivalent rates. Brady (41) indicated that radioactive 2,4,5-T per- sisted three to seven times longer in treated woody plants as in forest soils. The half-life of 2,4,5-T was 5.5, 5.8, 6.7, and 12.4 weeks in loblolly pine (Pinus taeda L.), post oak (Ouercus stellata Wangenh.), sweetgum (Liq- uidambar styraciflua L.), and red maple (Acer rubrum L.), respectively. All four species decarboxylated 2,4,5-T and released CO2 with no significant difference among species or doses. Modes of Breakdown in Plants Basler et al. (4) established that the 2,4-D and 2,4,5-T breakdown in excised blackjack oak (Quercus marilandica Muenchh.) leaves was 50% or more in 24 hours. Morton (74) showed that approximately 80% of the 2,4,5-T absorbed by mesquite leaves was metabo- lized in 24 hours. Numerous other investigations also have shown the importance of metabolism in detoxifica- tion and loss of phenoxy herbicides in many plant species (40). Leaves and stems of plants are main receptors of foliar-applied herbicides. Aside from their function in decarboxylation, breakdown, and conjugation of the herbicide, leaves and plant parts may abscise or abort from the plant and fall to the soil, where the tissue and any residual herbicide may weather and decay. Aerial parts of plants may be removed by mowing machines or clipped and consumed by grazing animals. If the herbi- cide does not kill or stop growth of the plant, such as happens in many grasses, the herbicide will be diluted by growth. On plant surfaces, phenoxy herbicides are lost by photodegradation and volatilization in a manner similar to loss from soils. Rainfall is also reported as an impor- tant means of accelerating herbicide loss from litter and plant surlaces (23, 24, 76). Persistence and Movement in Water Sources Trichell et al. (109), using gas chromatographic and bioassay detection techniques, investigated the loss of 2,4,5-T, dicamba, and picloramfrom bermudagrass and fallow plots of 3 and 8% slope. When determined 24 hours after application of 2.24 kg/ha, a maximum of about 2, 3, and 5 ppm picloram, 2,4,5-T, and dicamba, respectively, were found in runoff water after 1.3 cm of simulated rainfall. Losses of dicamba and picloram were greater from sod than from fallow plots, whereas 2,4,5-T losses were approximately equal. Four months after application, picloram, 2,4,5-T, and dicamba con- centration in runoff waterfrom sod plots had diminished to 0.03, 0.04, and 0 ppm, respectively. Maximum loss of any herbicide from the treated area was 5.5% and averaged 3%. Bovey et al. (24) sprayed a 1 :1 mixture of the triethylamine salts of 2,4,5-T plus picloram at 1.12 kg/ha every 6 months on a nativegrass watershed for a total of five treatments. Plant “wash-off" was the main source of herbicide detected in runoff water. Concentrations of both herbicides was moderately high (400 to 800 ppb) in runoff water if 3.8 cm of simulated rainfall was applied immediately after herbicide application. lf major natural storms occurred 1 month or longer after herbicide treat- ment, concentration in mnoff water was <5 ppb. Norris and Moore (86) and Norris (83) indicated that concentration of 2,4-D, 2,4,5-T, picloram, and amitrole seldom exceeds 0.1 ppm in streams adjacent to care- fully controlled forest spray operations in Oregon. Con- centrations exceeding 1 ppm have never been observed and are not expected to occur. Chronic entry of these 1O herbicides into streams did not occur for long periods after application. Impounded Water Bovey and Young (40) summarized the literature on the fate of 2,4-D and other phenoxys in impounded water. ln general, phenoxy decompose rapidly, espe- cially if adapted microorganisms are present. Photodegradation of phenoxys in impounded water is also an important means of breakdown. Groundwater Wiese and Davis (1 16) applied 500 ml of water to wet tubes (7.6 x 61 cm) of dry Pullman silty clay loam topsoil to a depth of 56 cm. The diethylamine salts of 2,3,6-TBA (2,3,6-trichlorobenzoic acid) and PBA (chlorinated ben- zoic acid) leached to about 51 cm, while the amine salt ot 2,4-D and the sodium salt of fenac (2,3,6- trichlorobenzeneacetic acid) leached to 38 cm. The amine salts of silvex and 2,4,5-T leached to approxi- mately 23 cm. Esters of silvex, 2,4,5-T, and 2,4-D remained in the top 8 cm of soil. When excessive water (34.4 cm) was used to wet soil in the tubes, all herbicides could be detected in the leachate except monuron (N'- (4-chlorophenyl){LN-dimethylurea) and the ester for- mulation of 2,4,5-T. O'Connor and Wiergenga (87) in New Mexico studied degradation and movement of 64 kg/ha of 2,4,5-T in Iysimeter columns in the greenhouse. They concluded that pollution of groundwater from normal application rates of less than 2 kg/ha of 2,4,5-T is unlikely because of its relatively slow rate of movement in soil and its rapid biological detoxification. Edwards and Glass (48) applied 11.2 kg/ha 2,4,5-T (excessively high rate) to a large field Iysimeter in Coshocton, Ohio. The total amount of 2,4,5-T found in percolation water intercepted at 2.5 m deep for as long as 1 year after application was insignificant. Bovey and Baur (20) found little or no 2,4,5-T 12 weeks after treatment in soils at five widely separated locations in Texas after treatment with the propylene glycol butyl ether esters of 2,4,5-T at 0.56 and 1.12 kg/ ha. Bovey et al. (23) conducted an investigation to deter- mine the concentration of 2,4,5-T and picloram in sub- surface water after spray applications of the herbicides to the surface of a seepy area watershed and Iysimeter in the Blacklands of Texas. A 1 :1 mixture of the triethylamine salts of 2,4,5-T plus picloram was sprayed '\ at 2.24 kg/ha every 6 months on the same area for a total of five applications. Seepage water was collected at 36 different dates, and 1 to 6 wells in the watershed were sampled at 10 different dates during 1971, 1972, and 1973. Concentration of 2,4,5-T and picloram in seepage and well water from the treated area was extremely low (<1 ppb) during the 3-year study. No 2,4,5-T was de- tected from 122 