B-‘l ‘l ll July ‘I971 esiclual Characteristics of Picloram a In Grassland Ecosystems if“ Texas A&M University Texas Agricultural Experiment Station H. O. Kunkel, Acting Director, College Station, Texas Contents Summary Introduction ____________________________________________________ __ Toxicity of Picloram _______________ __; ______________ j Methods of Residue Detection Picloram Residues and Susceptible Crops ..................................... .. Photolytic Degradation of Picloram Vegetational Areas of Texas Studied ___________________________________________ __ Bf Theoretical Routes of Dissipation of Picloram from Rangel -: Dissipation from Soils ............................ .. Role of Temperature .................................................................. ..: Influence of Soil Properties Movement and Loss of Picloram Through Soil Profiles ............. Subhumid Rangeland Sites ...... _- “I Vegetated Areas _ ..... ._ Fallowed Areas Tropical Sites ............................... .. Vegetated Areas . . . _ . . . . . . .. Fallowed Areas .................................................................. ..'.l Semiarid Sites ............................ .. Influence of Slope on Distribution of Picloram in Soil P Persistence of Picloram in Soils Influence of Formulation of Residual Properties of Picloram in Soil. Movement of Picloram in Surface Water Dissipation of Picloram from Impounded Water Sources ........ .. Dissipation of Picloram from Grasses ............................................. a; Subhumid Areas ........................................................................ Semiarid Areas ........................................................................ .. Dissipation of Picloram from Broadle-af Herbs ........................... .. Dissipation of Picloram from Woody Species ............................... .. p? Subhumid Areas i Tropical Areas Semiarid Areas Acknowledgment Literature Cited Summary Picloram, applied alone or in combination with 2,4,5-T, has potential for control of many species of herbaceous weeds and woody plants on grass- lands. Picloram is relatively persistent, and complex factors influence the rate of dissipation from various segments of the ecosystem. Its persistent nature is one quality responsible for its effectiveness as an herbicide. Little data are available concerning the mode or degree of loss of picloram from the time it leaves the spray boom until it reaches the target areas. How- ever, once picloram reaches plant and soil surfaces, there is progressive decline within the ecosystem. It is susceptible to photodecomposition. Picloram is mobile within the ecosystem and follows the movement of water. Theoretically, picloram reaching the soil surface, if not photodecom- posed, may move vertically through the profile, laterally on the soil surface and to a limited degree laterally through the profile. The degree, direction and rate of movement are dictated by explicit characteristics of the vege- tation and soil and the rate of picloram applied. In general, when a low rate (0.5 pound per acre or le-ss) is applied to rangeland, especially those with heavy-textured soils, downward movement is much less than where higher rates are applied to highly permeable, sandy soils. In fine sandy soils, detectable residues were rarely moved beyond the top foot of the profile following the application of only 0.25 pounds per acre to range- land in North and West Texas. Present data from various sources do not indicate extensive sorption of picloram by the soil colloid or rapid detoxification by microorganisms. Dilution in the soil may be one of the most important, practical means of dissipating picloram. Subsurface lateral movement is also dependent on direction and rate of soil water flow. Subsurface lateral movement, however, is apparently of lesser importance than vertical mobility in the soil profile. On slopes exceeding 3 to 4 per- cent, lateral subsurface movement may be more important than indicated by available data especially following high application rates and heavy rainfall. Movement over the soil surface is governed primarily by intensity of rainfall, time-lapse from application to the first rain, rate of picloram applied, density and botanical composition of vegetation cover, texture of soil, and slope of the land. The longer the exposure on the soil surface before rainfall, the less picloram is available for movement. Rainfall of low intensity, especially before heavy rainfall, lessens the chance of surface runoff due to penetration of the soil by picloram. In NorthTexas, 17 parts per billion (ppb) of picloram occurred in surfacerunoff after application of 0.25 pound per acre to highly permeable, sparsely vegetated soils. Water samples were collected l0 days after treatment, immediately after applying 4 inches of simulated rainfall over a 9-hour period. Applications of l and 2 pounds per acre of picloram to rangeland in South and Southwest Texas did not result in detectable residues in domestic water wells where samples were taken for up to 2 years following application. ' Once runoff water was moved to surface watering ponds under experi- mental conditions, dilution of picloram residues in the ponds prevented detection. Current methods allow detection of one part picloram per billion parts of water. Direct application of picloram, under experimental 3 conditions, equivalent to l or 2 pounds per acre showed that dilution was an important mode of picloram dissipation. Dissipation curves, con- centration of picloram in pond water versus time, indicate loss to be concentration dependent. Initial loss rates are rapid, then concentrations “level off” at l or 2 ppb. However, these low concentrations may be detectable at up to a year after treatment of ponds. Research indicates vegetative growth of sensitive field crops would probably not be reduced using a single irrigation of water containing l to 4 pp'b of picloram. Large volumes of water containing 4 ppb of picloram repeatedly applied to seedlings would probably reduce crop growth. However, residues of l0 ppb or more in irrigation water could severely affect the growth of some sensitive crop seedlings. Recent research in dry areas of Texas indicate rapid initial loss of picloram from grasses. Loss rates of 2.5 to 3 percent per day for 30 days were measured. There are indications, however, that grass roots may extract picloram from the soil profile and transport it to aerial portions of the