drainage samples from a field lysimeter at another site sampled for 1 year after treatment with 1.12 kg/ha of a 1:1 mixture of the triethylamine salt of 2,4,5-T plus picloram. Picloram was detected in lysim- eter water at only 1 to 4 ppb during 2 to 9 months after treatment. Supplemental irrigation in addition to 85.5 cm natural rainfall leached 2,4,5-T and picloram into the subsoil. Surveys An extensive analysis of surface waters of Texas in 1970 for 2,4-D, 2,4,5-T, and silvex revealed zero or trace levels of these herbicides (47). Hectares of bmsh sprayed with 2,4,5-T annually in the 1960's was generally less than 0.4 million. Out of a total of 43 million ha of range and pasturelands, approximately 0.8 million ha of pas- ture weeds were sprayed annually with 2,4-D in Texas (111). Some herbicide was introduced into the environ- ment each year but did not contaminate surface waters. Mode of Breakdown in Water Phenoxy herbicides do not persistent in water sources, and significant concentrations, if found, occur within a short time after treatment (40). Loss of herbi- cides from treated areas by movement in runoff water is a very small percentage of the total amount applied even under intensive natural or simulated rainfall. Phenoxy herbicides rapidly dissipate in streams and are not detected downstream from points of applica- tion. In impounded water, phenoxys decompose rap- idly, especially if adapted microorganisms are present. Even under large-scale applications to surface water sources, 2,4-D disappeared rapidly after application, and concentrations remained low or undetectable. ln surveys of major river systems in the United States, 2,4-D appeared infrequently and in minute concentra- tions. Spray Drift Potential Maybank and Yoshida (68) indicated that a typical droplet-size distribution produced by herbicide spray nozzles using water diluent contained droplets of <100 11 um in diameter that were subject to drift. This could amount to 20% of the total spray volume, depending upon the type of nozzles and pressures used. Smith and Wiese (1 06) found that application of 2,4-D at 0.05 to 0.1 kg/ha to cotton caused significant yield loss. The earlier the cotton was sprayed, the more severe the damage. Studies by Maybank and Yoshida (68) indicated that drift of herbicide at 0.04 kg/ha approached those con- centrations causing injury to cotton. If precautionary measures are not taken, spray droplets of <100 um may drift several hundred meters, and application rates of 2,4-D at 0.5 kg/ha or higher may damage adjacent sensitive crops. At four locations in Texas, Behrens et al. (17) found that 2,4-D caused more leaf malformation in cotton than did 2,4,5-T and MCPA. Silvex did not cause leaf malfor- mations. Similarly, 2,4-D caused greatest reduction in cotton yield followed by 2,4,5-T and MCPA. Silvex caused the least reduction in yield. Smith and Wiese (106) compared 2,4-D to dicamba, picloram, bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), and 2,3,6TBA. The orderof damage to cotton was 2,4-D ester> 2,4-D amine >> dicamba > MCPA > picloram >> bromoxynil >> 2,3,6-TBA. Sprays of 2,4-D, dicamba, or MCPA at 0.1 kg/ha reduced lint yields from 20 to 97%. Yield losses were most severe when cotton was sprayed before blooming. However, lint quality (micronaire and length) was not affected by these herbicides. ‘Tamcot’ cotton seedlingswereinjured byfoliarspraysof2,4,5-T,triclopyr, and clopyralid at 0.03 kg/ha in the greenhouse (29). No new growth occurred when cotton was treated with 0.14 or 0.56 kg/ha of 2,4,5-T or triclopyr. Clopyralid was less injurious to cotton than were triclopyr and 2,4,5-T, and only slight leaf malformations occurred at clopyralid rates of 0.03 kg/ha or less. Because clopyralid has shown excellent control of honey mesquite (Prosopis glandulosa Torr.) in Texas, damage from spray drift of this herbicide should be minimal. Methods to control spray drift and volatility of 2,4-D and other herbicides are discussed elsewhere (115). Injury of cotton and othercrops from 2,4-D has occurred in Texas. Such residues in soils or plants are short lived but can cause significant injury the season of applica- tion. Dicamba Persistence in Soil Scifres and Allen (91) indicated that dicamba applied at 0.28 kg/ha dissipated from grassland soils of Texas in 4 weeks and in 9 to 16 weeks at 0.56 kg/ha. Dicamba residues were generally detected no deeper than 120 cm in clay or sandy loam soils. However, dicamba residues were detected at 120 cm deep 53 weeks after application of granules at 1.68 or 2.24 kg/ha to sand in semiarid grassland. Under moist, warm soil conditions, dicamba has a half-life of <14 days (103) as a result of microbial degradation (62). Dicamba readily converts through microbial activity to 3,6-dichlorosalicyclic acid (DCSA) (103,104). DCSA can undergo breakdown but breakdown has been reported to be slower than for dicamba (103,104). DCSA adsorption to soils is signifi- cant (79). Although dicamba is minimally adsorbed to soils, its residues are short lived and unlikely to become a problem in groundwater. Modes of Breakdown ln Soil Microbial degradation is highly important in disap- pearance of dicamba (62, 103, 104). Bacteria that utilize dicamba have been isolated and identified (62). Persistence in Plants Herbaceous Plants Morton et al. (76) studied the disappearance of 2,4-D, 2,4,5-T, and dicamba over a 3-year period from a pasture containing silver beardgrass, little bluestem, dallisgrass, and sideoats grama. No important differ- ences were found in persistence of different herbicide formulations. The half-life of 2,4-D, 2,4,5-T, and dicamba in green tissue was from 2 to 3 weeks after application. Half-life in grass litter was 3 to 4 weeks. The short residual of herbicides in green tissues was attributed to dilution by growth. Rainfall hastened herbicide disap- pearance. Effect on Forage Grasses and Cotton Vine mesquite tolerated 0.28 kg/ha dicamba applied preemergence (54). After emergence, ‘Premier’ sideoats grama tolerated 0.56 kg/ha dicamba. Pre- or postemer- gence applications