plants. Such accumulations have not been observed in more humid areas of the state where rates of four to eight times those used in drier areas ‘were applied and the picloram was rapidly leached to the lower portions of the soil profile. Picloram residues in the environment do not appear harmful to mammals, fish, birds or insects which inhabit the ecosystem. Mammals are tolerant to relatively high concentrations of the herbicide without displaying detrimental affects. Picloram passes rapidly, intact, through mammalian systems. Biological significance is related primarily to suscep- tible plant life, especially cultivated food and fiber crops, growing adjacent to rangeland. Damage to cotton and vegetable crops in adjoining areas is of the greatest concern to rangeland managers when chlorophenoxy herbicides are applied because of possibilities of drift. Similar or more extensive crop injury could result from picloram if drift were allowed. However, for many weed species, picloram may be applied in the fall when susceptible crops are not grown. Pelleted and granular formulations of picloram would minimize the drift hazard. 4 l?“ -. i“ ODUCTION (34) AS A GROWTH REGULATOR, ,5,6-trichloropicolinic acid (picloram) l1as J phytotoxicity over prolonged periods _~ widely evaluated for control of herba- y and woody plants in grasslands. An of the 107 million acres of Texas grass- “ ‘ ted with woody plants that reduce total fiction (62). Only about 2 percent, or gacres, of these grasslands were treated in sh control} About 0.98 million acres W with herbicides; the remainder were O y plants by mechanical methods. The ‘ treatment of extensive acreages with erbicides exists. However, the chemicals T otoxic to a broad spectrum of undesir- * yet safe for use near desirable plants and __'~ enoxy herbicides such as (ZA-dichloro- i 'c acid (2,4—D) and (2,4-5—trichlorophe- acid (2,4,5—T) are widely used for pasture H. improvement. In recent years, picloram i - 'ed extensively by research and extension Consequently, a wealth of information e proper use and potential hazards of l». been accumulated. a - tibility of some problem woody species _,4,5—T, picloram, and picloram combined A‘: is shown in Table 1. Many species, qeconomical rates of 2,4,5—T and related " yacetic acid herbicides, can be controlled _. picloram or mixtures of picloram with p» effectiveness of a 1:1 ratio of picloram for control of honey mesquite Prosopis ) DC. var. glandulosa (Torr.) Cockrell) as (54) and. for control of other specie-s regions (9) has been established. Piclo- iunication, Garlyn Hoffman, Extension range “" control specialist, Texas A8cM University, Col- ‘ - t given herein is for research purposes only. n ercial products or trade names is made with g that no discrimination is intended and no ‘ Texas A8cM University, The Texas Agricultural _‘ tion or U.S. Department of Agriculture is implied. i iclual Characteristics of Picloram i In Grassland Ecosystems R. W. Bovey and C. J. Scifres* ram alone, applied in the spring or fall, effectively controlled huisache (Acacia farnesiana (L.) Willd.) (15). A portion of the picloram could be replaced with 2,4,5—T and huisache control maintained if 1 pound per acre of each herbicide was applied. Picloram, applied to the soil in granular or pelleted form, controlled some species such as lotebush (Condalia spp.) and agarito (Berberis greggii A. Gray) that are not controlled by foliar sprays (Table 1). However, honey mesquite can be controlled by foliar sprays only. Some species such as Texas persimmon (Diospyros texana Scheele) and lime prickly ash (Zanthoxylum fagara (L.) Sarg.) are not effectively controlled by 2,4,5-T or picloram. Most woody species susceptible to 2,4,5—T are more effectively controlled in the spring at specific growth stages than in the fall. Most species are also most susceptible to picloram at the growth stage of maximum susceptibility to 2,4,5—T. However, species such as live oak (Queircus virginiana Mill.) and hui- sache can be controlled by fall applications of picloram after susceptible crops such as vegetables, soybeans and cotton are harvested (l3, l4, 15). Chemical drift from aerial sprays may damage susceptible species in adja- cent cropping areas in spring and summer months. Granular formulations of picloram may minimize the drift hazard encountered with sprays, and it can some- times be used in fall, winter and early spring months before susceptible crops are planted (l4). Picloram also controls a large number of herba- ceous broadleaf weeds on rangelands and pastures in addition to brush? Control of ‘broomweed (Gutierre- zia dracunculoides (DC.) Blake) may often be a con- comitant benefit from spraying honey mesquite when 2,4,5—T + picloram are used (57). Common broom- weed, controlled by low rates of phenoxy herbicides for a short period in the spring, may be controlled with 0.25 to 0.5 pound per acre of picloram through- *Research agronomist, Crops Research Division, Agricultural Research Service, U.S. Department of Agriculture, and assist- ant professor, Department of Range Science, Texas A8cM Uni- versity, College Station. ‘Ritty, P. M. 1967. Unpublished list. Susceptibility of plants to Tordon herbicides. The Dow Chemical Co., Midland, Mich. 5. out the growing season. Picloram also controls other herbaceous weeds which are resistant to 2,4—D or 2,4,5—T, such as western whorled milkweed (Asclepia- daceae ve'rticillata L.) (46). Chlorophenoxy herbicides, including 2,4—D and 2,4,5—T, dissip-ate rapidly in warm, moist soils. Kling- man (41) stated that 1 to 3 pounds per acre of 2,4,5—T will last only 1 to 5 weeks under such co-nditions. Picloram is more persistent than chlorophenoxyacetic acid herbicides in soil (24, 48, 49). Some residual picloram may be necessary for control of several species of undesirable range and pasture plants. Huisache can absorb lethal concentrations of picloram through the roots or leaves (4, 7). Granular applications of picloram, by virtue of the herbicide’s activity in soil, TABLE 1. SOME PROBLEM WOODY SPECIES IN TEXAS AND THEIR RESPONSE TO SINGLE SPRAY APPLICA- TIONS OF PICLORAM, 2, 4, 5-T, OR MIXTURES OF PIC- LORAM PLUS 2,4,5-T‘ Controlled by- Picloram Species Picloram 2,4,5-T + 2,4,5-T Honey mesquite (Prosopis juli- flora (Swartz) DC. var. glan- dulosa (Torr.) Cockerell) yes yes yes Live oak (Quercus virginiana Mill.) yes no yes Post oak 8c blackjack oak (Quer- cus stellata Wangenh.) 