of 1.12 and 2.24 kg/ha severely retarded shoot production of all species including ‘Blackwell’ switchgrass. All species germinated and grew without reduction in shoot production in soil con- taining as much as 63 ppb of dicamba. In greenhouse studies, dicamba applied preemer- gence and postemergence at 0.14 to 2.24 kg/ha injured seedling kleingrass (21). Mature plants of kleingrass, buffelgrass, King Ranch bluestem [Bofhriochloa 12 ischaemum(L.) var. Songarica (Rupr) Celarier& Harlan], green sprangletop (Leptochloa dubia H.B.K.), sideoats grama, common bermudagrass [Cynodon dactylon (L.) Pers.], and plains bristlegrass (Setaria macrostachya H.B.K.) tolerated dicamba and 2,4-D at rates as much as 2.24 kg/ha. Rates of 2,4-D at 1.12 kg/ha injured buffelgrass, which tolerated dicamba (21). In the field, dicamba, 2,4-D, or 2,4,5-T generally did not reduce vegetative production of common, Coastal, or coastcross-1 bermudagrass when applied in spring or fall (31). In central Texas, herbage production of native forage grass was increased when whitebrush [Aloysia gratissima (Gillies & Hook.) Troncoso] was controlled by dicamba or picloram plus dicamba (32). Smith and Wiese (106) indicated that sprays of 2,4-D, dicamba, or MCPA at 1.12 kg/ha reduced lent yields of cotton from 20 to 97%. Yield losses were most severe when cotton was sprayed before blooming. However, these herbicides did not affect lint quality (micronaire and length). Modes of Breakdown in Plants Dissipation of dicamba from plants can occur by exudation through roots into the surrounding soil, by metabolism within the plant, or by loss from leaf surfaces (3). Loss by ultraviolet light is also suggested (13). Persistence and Movement in Water Sources Trichell et al. (109) studied dicamba runoff from sloping sod plots in Texas. They found that as much as 5.5% of the applied dicamba was recovered in runoff water when 1.3 cm artificial rain was applied 24 hours after herbicide application. No dicamba was found in runoff water from a similar artificial rain application 4 months later after a 21.6-cm natural rainfall event. Approximately 8% of the artificial rain was recovered as runoff. Norris (84) found maximum dicamba levels of 37 ppb about 5.2 hours after treatment at 1.3 km from the point where the sample stream entered the treatment unit in Oregon. Dicamba residues detected the first 30 hours after application resulted from drift and direct application to exposed surface water. By 37.5 hours, residue levels had declined to background levels; no dicamba residues were found more than 1 1 days after application. Dicamba levels found in streams were several orders of magni- tude below threshold response levels for fish and mam- mals. \ ln 1984, Muir and Griff (78) sampled the Ochre and Turtle Rivers which flow into Dauphin Lake in western Manitoba, Canada, to determine levels of MCPA, diclofop {(1)-2-[4-(2,4-dichlorophenoxy) phenoxy] propanoic acid}, dicamba, bromoxynil, 2,4-D, triallate [s-(2,3,3- trichloro-2-propenyl)bis(1-methylethyl)carbamothioate], and trifluralin [2,6-dinitro-Nfl-dipropyl-4-(trifluo- romethyl)benzenamine], which were used widely in each watershed. Dicamba and 2,4-D were detectable throughout most of the sampling period in both rivers at low levels of <1 ppb. Levels of <6 ppb of dicamba and 2,4-D were detected in water from the Turtle River before a high-water event, possibly from sprayed ditches or rights-of-way nearthe river. Even so, discharges of all herbicides monitored in the study were <0.1% of the amounts used in each watershed. Levels of dicamba and 2,4-D in June were still far below toxic levels forfish or fish food organisms and below levels affecting water quality standards. Impounded Water Dicamba dissipated most rapidly from water under non-sterile, lighted conditions (92). Pond sediment evi- dently contained microbial populations capable of de- composing the herbicide. Temperature was crucial in dicamba dissipation, especially in the presence of.sedi- ment. In some cases, influence of sediment on dissipa- tion rate of dicamba was apparently augmented by light. Under su mmer conditions, dicamba at 4.4 kg/ha/su riace area of ponds dissipated at about 1 .3 ppm/day. Dicamba dissipated as a logarithmic function of concentration with time. Influence of Dicamba in Irrigation Water on Seedling Crops Crops varied in their response to one irrigation of water containing dicamba (92). ‘Dunn’ was the most susceptible cotton cutlivar. Fresh weights of Dunn seed- lings were reduced at 100 ppb of dicamba, whereas concentrations of 500 ppb were required for weight reduction in ‘Paymaster'. ‘Blightmaster' was the most tolerant cultivar studied. ‘Pioneer 820’ and ‘RS-626’ grain sorghums seedlings also tolerated all dicamba treatments. RS-626 at 500 ppb showed increased fresh weight. ‘Straight-eight‘ cucumber seedlings tolerated irrigation water containing as much as 50 ppb dicamba but were injured or killed by 100 and 500 ppb, respec- tively. Crop tolerance to dicamba in irrigation water from greatest to least were sorghum > cotton > cucumber. 13 Triclopyr Relatively little research has been done with triclopyr in Texas because of preoccupation with other herbicides and because studies on triclopyr residues were being done at other locations. Persistence in Soil In Texas, Moseman and Merkle (77) determined that triclopyr when applied in the fall persisted about 6 months in a Miller clay soil but dissipated 3 months after summer application. Jotcham et al. (61) in Canada indicated that triclopyr was slightly less persistent than 2,4,5-T but neither herbicide was biologically active during the next season. In four different soils, triclopyr and 2,4-D had similar mobilities as determined by soil TLC. Schubert et al. (90) reported that triclopyr residues in soil decreased from a maximum of 18 to 0.1 ppm in 166 days in a West Virginia watershed. At two sites in Oregon, Norris et al. (85) found that triclopyr and its metabolites persisted for 1 year or more in small concentrations. They speculated that dry sum- mers in Oregon