8c (Q. ma-rilandica Muenchh.) yes yes yes Macartney rose (Rosa bracteata Wendl.) yes no yes Winged elm (Ulmus alata Michx.) yes partial yes Cactus (pricklypear and tasajil- lo) (Opuntia spp.) yes partial yes Yaupon (Ilex vomitoria Ait.) yes no yes Whitebrush (Aloysia lycioides Cham.) yes no Blackbrush (Acacia rigidula Benth.) yes partial yes Huisache (Acacia farncsiana (L.) Willd.) yes no yes Mixed hardwoods yes yes yes Texas persimmon (Diosypros texana Scheele) no no no Spiney hackberry (granjeno) (Celtis pallida Torr.) yes no yes Twisted acacia (Acacia tortuosa (L.) Willd.) yes no yes Lotebush (Condalia spp.) no’ no partial Catclaw (Acacia greggii A. Gray) yes no yes Agarito (Berberis trifoliolata Moric.) no’ no no Lime prickly ash (Zanthoxylum fagara (L.) Sarg. (Z. pterota (L.) H.B.K.) no no no Yucca (Yucca spp.) no yes no ‘Table prepared with the aid of G. O. Hoffman, Extension Range Brush and Weed Control Specialist, Texas A&M Uni- versity, College Station. From Bovey, R. W. 1971. Hormone- like herbicides in weed control . Economic Bot. (In press). ‘Controlled with granular soil applications of picloram. 6 can be used to control species such as hui oak, yaupon (Ilex vomitoria Ait.) and (Aloysia lycioides Cham.) (1s,14,51). goal, then, should be to regulate the len 1; that herbicides such as picloram are activi ecosystem for maximum control of undesira and minimal damage to desirable species. This report will summarize data availa rate and routes of picloram dissipation fr) land ecosystems and will report addition’ lished data where appropriate. The discu; include data from many sources to emp explain research on Texas grasslands. ' Toxicity of Picloram “Picloram has a low order of toxicity , and fish” (44,65). The test animals have; avian, mammalian and aquatic species (44). V’ of the biological food chain, algae-daph adverse effects were observed from the in _ picloram into the water (35). Research onf of fish including rainbow trout (Salmo. Richardson), channel catfish (Ictalurus Rafinesque) and bluegill (Leponis macrocl nesque), and on game birds such as mall platyrhynchos) and bobwhite quail (Colin anus) showed that herbicides containing; present a low potential hazard, if any, to v _ fish (39). Acute toxicity expressed as u.._~ to half the test population (LD50) are ; Q 750 milligrams per kilogram body weight.‘ and greater than 1,000 milligrams per k' .3 sheep (65). Approximately 6,000 milligr n ; gram is listed as the LD5O for chicks. Chro f to rats over a 90-day period require-s d .;‘ parts per million (ppm) (0.3 percent) for noted in liver. No effects were noted |_ ppm (0.1 percent) over 90 days. Skin i -“ absorption of picloram through the skin 7 LD5O for rabbits by skin absorption was ; f 4,000 milligrams per kilogram. No c0 _ occurs from contact with eyes, and the; moderate. irritation heals readily. There if no known reports of human sickness res the handling or application of picloram. "‘ erally concluded that the use of piclora no hazard to humans, livestock or wildlife. , the treatment of rangeland or of small ;1 with effective dosages of picloram would“ in levels of residue in food or feed toxic‘ _’ or live-stock (47). I * Methods of Residue Detection Picloram residues are usually determi j, assay or gas-liquid chromatography (GLC)._§ methods, measurement of picloram c if through the reaction of a susceptible org j described by Leasure (43). The most ' procedure is to compare weights or sympto,’ toxicity of susceptible crops such as soybe max L.), sunflowers (Helianthus annus L.), i, '3 i" ‘c. Con clppb) =31270- 27:143(s0ybean wt.) n | I l ~ 502 .903 1.505 2.007 2090 1.806 2.398 3.000 Log Piclorcm Concentration (ppb) I I I 4| Example of a standard curve developed from soy- _ y for determining concentration of picloram in n, 1 vulgaris L.), or cucumbers (Cucumus sativa 1 in field samples with those of plants grown picloram concentrations (18, 43, 60). Stand- _ are developed with crop response versus YJCrop response in field samples is then con- ie~=picloram concentration. A typical standard _, g ovendry weights of soybeans versus piclo- e-v tration is shown in Figure l. Rarely is i: - ponse curve linear, regardless of bioassay method of evaluation (60). Using criteria g weight of the assay species, a logarithmic p 'p usually exists. Recent research describes y system using sesbania (Sesbania. punicea 'eld) for determination of picloram in the *'of 2,4,5—T in water (22). The high sensitivity 'a seedlings to picloram and insensitivity to it trations of chlorophenoxy herbicides makes possible. e, Bovey and Hall (48) described extraction ‘ ' for determination of picloram residues in les. by electron-capture, GLC. Twenty to 0f soil are usually needed for this analysis. ‘Hod has gained wide acceptance and has been t water and vegetation samples. The piclo- tracted from soil with equal volumes of a i illed waterzmethanol solution. The water: a extracts are evaporated to about 25 milli- f_ are acidified. The acidified residue is ex- 'ce in ether and the aqueous portion dis- TBoron trifluoridezmethanol reagent is then 1 the samplesi-‘heated until only a trace of I remains. In the last step, picloram acid w to its methyl ester. Sample containers Washed with distilled water followed by an i,- me of hexane. The water and hexane are fin a separatory funnel, the aqueous portion t and one or two microliters of hexane injected into the gas chromatograph. Recorder re- sponse from the field samples is compared to that of known concentrations extracted by the above pro- cedure (Figure 2). Recorder response is adjusted for extraction efficiency by adding known amounts of picloram to field samples and extracting by the above procedure. Usually, more than 80 percent of the picloram in soils can be recovered by the extraction procedure. Instrument temperatures, column materi- als and other details are given by Merkle et all. (48). For analysis of water samples, 1 liter is evaporated to 100 milliliters, acidified with hydrochloric acid (HCl) and extracted with ethyl ether. The remainder of the procedure is as described for soils.‘ Plant tissue is analyzed by grinding samples twice in acidified ace- tone. The suspension is then suction filtered, methyl- ated and transferred to hexane in the same manner as described for soils. " Both bioassay and gas-liquid chromatography are useful methods of picloram detection, and results from soils are comparable (10, 60). Perhaps the most satis- factory procedure is to combine methods when pos- sible. Determination. of picloram residue in soil by GCL, verified by phytotoxicity to a susceptible crop, “w, i ‘liREATED STANDARD FIELD CHECK SAMPLE SAMPLE Figure 2. Recorder response after injection of extracts of a picloram standard, of a field sample treated with picloram, and of untreated field samples into electron-capture gas chro- matograph. Concentration of picloram in field sample of soil is about 5 ppb. 7 leaves little question as to its presence. Bioassay and gas chromatographic technique-s were compared in Figure 3 for Nipe and Fraternidad clays in Puerto Rico receiving 3 pounds of picloram per acre (10). Both methods showed similar trends in piclora.m con- centrations at most depths of sampling. Picloram Residues and Susceptible Crops Most broadleaf crops are susceptible to very low rates of picloram. Alley and Lee (1) in Wyoming found that soybeans and other broadle-af crops showed symptoms of picloram in 1965 from applications of 0.5, l, 2 or 3 pounds per acre in 1964. Near Alliance and North Platte, Nebraska, field beans failed to yield seed in 1966 where 2 to 5 pounds of picloram per acre were applied in 1964 to soils (20). lViese3 planted sorghum (RS 626) and soybeans (Clark 63) in June 1967, 1968 and 1969 in Pullman clay loam at Bush- land, Texas, immediately, l and 2 Years after app-li- cation of picloram at rates up to 3 pounds per acre. ‘Wiese, A. F. 1969. Unpublished data. FRAERN/DAD CLAY 20o - I BIOASSAY 7/, GAS CHROMATOGRAPH I60- - IV/PE CLAY 20° I BIOASSAY _ GAS CHROMATOGRAPH PICLORAM, ppb o-s I 6-12 2|- 33-39 45-5: DEPTH, Inches Figure 3. Comparison of cucumber bioassay and gas chroma- tography in determining picloram content in Nipe and Fra- ternidad clay in Puerto Rico 3 months after application of picloram at 3 pounds per acre. (Data from Bovey et al. (10) ) . Sorghum grain yields were not significantly in plots» receiving 0.25, 2 and 3 pounds of" H» per acre in 1967, 1968 and 1969, respective bean yields were reduced at rates of picloram 0.0156 and 0.125 pounds per acre in~l967 a p but were not reduced in plots treated with 3 ; per acre when planted 2 years after treatment i ’ Wheat (Tascosa) was not significantly reduced '_ when planted after 1 and 2f: years in plots up to 3 pounds per acre of picloram in the , 1966. Most plots received 2 to 4 irrigation t w of 3 to 4 inches of water per acre in addition to _i rainfall during the growing season. Piclo ~ usually present in small amounts 2 years aft" ment in the soil profile at depths to 12 feet. _ i, Since picloram is an auxin-like growth *-;_l (19, 23, 34, 38), low concentrations may s growth of certain species. Scifres and Bovey ‘i ied the reaction of seven varieties of sorghum ~ to picloram in the greenhouse and found l ovendry weights of some varieties were increa treatment, some varieties were not affected ~_ growth of other sorghum varieties was reduced A tively low picloram concentrations. Bovey, Miller and Diaz-Colon (8) found u, sorghum, wheat, rice and cotton could be ; o out reduction in fresh weight as early as 3 , after application of picloram up to 6 pounds i in a tropical environment in Puerto Rico. A soybeans were more susceptible to residues w native cropping was suggested. Picloram, one-half ounce per acre just before planting in; reduced soybean (var. Harosoy 63) yields -~ percent (64). Picloram symptoms were no, soybeans planted a year following applicati _‘ to 2 ounces per acre of picloram, but yields -, reduced. a Baur, Bovey and Benedict (6) found increases in the fresh weights of greenho l, corn, sorghum, cotton, cowpea and soybean? with water containing 0.25 to 0.5 part per (ppb) and wheat treated with water conta' ppb picloram. Herbicide treatment decrea protein concentrations in aerial portions in t; cots studied and in sunflower. However, and cotton, soluble protein was significantly ‘f by picloram treatment. In Ohio (37) field studies, the relative 1 of agronomic crops to picloram residues > barley > alfalfa z soybean. Two o ~ eighth pound) per acre applied 9 months béf 5 ing had no visible effect on any of the crops. at 2 pounds per acre applied 9 months before: had no affect on corn or oats, but reduced . barley by 40 to 50 percent and killed alfalfa: beans. However, neither yield nor stand of l planted 20 months after application of 2 u acre of picloram were affected. Klingman . » (42) found that one-half gram per acre of Y shortly after setting tobacco reduced the dol- l ed from the crop by 50 percent. J and Santelmann (2) studied the effect of M on native grasses at various stages, of growth i ld and in the greenhouse. Picloram applied i ;- ce at 0.75, 1.5 and 3 pounds per acre pre- big bluestem (A ndropogon gerardi Vitman) " grass (Panicum virgatum L.) When applied ‘X significantly reduced the density of all spe- f- ever, picloram applied to established native rates up to 4 pounds per acre did not reduce of desirable forage grasses. All treatments l “(production of forbs. i (Bromis inermis Leyss.) tolerated up to ,, d per acre of picloram applied in the spring, 's were reduced from the same rate applied (fall. This response was correlated with root picloram by smooth bromegrass at initiation tive growth. 4x t studies‘ indicate that l ppm of picloram pper inch of soil prevents the growth of side- m- and switchgrass seedlings. ever, in the field, native grass yields were itly increased in plots receiving up to 4 picloram per acre in the subhumid areas _< as early as 6 months after treatment? Sus- ss production was dependent upon effective tIOl. Photolytic Degradaiion of Picloram A ‘pt in the ultraviolet range will degrade pic- 'olecules on plant or soil surfaces (30, 49). iuces the amount of picloram available for "t in the ecosystem. The chemical structure A m is shown in Figurei4. According to Hall, _d Merkle (30), photodecomposition freed ‘f ions, two for each picloram molecule de- I“: d- acids were formed in the process. The =. ‘ted that the pyridine nucleus was destroyed folet light." About 20 percent of a 2x102 f; centration was degraded per 48-hour expos- f‘ tmviolet light at 253.7 nanometers. Decom- Qof the molecule occurred in ‘sunlight but was 1- more variable than under controlled ultra- ; t sources in the laboratory. l ' isooctyl ester of picloram was degraded more "it percent) by ultraviolet light than the potas- formulation (g6 percent) after 72 hours in ' dishes under laboratory conditions (l6). of loss from esters applied to soil was lower from open petri dishes. Bovey, Dowler and g 1971. Unpublished data. ;W., R. E. Meyer and H. L. Morton. 