may retard dissipation of triclopyr com- pared with West Virginian summers. Triclopyr residues were confined to the top 30 cm of soil. Newton et al. (82) in Oregon also found that triclopyr persisted in small amounts for 1 year in soil after aerial application. How- ever picloram, triclopyr, and 2,4-D residues decreased rapidly after application, leveled off 79 days after treat- ment, and then began a period of slow loss that contin- ued until the following summer. Newton et al. (82) found that picloram was lost quicker than triclopyr or 2,4-D as contrasted to results reported by Norris et al. (85), who observed that picloram persisted longer than 2,4-D. Norris et al. (85) was working in a nearby but drier area. Newton et al. (82) suggested that triclopyr is similar to 2,4-D in movement and persistence. Mode of Breakdown In Soils Leaching, photodegradation, and microbes degrade triclopyr (3). Persistence in Plants Bovey et al. (28) found more picloram than triclopyr in greenhouse-grown huisache [Acacia farnesiana (L.) Willd.] 0, 3, 10, and 30 days after treatment with foliar sprays, soil application, or soil-plus-foliar treatments. In field-grown honey mesquite, more clopyralid and piclo- ram than triclopyr or 2,4,5-T was detected in honey mesquite stem tissue (27). Triclopyr and 2,4,5-T resi- dues were generally <2 ppm by 3 days after application, whereas picloram and clopyralid residues were as high as 11 and 22 ppm, respectively. Concentrations of triclopyr and picloram recovered from honey mesquite stems were about 25% greater at 3 than at 30 days alter treatment, whereas concentrations of 2,4,5-T and clopyralid were about 50% greater at 3 than at 30 days after application. Concentrations of 2,4,5-T in standing dead stems were 0.2 and 0.4 ppm dry weight in upper stem phloem and upper stem xylem, respectively, 20 months after application (27). Phloem and xylem tissue taken from the base of dead stems had <0.01 ppm of 2,4,5-T and had little or none in live resprouts. Concen- tratio ns of triclopyr in dead stems ranged from 0.06 to 0.9 ppm, but generally the herbicide could not be detected in live resprouts. After 22 to 26 months, as much as 0.4 and 0.9 ppm of 2,4,5-T and triclopyr could be detected in dead honey mesquite stems that had fallen to the soil surface. Thorns also contained detectable concentrations of 0.1 ppm each of 2,4,5-T and triclopyr. ln comparison, concentrations of picloram ranged from 0.3 to 1.3 ppm dry weight 20 to 26 months after treatment in dead honey mesquite stems-standing or fallen on the soil (27). Concentrations of clopyralid in the same tissues ranged from 0.7 to 3.3 ppm. No clopyralid was detected in treated live stems, but concentrations of 0 to 0.04 ppm of picloram were detected. Picloram and clopyralid, at 0.3 and 0.8 pPlh, respectively, were de- tected in thorns from several dead stems. Norris et al. (85) found that triclopyr decreased rapidly from grasses in Oregon. Initial average concentrations of 527 ppm immediately aftertreatment were reduced to <0.3 ppm by 158 days aftertreatment. Newton et al. (82) found that 2,4-D, triclopyr, and picloram persisted in evergreen foliage and twigs for nearly 1 year. Crowns and browse layers showed similar rates of loss, but browse layer concentrations of 2,4-D and triclopyr were only about one-third of those in crown foliage. Despite shading, picloram decreased to low levels before rainfall and remained low but detectable. Salt formulations of the herbicides were lost faster than ester formulations, and herbicide residues decreased rapidly in litter and soil. Whisenant and McArlhur (114) showed the dissipa- tion of triclopyr from several herbaceous and woody species in northern ldaho. Triclopyr concentrations in foliage varied among species at two sites. The highest concentration of 362 ppm occurred in shinyleaf ceano- 14 thus (Ceanothus velutinus Dougl. ex. Hook.) 1 day after treatment, but by 365 days more than 98% of the triclopyr had dissipated from all species. Triclopyr resi- due data from the study and large herbivore toxicologi- cal data from other studies indicate that toxicity from proper use of triclopyr is unlikely. Effect on Plants Triclopyr was generally more phytotoxic to seedling ‘5855X127C’ corn, ‘TAM 0312’ oat, ‘MS 398' grain sorghum, and ‘Selection 75' kleingrass than was either 2,4,5-T or clopyralid (29). ‘Caddo’ wheat tolerated tri- clopyr at 0.56 kg/ha. Triclopyr and clopyralid caused greater injury to ‘Florrunner’ peanuts than did 2,4,5-T, whereas 2,4,5-T and triclopyr were more damaging to ‘Tamcot' cotton and ‘Liberty’ cucumber than was clopyralid. All three herbicides at 0.14 and 0.56 kg/ha killed ‘Gail’ soybean. Kleingrass was not affected by any rate of clopyralid. Factors Affecting Degradation Triclopyr is lost from grasses because of metabolism, growth dilution, wash-off, volatilization, and photode- gradation (85). Persistence and Movement in Water Sources Surface Runoff Water Schubert et al. (90) using a helicopter treated the upper part of a watershed in West Virginia with 11.2 kg/ ha triclopyr. Two streams transversed the treated area. Movement of triclopyr residues in soil and water downslope from the treated area was insignificant. Maximum concentration of triclopyr in stream water was 95 ppb the first 20 hours after application, similar to that observed for other herbicides applied to forest streams (85). Reduction in concentration the first 20 hours after application was attributed to photodecompo- sition. In September during thefirst significant rains after application in May, maximum triclopyr residues of 12 ppb were found in a small pond at the site. A 6-cm rain on November 9, causing a 6,500-L stream discharge, increased triclopyr concentrations to 15 ppb, but after November 11 no more triclopyr was detected. Groundwater Triclopyr was applied in both ester and amine formu- lations on October 24, 1986, to Coastal Plain flatwood '\ watersheds near Gainesville, Florida (43). Panicum grasses (Panicum spp. and Dichanthelium spp.), wiregrass (Arietida stricta), gallberry (llexglabra), and