1970. Unpub- v e seedling growth of germinating blue’ grama. T ts grama (Bouteloua curtipendula (Michx.) ‘ o and four-leaf stage at 1.5-pounds per acre, lies in Nebraska (45) indicated that smooth i Cl / CI / \COOH PICILORAM Figure 4. Structure of picloram acid. Ultraviolet light decom- poses the molecule by removing the chlorines and cleaving the ring (30) . N Merkle (10) felt that loss of picloram by photodecom- position would occur in field applications unless the herbicide was leached into the soil (Figure 5). Once in the soil profile, picloram is protected from photo- decomposition. One milligram of picloram as the potassium salt (liquid) and 2 percent picloram granules remained active for at least 6 months in petri dishes under tropical environment and protected from rainfall at Mayaguez, Puerto Rico. Dishes were exposed daily to natural sunlight and forest shade conditions. How- ever, the isooctyl ester of picloram completely dis- sipated after 6 months under sunlight, but not when kept under a forest canopy“. Hours of radiation and light intensity are less in Puerto Rico in the summer than in Texas. In Texas (10), l pound per acre of the potassium salt of picloram sprayed on a denuded sandy soil receiving no rainfall could not be detected 6 weeks after treatment. Vegetational Areas of Texas Studied Variation in the Texas environment has been dis- cussed by Gould (26). Climate performs an important role in dissipating picloram. According to Gould's compilation, annual precipitation in Texas increases from west to east from about 8 inches at El Paso to ‘Bovey, R. W. and]. R. Baur. 1968. Unpublished data. Aerial Spray Figure 5. Th; routes of mov dissipation of 1' Q 6g?’ QIQ§§Q°Q Y Y i’ K i i <3‘? 6° Q_____ Y k + over 55 inches at Port Arthur. Most of the precipita- tion’ comes as rainfall although snowfall contrib-utes significant moisture in some‘ areas of North and West Texas. The growing season increases from northwest to southeast with 179 frost-free days at Dalhart to 341 days at Galveston. According to Gould, Cenozoic clay and sand sediments influenced edaphic characteristics of the eastern and western thirds of Texas, whereas, central Texas was primarily influenced by limestone-s, marls, sands and clays of Mesozoic and Paleozoic eras. Gould recognizes 10 major vegetation areas of Texas (Figure 6). Research on dissipation of picloram has empha- sized several vegetational areas (Figure 6). One such area is the Rolling Plains, a southern extension of the Great Plains region of the central United States. This physiographic province occupies some 24 million acres, two-thirds of which is rangeland. Elevation range-s from 800 to 3,000 feet. Average annual precipitation is often less than 22 inches, and seasonal precipitation is highly variable. The province is characterized by hot, dry summer periods with high evaporation rates. Winter temperatures often drop below freezing. Soils are neutral to slightly calcareous, range from fine sands to clays and are all invaded by honey mesquite. Original vegetation included midgrasses. However, due to grazing pressure accentuated by periodic drouth, much of the area is presently shortgrass plains with a predominance of buffalograss (Buchloe dac- tyloides (Nutt.) Engelm.), tobosagrass (Hilaria mutica (Buckl) Benth.) and ‘blue grama (Boute-loua gracilis (Willd.) ex Lag. ex Griffiths). 710 rie, a slowly drained plain less than 150 feetf tion. Average annual rainfall reaches 50 in the eastern portion and is fairly evenly dis g Figure 6. Vegetational areas of Texas as described (26). Black dots indicate locations where picloram I studies have been or are being conducted. "1 after application =10 land ecosystems. if Two study sites were located in the Coas 1O VEGETATIONAL AREAS OF TEXAS 1. Pinwwoods 2. Gulf Prairie and Marshes 3. Post Oak Savannah 4. Blackland Prairies 5. Cross Timbers and Prairie: 6. South Texas Plains 7. Edwards Plateau \ B. Rolling Plains 9. High Plains 10. Trans-Pecos, Mountains and Basins is characterized by an average 300-day grow- , warm temperatures and high relative hu- p, he soils are acid sands, sandy loams and clays. t eable soil profiles attribute to the forma- f lt meadows and marshes. Climax vegetation I ss prairie b-ut has been invaded by several brush. Tame pastures are common to the Tdy area was also located in the Post Oak Topography is gently rolling to hilly lation 300 to 800 feet. Annual rainfall is inches with seasonal highs in May or June. p tory species are blackjack oak (Quercus i; a Muenchh.) and post oak (Que-rcus stel- enh.) with tall grasses in the understory. range from acid sandy loams or sands to ' proved pastures are widely used, especially effective brush control. t, Blackland prairies are dark colored, cal- ays supporting short, mid or tall grasses ,; on latitude and site. The gently rolling level topography affords rapid surface drain- .; annual rainfall varies from 30 to 40 fr" west to east. Vegetation typically would §= 'rie, but much of the are-a is presently tivation or established in tame pastures. Edwards Plateau is located in Central Texas tions of 100 to more than 3,000 feet. Pre- " varies from 15 to 33 inches from west to i 1 surfaces are rough, and profiles are shallow 4 lain by limestone, caliche or granite. Brush - includes juniper (juniperus sppt), honey ,0, live oak and sand shinnery oak (Quercus i’ ydb.). Rough rocky areas typically support Qmidgrass understory grading into buffalo- i tobosagrass in the heavier, northwestern ,1 Texas Plains is located in the south- i of Texas with elevations from sea level ,9 . Precipitation varies from 18 to 33 inches to east. The frost free period is 260 days i and 340 days or more in the south. The k calcareous to neutral clays with a firm _] . The bottomlands are silt clay loams of 'ls. The vegetation is a mixture of thorny p» ts and cacti. i Theoretical Routes of Iation of Picloram From Rangeland fch on dissipation of picloram from range- Qtems has been influenced by the inforrna- i d in the flow chart in Figure 5. A typical, range might consist of a woody plant 'th ‘low-growing shrubs and herbaceous e understory. Most of the picloram re- it aerial spray will be intercepted by vegeta- with the remainder reaching the soil or i e on the soil surface. Studies have in- cluded measurement of losses from such site-s. Pic- loram formulations are water-soluble, and presumably, the applied herbicide is mobile within the environ- ment and may move vertically into the soil profile, or laterally on the soil surface. There has been concern as to the ultimate destiny of picloram in the rangeland environment with particular interest to movement and longevity in soils and water supplies. Dissipation From Soils Role of Temperature The persistence of picloram has been studied in a wide array of soils. Hamaker, Youngson and Gor- ing cited half-order reactions and Michaelis-Men- ton kinetics as most satisfactorily describing the detox- ification of picloram in soils. Grover (27) showed a definite, early lag period in the dissipation of picloram from incubated soils after which degradation followed at first-order reaction. Thus, dissipation of picloram from soils after the lag period was over was independ- ent of concentration. However, duration of the lag period was dictated ‘by concentration. Presumably, the duration of the lag period in cool, dry environ- ments would be longer than under more mesic con- ditions. Hamaker, Youngson and Goring (32) found picloram loss from soils collected from 18 states to be correlated with the number of days over 90° F and with annual precipitation. Thus, dissipation rates increased in the southern and southeastern states under prolonged, warm, moist conditions. Bovey, Ketchersid and Merkle (l6) found that 45 percent of the isooctyl ester of picloram was lost from open pe-tri dishes in a dark oven at 60° C, whereas only 2 per- cent of the potassium salt were lost. According to Merkle, Bovey and Davis (49), picloram dissipated more rapidly from Houston clay at temperatures of 38° C than at 4° or 20° C (Figure 7). Within a given temperature, picloram dissipation was more rapid at field capacity moisture than at 0.1 field capacity. Although the specific role of temperature in the dissipation of picloram has not been established, the general conclusion by most researchers is that dissipa- 100 HOUSTON CLAY FIELD CAPACITY 96 PICLORAM RECOVERED Mourns Figure 7. Dissipation of picloram from Houston clay as af- fected by temperature. (Data from Merkle et al. (49) ). 11 tion is accelerated at higher temperatures. Since ini- tial dissipation rates are concentration dependent, half-life values for different regions would be of little use (27). However, from a practical standpoint, resid- ual life of picloram can be estimated with good ac- curacy based on degree and duration of temperature following application. Influence of Soil Properties Merkle et al. (49) reported that picloram was more persistent in clay or sandy loam than sand, at- tributing dissipation in light textured soils to removal by leaching. Grover (28) correlated the soil activity of picloram with pH and organic matter content but not with cation exchange capacity or percent clay. These data indicate that picloram is not inactivated by the soil colloids. Hamaker et al. (31) reported that sorption of picloram by soils was primarily due to organic matter and hydrated metal oxides. They also felt that clays were relatively minor in sorption of picloram. Both unionized picloram and the anion were involved in sorption on soils. However, Bovey and Miller (ll) reported that up to 600 pounds per acre of activated charcoal was not sufficient to detoxify picloram after application of 0.5 pound per acre to a silty clay to allow growth of cucumber and field beans. Grover (28) also indicated that soil reaction, as influenced by ionic strength of the soil solution and organic matter, may influence phytotoxicity from residual picloram. This may indicate that availability of the acid or the salt form is important in the dissipa- tion of picloram. Bovey et al. (l6) illustrated relative stability of the salt formulation of picloram which indirectly supports this hypothesis. However, Young- son et al. (66) showed that pH and percent clay did not influence the rate of decomposition, but percent organic matter, percent moisture and temperature were important. Little data are available substantiating micro- bial breakdown of picloram in soils. Resistance to microbial degradation may be responsible for the longer persistence of picloram in soils as compared to the rapid loss of 2,4-D. Youngson et al. (66) found that several varieties of bacteria and fungi decomposed picloram in laboratory studies using culture solutions and natural soils.. It was generally concluded that decomposition was not via utilization of picloram as an energy source by microbes. Picloram at 100 ppm of soil did "not affect carbon dioxide evolution, urea (hydrolysis, population counts of bacteria and fungi, and conversion of ammonia to nitrate or nitrite to nitrate (25). This would indicate picloram did not affect life processes of soil microorganisms. Movement and Loss of Picloram Through Soil Profiles Subhumid Rungelund Sites Vegetated Areas. Picloram concentrations in soils from brush research plots in Texas sprayed in 1963 and. 1964 were determined with bioassay by safflower l2 "Baur, j. R. and W. Bovey. 1967. Unpublished dad (Carathamtts tinctoria L. var. U.S. l0) (Table sampled at several locations approximately i years after treatment at rates up to 4 and 8 per acre usually contained less than 1 ppb; loram. Most of these soils were sandy loams files were sampled to only 2 or 3 feet deep. More frequent soil sampling at Vict‘ Carlos indicated by gas chromatographic anal ' 2 pounds of picloram per acre disappeared top 2 feet of soil at 6 and 12 weeks after respectively (48) (Table 3). Picloram was not i in soil from Victoria and Carlos after l year less of application rate or sampling depth. _ that any variation within each plot would be A 25 samples from a plot sprayed with 8 po {I acre of picloram were taken to a depth ti? Picloram residue was not detectable in any.’ As a further check, field beans planted in‘ ‘ soils from Victoria and Carlos developed l! indicating absence of residues. 8' Approximately 25 percent of the piclora u. the soil surface at Carlos and l0 percent at 1 Foliage of trees and other plants intercepted i: Dense stands of live oak covered the Vic whereas the yaupon and herbaceous vegeta A more scattered at Carlos. Rapid disappearan loram from the light textured soils was parti ‘ . uted to leaching by rainfall (Table 4). Additional studies were conducted at t’ study picloram residues in soil after spray t’) of the potassium salt at l pound per acre. concentrations were usually less than 2 n; levels down to 8 feet 1 year after treatment. , levels ranged from about 1 to l0 ppb 6 m t‘ treatment. The highest concentrations were with layers containing highest clay content kiy ‘IOO _ 8O 6O 4O Picloram Conc. Detected in Grass (ppb) I I ' I 00.003 0.25 0.50 _0.75 1.00 Picloram Field Rate (lb/A) Figure l0. Concentration of picloram in grasses 5 months after the application of several rates in the Rolling Plains of Texas. 