most herbaceous plant species were controlled by both formulations. Triclopyr application resulted in a shift towardabluestem-dominated understory. Triclopyr resi- dues were detected at trace levels of 1 to 2 ppb in storm runoff during the first runoff event after application. No triclopyr residues were detected in subsequent runoff events or in any groundwater wells for 6 months after application. Impounded Water Examination of triclopyr and by-product 3,5,6- trichloro-2-pyridinol (TCP) residue dissipation following application of the triethylamine salt of triclopyr at pre- scribed rates showed that no adverse effects should be produced on the aquatic environment (52). The results showed that detectable triclopyr levels in water were variable from 3 to 21 days, residue half-life being less than 4 days. Residue accumulation in sediment, plants, and fish was negligible. TCP concentrations and persis- tence weretransitory. However, results of crayfish evalu- ation indicated prolonged persistence of triclopyr and TC P. Further evaluation of triclopyr and TCP accumula- tion in clams and crayfish, separating the edible parts of the crayfish from the nonedible parts, must be accom- plished before a tolerance level can be established. MOGG Of Breakdown |l1 Water Photodegradation is a major means of triclopyr de- composition in water (3). Tebuthiuron Persistence in Soil Pelleted tebuthiuron was applied aerially on dupli- cate plots at 2.2 and 4.4. kg/ha in spring, summer, fall, and winter of 1978 and 1979 for mixed brush control (30). Soil was predominantly an Axtell tine sandy loam (Udertic Paleustalfs). Tebuthiuron persisted for more than 2 years in the Claypan Resource Area of Texas at depths of 0 to 15 cm and 15 to 30 cm as determined by ‘Tamcot’ cotton and ‘Caddo’ wheat bioassays. Tebuthiuron content ranged from 0.08 to 0.49 ppm. Deeper soil depths were not sampled. On a Houston Black clay (Udic Pellustert), pellets were broadcast and applied in bands at 2.24 kg/ha (22). Tebuthiuron was also detected to depths of 46 to 61 cm but not at 76 to '\ 91 cm deep. After 6 months, most tebuthiuron was 15 found in the 0- to 15-cm soil layer. Whethertebuthiuron leached deeper after 6 months is unknown. Tebuthiuron applied as a broadcast spray also resided mainly in the 0- to 15-cm layer. In another study, tebuthiuron at 2.24 kg/ha persisted in the 0- to 15-cm and 15- to 30-cm layers of soil for 25 months on a Lufkin fine sandy loam (Vertic Albaqualfs) but not in a Wilson Clay loam (Vertic Ochraqualfs) (73). The Lufkin fine sandy loam, how- ever, was underlain by a claypan at 15 to 30 cm deep, whereas the Wilson clay loam was more permeable. All studies mentioned are in an area with approximately 75 to 90 cm or more annual rainfall. In semiarid rangeland soils in north central Arizona, Johnsen and Morton (59) detected most tebuthiuron in the surface 30 cm of soil during the first 5 years after treatment, but small concentrations were detected as deep as 105 cm 6 and 9 years after treatment. After 9 years, from 55 to 75% of the tebuthiuron detected was at the depth of 60 to 90 cm. Factors Affecting Dissipation and Leaching Tebuthiuron has a half-life of 12 to 15 months in areas receiving 100 to 150 cm rainfall annually (3). Photode- composition and volatilization loss from soil is negligible. Some microbial breakdown occurs, but the half-life of tebuthiuron is considerably greater in low rainfall areas and in soils of high organic matter regardless of rainfall. Chang and Stritzke (44) found that after six successive desorption extractions, 40% of the tebuthiuron was adsorbed to soil with 4.8% organic matter, but less than 1% was adsorbed to soil with 0.3% organic matter. Soil mobility of tebuthiuron was greater in soil with low organic matter and low clay content. Greater dissipation occurred at 15% soil moisture and 30 °C than at lower moisture and temperature levels. Baur (6) also found that leaching of tebuthiuron was inversely related to clay content of soil and directly related to rate of application. Tebuthiuron is more phytotoxic in soils low in clay and/ or organic matter (44, 46). Therefore for these reasons, one could expect greater tebuthiuron persistence in semiarid soilsthan soils in humid areas, as discussed by Johnsen and Morton (59). Distribution and Dissolution of Pellets Whisenant and Clary (113) indicated that using a 40% active extruded pellet at 0.6 and 1.1 kg/ha left residues of 9 to 21% and 17 to 38% of the treated areas, respectively. The lower percentages were from a soil with 47 g/kg soil organic carbon (OC), and higher per- centages were on loam soils with 17 and 18 g/kg OC. Van Pelt and West (110) placed large tebuthiuron bri- quettes of 1.8 g, 13% active ingredient (a.i.) beneath pinyon pine trees at the dripline, midcrown, and stem base. Residues analysis indicated that overland runoff, wind, and animals did not move briquettes. Effect on Plant Growth Greenhouse. Baur and Bovey (10) compared the growth inhibition ot five herbicides by applying 1.4 to 1,434 ug/plant to one unifoliolate leaf of ‘Southern blackeye’ cowpea and the partly unfurled true leaf ot ‘Topland’ sorghum seedlings. The order of decreasing effectiveness for growth and herbicidal effectiveness for cowpea and sorghum was paraquat (1 ,1'-dimethyl-4,4'- bipyridinium ion), glyphosate, tebuthiuron, 2,4-D, and endothall (7-oxabicyclo [2.2.1] heptane-2,3-dicarboxy- lic acid). Tebuthiuron and glyphosate had little inhibitory effect on germination of sorghum, cowpea, or ‘Era’ wheat. Tebuthiuron applied preemergence or early poste- mergence at 0.6 kg/ha injured buffelgrass (26). Buftelgrass became more tolerant with age to as old as 150 days, but plants were still injured at 1.1.kg/ha ot tebuthiuron applied as foliar sprays to plants growing in pots. Buftelgrass [Pennisetum ciliare (L.) Link] shoot and root weights were reduced by 2 to 4 ppm of tebuthiuron placed 0 to 3, 8 to 11, or 15 to 18 cm deep in soil columns 30 days after emergence (89). Plains bristlegrass seedling shoot weights were not reduced when 2 ppm of tebuthiuron were placed 8 to 1 1 cm deep or deeper. Field. Common bermudagrass and kleingrass toler- ated March and April applications of tebuthiuron at 2.2 kg/ha using an 80% wettable powder formulation, but June applications reduced production (12). Buftelgrass and buffelgrass X birdwood hybrid tolerated tebuthiuron at 0.4, 1.1, and 2.2 kg/ha with March, April, and June applications. Coastal bermudagrass tolerated March but not April or June treatments. Tebuthiuron had little effect on protein concentrations of common or Coastal bermudagrass, buffelgrass, and kleingrass but reduced protein concentrations in the buttel X birdwood hybrid. Masters and Scifres (67) reported that application of tebuthiuron pellets (20% a.i.) at rates as much as 2.2 kg/ ha did not affect in vitro digestible organic matter of little bluestem, Bahiagrass (Paspalum notatum Flugge), Bell rhodesgrass (Chloris gayana Ku nth), and green sprangletop but did increase foliar crude protein concen- trations of little bluestem the growing season of applica- tion. ln the South Texas Plains, tebuthiuron pellets (20% a.i.) at rates as much as 2.2 kg/ha at three locations did 16 not significantly decrease buffelgrass standing crop or foliar cover compared with untreated areas (55). Natural Areas. In the Texas Post Oak Savannah during spring, aerial application of tebuthiuron pellets (20% a.i.) at 2.2 and 4.4 kg/ha to heavy brush cover increased grass production the second growing season atter application (102). Treated native-grass stands consisted of a higher proportion of perennial species of good-to-excellent grazing value than stands on un- treated rangeland. Aerial application of tebuthiuron pellets at 2.2 kg/ha to mixed brush in south Texas significantly increased grass standing crop at 1, 2, and 3 years atter treatment (100). Overall grazing of the grass stand was improved, but forb production and diversity were decreased where 1 kg/ha or more of herbicide was applied. Forage stand recovered after 3 years regardless of herbicide rate used. Reseeding on Treated Areas In January 1976, Baur (7) treated areas in the Texas Claypan Resource area near Leona, Texas, with 1.1 or 2.2 kg/ha of tebuthiuron using the wettable powder as a spray or 20% a.i. pellets. Tebuthiuron at 1.1 kg/ha suppressed weed cover and produced a 71% kleingrass cover. Tebuthiuron at 2.2. kg/ha prevented kleingrass establishment. ln 1977, kleingrass production in plots treated with 1.1 kg/ha tebuthiuron the same year ex- ceeded untreated areas, but 2.2 kg/ha of tebuthiuron markedly reduced kleingrass production. No Coastal bermudagrass survived in tebuthiuron-treated areas on the deep sand. ln other work, Baur (5) showed that annual ryegrass could not be established until 261 days and 68 cm of rainfall after treatment of 1.1 kg/ha of tebuthiuron as either spray or granule. Rates of 3.4 kg/ha prevented revegetation by johnsongrass on 95% of the area atter 499 days on the black clay loam soil. Persistence in Plants Herbaceous Plants Concentration of tebuthiu ron in Coastal bermudagrass was 438 ppm from spray applications ot 2.2 kg/ha but was <1 ppm from broadcast- or band-applied pellets at 2.2 kg/ha (22). Low concentrations are desirable in forage because livestock or wildlife may graze treated areas immediately attertreatment. Tebuthiuron concen- trations in forage from sprays decreased rapidly with time, and residues from sprays or pellets were <2 ppm \ vb -\ within 3 months aftertreatment in the Texas Blacklands prairie. In semiarid areas, tebuthiuron or its metabolites per- sisted as long as 11 years after treatment (60). Tebuthiuron was detected in sideoats grama and blue grama [Bouteloua graci/is (H.B.K.) Lag. ex Griffiths] 10 years after application of 6.7 kg/ha. Metabolites of tebuthiuron were detected in blue grama 1 1 years alter applications of 2.2, 4.5, and 6.7 kg/ha. Highest concen- trations of tebuthiuron plus metabolites were 25 ppm in blue grama 10 years after application of 4.5 kg/ha and 21 and 23 ppm in sideoats grama 9 and 10 years, respec- tively, after applications of 6.7 kg/ha. Only these 3 samples of 120 samples exceeded legal limits of 20 ppm of tebuthiuron plus metabolites in forage plants. No samples from plots treated with 4 kg/ha or less exceed- ed 10 ppm of tebuthiuron plus metabolites, and only 10% of them exceeded 5 ppm. Woody Plants Foliage, twigs, stems, and litter from recently killed Utahjuniperwuniperus osferosperma(Torr.) Littleltrees averaged 13.3, 0.4, 0.4, and 4.0 ppm of tebuthiuron plus its metabolites, respectively (58). Dead stems averaged 0.5 ppm in sapwood, 0.1 ppm in heartwood, and 0.4 ppm in bark 3 to 9 years after treatment. Root bark averaged 1.1 ppm and root wood averaged 0.5 ppm. The investi- gator concluded that residues have little potential harm when used as firewood or fenceposts. Persistence and Movement in Water Sources Surface Runoff Water Pelleted tebuthiuron was applied at 2.24 kg/ha to a 1.3-ha rangeland watershed. A 2.8-cm rain, 2 days after application, produced 0.94 cm of runoff, which con- tained an average of 2.2 ppm of tebuthiuron (22). Tebuthiuron concentration decreased rapidly with each subsequent runoff event. After 3 months, tebuthiuron concentration was <0.05 ppm; none was detected in runoff water 1 year after treatment. Concentration of tebuthiuron, applied as a spray at 1 .12 kg/ha, decreased to <0.01 ppm in runoff within 4 months from a small plot receiving simulated rainfall. 