22 TABLE 14. HERBICIDE CONCENTRATIONS (PP MAINING IN GRASS TISSUE IN 196s AFTER THE j CATION OF 2 OR 4 POUNDS PER ACRE OF GRAQ PICLORAM To REDBERRY JUNIPER IN 1966 1 MERTZON, TEXAS A Picloram con ~-¢--_i Position Relation of » on grass to (ppb) j hill juniper canopy ._ 2 lb./acre 4 ll Top Under canopy 34 Top Open area 82 Bottom Under canopy 29 Bottom Open area 73 difference in the amount of detectable picl___ grass tissue collected from the upper and lo * tions of the slope where 2 pounds per acre of f were applied originally (Table l4). Howev J tissue collected from under redberry juniper g contained less detectab-le picloram than grass A in the open spaces where 2 pounds per a applied. Approximately half as much detectable v f residue remained in grass tissue from the tot slope than at the lower end of the hill _ pounds per acre of the herbicide was used. l” mately half as much picloram was recov f grass tissue sampled from under the redberry- canopies (57 ppb) than occurred in grasses » from adjacent open spaces at the upper part‘ treated with 4 pounds per acre. However, f no difference in the amount of picloram _ from grasses growing under the redbeny juni opies and that growing in an adjacent open the lower end of the plot treated with 4 acre. Since the picloram was applied in pel » concentrations in aerial portions of grass r-v. undoubtedly due to root uptake. Analyses showed highest amounts of detectable piclo - = lower end of this slope (Figure 9). These I, cate that position of the forage species in a? the woody plants at time of treatment will -i amount of picloram uptake by grasses. Dissipation of Picloram From Broadleof i Scifres, Hahn and Merkle (58) reportedt tectable picloram was reduced by 93 percent leaf plants within 30 days following picloram; tion. Since most of the leaves were killed by g’ they became a part of the surface litter wi ,' j after treatment. However, the deposition Hi mains of the broadleaves to the surface debri increase picloram content in surface litter ,- Dissipation of Piclorum From Woody Si‘ Subhumid Areas , i Baur, Bovey and Smith (5) reported that of 2,4,5-T in live oak at Victoria were w ;p_ applied in the presence of picloram than Q was applied alone. However, 99 percent of cide detected at l month after treatment § sipated 6 mo-nths later. 9 , DISSIPATION or THE POTASSIUM SALT .1 M APPLIED AT 2 POUNDS PER ACRE ON 5| TALL GUAVA IN JULY 1967, MAYAGUEZ, A e01 Picloram (mg/g fresh wt.) Time sampled (Immediately) (l month) 0.51 654.3 0.76 121.9 0.38 0.99 686.7 0.34 31.3 0.33 0.55 tions with two plants per replication with two ples per tree. Picloram applied during rainy and Bovey9 studied picloram residues in irons and roots of yaupon in plots sprayed 1969. Residues were usually less than 0.2 _ys after spraying 1 pound per acre of , t were not detectable 1 year after treat- i, were retreated in May 1970 with 1 pound f picloram. Six months after retreatments, Jyaupon averaged 0.2 to 0.3 ppm, indicat- 'no accumulation of picloram from l year i '_ (Psidium guajava L.) contained from 31 l") picloram in various plant parts after spray- 9e potassium salt at 2 pounds per acre in pi" (Table 15). One month after treatment, gall plant parts averaged less than 1 ppm. ' 1 s imesquite leaves contained about 25 ppm j- the day of application of one-fourth acre. Leaves contained 0.3 ppm picloram j- ; Plains environment (58). Time of irri- 1 ntly had little affect on loss of picloram Qtissue. According to Bovey and Diaz- fherbicides applied in oil are difficult to h; f surfaces with rainfall immediately after 7Picloram was more slowly dissipated fnnery oak leaves than from honey mes- (58). Deposition of honey mesquite ‘ increase the picloram content of surface i, er, the cumulative addition of remains W herbs, honefylmeisquite and sand shin- ves caused an increase in the picloram ‘<1 ace litter at 60 days, as opposed to 30 tment. lid J. R. Baur. 1968. Unpublished data. (R. W. Bovey. 1970. Unpublished data. Acknowledgment This study was a cooperative investigation of The Texas Agricultural Experiment Station, and Plant Science Research Division, Agricultural Re- search Service, USDA, Texas A8cM University, Col- lege Station. literature Cited 1. Alley, H. P. and G. A. Lee. 1966. Crop tolerance to pic- loram residual. West Weed Contr. Conf. Res. Rep. p. 102. 2. Arnold, W. R. and P. W. Santelmann. 1966. The response of native grasses and forbs to picloram. Weeds 14:74-76. 3. Barnett, A. P., E. W. Hauser, A. W. White, and J. H. Holiday. 1957. Loss of 2,4-D in washoff from cultivated land. Weeds 2:133-137. , 4. Baur, J. R. and R. W. Bovey. 1969. Distribution of root- absorbed picloram. Weed Sci. 17:524-528. 5. Baur, J. R., R. W. Bovey, and J. D. Smith. 1969. Herbi- cide concentrations in liveoak treated with mixtures of picloram and 2,4,5-T. Weed Sci. 17:567-570. 6. 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. 7. Bovey, R. W., F. S. Davis, and M. G. Merkle. 1967. Dis- tribution of picloram in huisache after foliar and soil application. Weeds 15:245-249. 8. Bovey, R. W., F. R. Miller, and J. Diaz-Colon. 1968. Growth of crops in soils after herbicidal treatments for brush control in the tropics. Agron. J. 60:678-679. 9. Bovey, R. W., F. S. Davis, and H. L. Morton. 1968. Herbi- cide combinations for woody plant control. Weed Sci. 16:332-335. 10. Bovey, R. W., C. C. Dowler, and M. G. Merkle. 1969. The persistence and movement of picloram in Texas and Puerto Rican soils. Pest. Monit. J. 32177-181. 11. Bovey, R. W. and F. R. Miller. 1969. Effect of activated charcoal on the phytotoxicity of herbicides in a tropical soil. Weed. Sci. 17:189-191. 12. Bovey, R. W. and J. Diaz-Colon. Effect of simulated rain- fall on herbicide performance. Weed. Sci. 17:154-157. 13. Bovey, R. W., H. L. Morton, and J. R. Baur. 1969. Con- trol of live oak by herbicides applied at various rates and dates. Weed Sci. 17:373-376. 14. Bovey, R. W., H. L. Morton, J. R. Baur, J. Diaz-Colon, C. C. Dowler, and S. K. Lehman. 1969. Granular herbicides for woody plant control. Weed Sci. 17:538-541. 