0n 0.6-ha plots, mean tebuthiuron concentration from sprays and pellets was 0.50 ppm or less in water when the first runoff event occurred 2 months after application. Concentrations of tebuthiuron in soil and grass from pellet applications ~. were <1 ppm and decreased with time. 17 Tebuthiuron applied at 1 kg/ha as 20% a.i. pellets to dry Hathaway gravelly, sandy loam soil in the spring diminished by 5% at the first simulated rainfall event, 37 mm, in runoff water and sediment (75). The second and third simulated rainfall events, 22 and 21 mm, respec- tively, removed an additional 2% of tebuthiuron. When tebuthiuron was applied to wet soil in the spring, the initial simulated rainfall events, totaling 42 mm, removed 15% of the tebuthiuron. When tebuthiuron was applied to wet soil in the fall, the initial rainfall events, totalling 40 mm, removed 48% of the tebuthiuron in runoff water and sediment. No significant differences were found in the total amount of tebuthiuron within the soil profile after application to dry and wet soils. More than half of the tebuthiuron had moved into the upper 7 cm 1 day after application. Tebuthiuron was not detected below 90 cm after 165 mm of simulated rainfall and 270 mm of natural rainfall. HYGFOIOQIC Effects Selected hydrologic variables were evaluated after conversion of heavily wooded sites to open grassland with a he rbicide-prescribed burning treatment sequence in east central Texas (66). Terminal infiltration rates and sediment production 3 years after aerial application of tebuthiuron pellets at 2.2 kg/ha for brush management differed little from values for untreated woody areas. Hexazinone Although hexazinone is used as a spot-soil treatment in Texas for control of honey mesquite and other woody plants (112), little work has been done in Texas on its residues in soils, plants, and water sources. Persistence in Soil The mobility of hexazinone in runoff water and its leachability in soil is well documented (1 , 18, 49, 50, 63, 118). Hexazinone movement downslope in runoff water can sometimes injure nontarget vegetation remote from the point of application (1). The high water solubility of hexazinone in water (3.3 g/100 g) contributes to its leaching potential. However, Prasad and Feng (88) found that after 1 year, hexazinone residues were re- duced to 1% at the treated spot and did not move laterally beyond 0.5 m on a sandy loam in Canada. Greenhouse studies in silt and sandy loam soils showed that half-life of hexazinone was 4 to 5 months (3). Under field conditions, half-life varied from 1 to 6 months depending upon location. Microbial breakdown contributes to decomposition in soil (3). Hexazinone photodegrades on the soil surface, but volatilization losses are negligible. Persistence in Plants Hexazinone and tebuthiuron were rapidly taken up by roots of seedling winged elm (Ulmus alata Michx.), bur oak (Ouercus macrocarpa Michx.), black walnut (Juglans nigra L.), eastern redcedar (Juniperus virginiana L.), and loblolly pine (70). Four hours later, “C was detected in all parts of winged elm treated with “C-tebuthiuron and “C-hexazinone. Root and foliar absorption varied with herbicide and species. However, the results indicated that selectively of tebuthiuron and hexazinone can be attributed to amount of intact herbicide translocated to the foliage. Loblolly pine and eastern redcedar pre- vented accumulation of the parent compound in the foliage within 24 hours. Demethylation was the primary detoxification mechanism of tebuthiuron by eastern redcedar, loblolly pine, and bur oak. Loblolly pine, a hexazinone-resistant species degraded hexazinone rap- idly into three unknown degradation products, thereby preventing its accumulation in foliage. Jensen and Kimball (57) using whole-plant metabo- lism studies with pear [Pyrus melanocarpa (Michx.) WiIld.] and bristly dewberry (Rubus hispidus L.) found no difference in “C accumulation in leaves but found a greaterformation of the mono demethylated metabolite, B,[3-cyclohexyl-6-methylamino-1-methyl-1 ,3,5-triazine- 2,4-dione] in the more tolerant P. melanocarpa. Effect on Plants Hexazinone controls many annual and biennial weeds, woody vines, and most perennial weeds and grasses, except johnsongrass (3). Hexazinone promises control of aquatic weeds and selective weeds in crops such as alfalfa, cacao, coffee, oil palm, pecans, sugarcane, rubber trees, tea, and certain conifers. Persistence and Movement in Water Sources Lavy et al. (63) found relatively small amounts of hexazinone in runoff water from a spot-gun application to a forest floor in Arkansas. Forest litter was highly effective in absorbing surface applications of hexazinone. ln another study, maximum concentration of hexazinone was 14 ppm in the stream that drained a 11.5-ha watershed treated with 2 kg/ha (18). Hexazinone resi- dues of <3 ppm were detected in stream discharge for 1 18 year after application. The amount of hexazinone trans- ported from the watershed in stream discharge repre- sented only 2 to 3% of the amount initially applied. Neary et al. (81) found only 0.53% loss of hexazinone in streamflow of the applied herbicide in Georgia. Resi- dues in streamflow peaked at 442 ppb in the first storm but declined rapidly and disappeared within 7 months. Total sediment yield increased by a factor of 2.5 be- cause of increased runoff associated with site prepara- tion using herbicide and salvage logging. However, sediment loading remained below those produced by mechanicaltechniques, and overallwaterquality changes were small and short lived. Leitch and Flinn (65) applied hexazinone at 2 kg/ha from a helicopter to a 46.4-ha catchment. Only 6 of 69 samples analyzed contained hexazinone, which was well below maximum allowable concentration of 600 ug/L for potable water. Aquatic Environment Polyethylene exclosures were located in atypical bog lake in north-eastern Ontario (107).Triclopyr, 2,4-D, and hexazinone were applied at 0.3 and 3, 1 and 2.5, and 0.4 and 4 kg/ha, respectively. Less than 5% of the triclopyr and 2,4-D remained in water after 15 days. As much as 25% of the 2,4-D absorbed to the side of the corrals. Triclopyr could not be detected after 42 days. At 0.3, 0.4, and 4 kg/ha, hexazinone could not be detected by 21 and 42 days after application. Hexazinone dissipated more rapidly than 2,4-D and was not absorbed to sedi- ments. Karbutilate Karbutilate is no longer available, but two papers demonstrate the fate of pelleted herbicides used in brush control. Karbutilate spheres containing 0.76 g a.i., were monitored in the Texas Post Oak Savannah soils (80). Karbutilate residues (1.1 ppm) were detected after 194 days in clay loam to 90 cm deep directly below the point of impact of the spheres. Vertical movementthrough the soil profile was more evident than lateral displace- ment. After 191 days, about twice as much karbutilate was detected in the upper 30 cm of a clay loam than detected in a loamy sand. Scifres et al. (101) applied karbutilate formulated as a ball 1.34 cm in diameter to brush-infested rangeland by aircraft and by hand in a grid pattern with 1.83-m spacing. The karbutilate balls eliminated all vegetation in a 24- to 45-cm diameter circle the year of application. Treated areas revegetated within 2 years after treatment of 2.24 kg/ha in the Post Oak Savannah but required 32 months in the Rolling \ w. \ Plains in sandy loam in sodgrasses such as tobosa [Hi/aria mufica (Buckl.) Benth.] and buffalograss. Glyphosate Glyphosate controls many herbaceous and woody plants (19). It is recommended that spray drift or mist of glyphosate not be allowed to contact green foliage, green bark, or suckers of desirable plants. Glyphosate has limited use on rangelands. Torstensson (108) indicated that glyphosate is rapidly adsorbed on soil. Adsorption occurs through the phos- phoric acid moiety that competes for binding sites with inorganic phosphates. Glyphosate is virtually immobile in soils. Half-life ranges from a few days to several months and is correlated with microbial activity of soils. Inactivation of glyphosate through soil adsorption is important. Bronstand and Friestad (42) concluded that regular use of glyphosate in agriculture or forestry allowed only very remote chances of contaminating the aquatic envi- ronment. The compound dissipates by microbial degra- dation, adsorption to sediments, and by photolysis. Literature Cited 1. Allender, W.J. 1991. Movement of bromacil and hexazinone in a municipal site. Bull. Environ. Contam. Toxicol. 46:284-291. 2. Altom, J.D., and J.F. Stritzke. 1973. Degradation of dicamba, picloram and four phenoxy herbicides in soils. Weed Sci. 21:556-560. 3. Anonymous. 1989. Herbicide handbook of the Weed Science Society of America. 6th Ed. Herbicide Hand- book Comm., N.E. Humbury, Chairman, WSSA. Champaign, IL. 301 p. 4. Basler, E., C.C. King, A.A. Badiei, and P.W. Santelmann. 1964. The breakdown of phenoxy herbicides in blackjack oak. Proc. South. Weed Conf. 17:351-355. 5. Baur, J.R. 1978. Effects of picloram and tebuthiuron on establishment and ryegrass winter pasture. J. Range Manage. 31:450-455. 6. Baur, J.R. 1978. Movement in soil of tebuthiuron from sprays and granules. Texas Agri. Exp. Sta. PR-3524. 14 p. 7. Baur, J.R. 1979. Establishing kleingrass and ber- mudagrass pastures using glyphosate and tebuthiuron. J. Range Manage. 32:119-122. 8. Baur, J.R., R.D. Baker, R.W. Bovey, and J.D. Smith. 1972. Concentration of picloram in the soil profile. Weed Sci. 20:305-309. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. Baur, J.R., and R.W. Bovey. 1974. Ultraviolet and vola- tility loss of herbicides. Arch. Environ. Contam. Toxicol. 2:275-288. Baur, J.R., and R.W. Bovey. 1975. Herbicidal effect of tebuthiuron and glyphosate. Agron. J. 67:547-553. Baur, J.R., R.W. Bovey, and C.R. Benedict. 1970. Effect of picloram on growth and protein levels in herbaceous plants. Agron J. 62:627-630. Baur, J.R., R.W. Bovey, and E.C. Holt. 1977. Effect of herbicides on production and protein levels in pasture grasses. Agron. J. 69:846-851. Baur, J.R., R.W. Bovey, and H.G. McCall. 1973. Thermal and ultraviolet loss of herbicides. Arch. Environ. Contam. Toxicol. 4:289-302. Baur, J.R., R.W. Bovey, and M.G. Merkle. 1972. Con- centration of picloram in runoff water. Weed Sci. 20:309- 313. Baur, J.R., R.W. Bovey, and J.D. Smith. 1969. Herbicide concentration in live oak treated with mixtures of piclo- ram and 2,4,5-T. Weed Sci. 17:567-570. Baur, J.R., R.W. Bovey, and J.D. Smith. 1972. Effect of DMSO and surfactant combinations on tissue concen- trafions of picloram. Weed Sci. 20:298-302. Behrens, R., W.C. Hull, and C.E. Fisher. 1955. Field responses of cotton to four phenoxy-type herbicides. Proc. South. Weed Conf. 8:72-75. Bouchard, D.C., T.L. Lavy, and E.R. Lawson. 1985. Mobility and persistence of hexazinone in a forest water- shed. J. Environ. Qual. 14:229-233. Bovey, R.W. 1985. Efficacy of glyphosate in non-crop situations. In E. Grossbard and D. Atkinson (eds.), The herbicide glyphosate. Butterworth & Co., Ltd. U.K. pp. 435-448. Bovey, R.W., and J.R. Baur. 1972. Persistence of 2,4,5-T in grasslands of Texas. Bull. Environ. Contam. Toxicol. 8:229-233. Bovey, R.W., J.R. Baur, and E.C. Bashaw. 1979. Toler- ance of kleingrass to herbicides. 32:337-339. Bovey, R.W., E. Burnett, R.E. Meyer, C. Richardson, and A. Loh. 1978. Persistence of tebuthiuron in surface runoff water, soil, and vegetation in the Texas Blacklands Prairie. J. Environ. Qual. 7:233-236. Bovey, R.W., E. Burnett, C. Richardson, J.R. Baur, M.G. Merkle, and D.E. Kissel. 1975. Occurrence of 2,4,5-T and picloram in subsurface water in the Blacklands of Texas. J. Environ. Qual. 4:103-106. Bovey, R.W., E. Burnett, C. Richardson, J.R. Baur, M.G. Merkle, and W.G. Knisel. 1974. Occurrence of 2,4,5-T and picloram in surface runoff water in the Blacklands of Texas. J. Environ. Qual. 3:61-64. Bovey, R.W., C.C. Dowler, and M.G. Merkle. 1969. 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Leaching of fluridone, hexazinone and simazine in sandy soils in the Netherlands. Nether- lands J. Agric. Sci. 37:257-262. M. w “w. [Blank Page in Original Bulletin] ‘ r .-‘.§- ‘ v I!’ » _, ~ ‘ i‘ 1 .. '1 1 n: [Blank Page in Orifiafl Bulletin] ' ‘ _ 4 ( .-..~ 1'4‘ ‘ . 4- r ~1 ._ . . 1*. a‘ “\ p ‘<4 [Blank Page in Original Bulletin] " Mention of a trademark or a proprietary product does not constitute a guarantee or a warranty of the product by The Texas Agricultural Experiment Station and does not imply its approval to the exclusion of other products that also may be suitable. All programs and information of The Texas Agricultural Experiment Station are available to everyone without regard to race, color, ; religion, sex, age, disability, or national origin. “*- Copies printed: 2,000