15. Bovey, R. W., J. R. Baur, and H. L. Morton. 1970. Con- trol of huisache and associated woody species in South Texas. J. Range Manage. 23:47-50. 16. Bovey, R. W., M. L. Ketchersid, and M. G. Merkle. 1970. Comparison of salt and ester formulations of picloram. Weed Sci. 18:447-451. 17. Dowler, C. C., W. Forestier and F. H. Tschirley. 1968. Effect and persistence of herbicides applied to soil in Puerto Rican Forests. Weed Sci. 16:45-51. 18. Dowler, C. C. 1969. A cucumber bioassay test for the soil residues of certain herbicides. Weed Sci. 17:309-310. 19. Eisinger, W., D. J. Morre, and C. E. Hess. 1966. Promo- tion of plant growth by Tordon herbicide. Down to Earth 21 (4) :8-10. 20. Fenster, C. R., O. C. Burnside, and G. A. Wicks. 1966. Comparison of the residual effects of dicamba, picloram and 2,3,6-TBA with field beans (Phaseolus vulgaris L.) . Proc. North Cent. Weed Cont. Conf. 20:20. 23 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 24 Getzenduner, M. E., J. L. Herman, and Bart VanGiessen. 1969. Residues of 4-amino-3,5,6-trichloropicolinic acid in “grass from application of Tordon herbicides. Agr. and Food Chem. 17 (s) =1251-12ss. Goodin, J. R. 1968. A new selectiye bioassay for Tordon in water. Down to Earth 24(4) :4-6. Goodin, J. R. and F. L. A. Becher. 1967. Picloram as an auxin substitute in tissue culture. Plant Physiol. 42:523. Goring, C. A. I., C. R. Youngson, and J. W. Hamaker. 1965. Tordon herbicide disappearance from soils. Down to Earth 20 (4) z3-5. Goring, C. A. I., J. D. Griffith, F. C. O’Melia, H. H. Scott, and C. R. Youngson. 1967. The effect of picloram on mi- croorganisms and soil biological processes. Down to Earth 22 (4) :14-17. Gould, F. W. 1962. Texas p1ants—-A checklist and eco- logical summary. Texas Agr. Exp. Sta. MP-585. 112 p. Grover, R. 1967. Studies on the degradation of 4-amino- 3,5,6-trichloropicolinic acid in soil. Weed Res. 7:61-67. Grover, R. 1968. Influence of soil properties on phyto- toxicity of 4-amino-3,5,6-trichloropicolinic acid (picloram) . Weed Res. 8:226-232. Haas, R. H., C. J. Scifres, M. G. Merkle, R. R. Hahn, and G. O. Hoffman. 1971. Occurrence and persistence of pic- loram in natural water resources. Weed Res. 11:54-62. Hall, R. C., C. S. Giam, and M. G. Merkle. 1968. The photolytic degradation of picloram. Weed Res. 8:292-297. Hamaker, J. W., C. A. I. Goring, and C. R. Youngson. 1966. Sorption and leaching of 4-amino-3,5,6-trichloropico- linic acid in soils. Adv. Chem. Ser., 60:23-27. Hamaker, J. W., C. R. Youngson, and C. A. I. Goring. 1967. Prediction of the persistence and activity of Tordon herbicide in soils under field conditions. Down to Earth 23 (2) : 30-36. Hamaker, J. W., C. R. Youngson, and C. A. I. Goring. 1968. Rate of detoxification of 4-amino-3,5,6-trich1oro- picolinic acid in soil. Weed Res. 8:46-57. Hamaker, J. W., H. Johnston, R. T. Martin, and C. T. Redemann. 1963. A picolinic acid derivative: A plant growth regulator. Science, N. Y., 141-363. Hardy, J. L. 1966. Effect of Tordon herbicides on aquatic chain organisms. Down to Earth 22(2) :11-13. Herr, D. E., E. W. Stroube, and D. A. Ray. 1966. The movement and persistence of picloram in the soil. Weeds 14:248-250. Herr, D. E., E. W. Stroube, and D. A. Ray. 1966. Effect of Tordon residues on agronomic crops. Down to Earth 21(4) :17-18. Kefford, N. P. and O. H. Caso. 1966. A potent auxin with unique chemical structure—4-amino-3,5,6-trichloro- picolinic acid. Bot. Gaz. 127:159-163. Kenaga, E. E. 1969. Tordon herbicides—Evaluation of Safety to fish and birds. Down to Earth 25 (1) :5-9. Keys, C. Haand H. A. Friesen. 1968. Persistence of pic- loram activity in soil. Weed Sci. 16:341-344. Klingman, G. C. 1964. Weed Control: As a Science. John Wiley 8c Sons, New York. 21 pp. Klingman, G. C. and H. Guedez. 1967. Picloram and its effect on field-grown tobacco. Weeds 15:142-146. Leasure, J. K. 1964. Bioassay methods for 4-amino-3-5,6- trichloropicolinic acid. Weeds 12:232-233. Lynn, G. E. 1965. A review of toxicological information on Tordon herbicides. Down to Earth 20(4) :6-9. 45. 46. 47. 48. 49. 50. 51. 52. 53. 54. 55. 56. 57. 58. 59. 60. 61. 62. 63. 64. 65. 66. McCarty, M. K. and C. J. Scifres. 1968. w grass response to herbicides as affected by y cation in relation to nitrogen fertilizationtr 16:443-446. ‘. McCarty, M. K. and C. J. Scifres. 1968. "-5 milkweed and its control. Weed Sci. 16:4-7. McCollister, n. n. and M. L. Leng. 1969. picloram and safety evaluation of Tord -j Down to Earth 25(2) :5-10. ' Merkle, M. G., R. W., Bovey, and R. H .1. determination of picloram residues in soil w? matography. Weeds 14:161-164. A Merkle, M. G., R. W. Bovey, and F. S. Da tors affecting the persistence of picloram inf J. 59:413-414. Meyer, R. E., H. L. Morton and T. O. ' truck sprayer for applying chemicals to ~__ 15:286-287. -. " Meyer, R. E. and T. E. Riley. 1969. Influ ' granules and sprays on whitebrush. Weed Moffat, R. W. 1968. Some factors affecting A ance of Tordon in soil. Down to Earth 23( 5; Morton, H. L., E. D. Robison, and R. E., Persistence of 2,4-D, 2,4,5-T and dicamba» grasses. Weeds 15:268-271. Robison, E. D. 1967. Response of mesquite 1 loram and 2,4,5-T picloram combinations. " Sci. Soc. 20:199. ' Scifres, C. J., O. C. Burnside, and M. K. 4 Movement and persistence of picloram in Nebraska. Weed Sci. 17:486-488. i’ Scifres, C. J., R. R. Hahn, J. Diaz-Colon, . w _ 1971. Picloram persistence in semi-arid ra water. Weed Sci. 19:381-384. , Scifres, C. J., R. R. Hahn, and J. H. B ~ p " cal control as related to phenology of co -~ J. Range Manage. 24:370-373. 1° Scifres, c. J., R. R. Hahn, and M. c. M sipation of picloram from vegetation of A lands. Weed Sci. 19:329-332. Scifres, C. J. and R. W. Bovey. 1970. A sponses of sorghum varieties to picloram. 777. i. Scifres, C. J., R. W. Bovey, and M. G. M i ation in bioassay attributes as quantitative“ loram in soils. Weed Res. (In Press). A Trichell, D. W., H. L. Morton, and M. 1 Loss of herbicides in runoff water. Weed U.S.D.A. 1964. Grassland restoration: 4, problem. Unnumbered Bu1., USDA, Soil ple, Texas. 17 p. Walter, J. P., R. W. Bovey, and M. c. Movement of picloram through soil p ~ Weed Sci. Soc. 23:249. Wax, L. M., L. A. Knuth, and F. W. Slife. of soybeans to 2,4-D, dicamba and picl 17:388-393. a Weed Sci. Soc. Amer. 1967. Herbicides _p Humphrey Press, Inc., Geneva, N.Y. 293 Youngson, C. R., C. A. I. Goring, R. . Scott, and J. n. Griffith. 1967. Factort: decomposition of Tordon herbicide in Earth 23(2) z3-ll. ’ v