8-1398
lune 1982

  

Woolly Locoweed and

Forage Response to

The Texas Agricultural Experiment Station, Neville P. Clarke, Director, The Texas A&M University System, College Station, Texas

CONTENTS

Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Introduction . . . . . . . . . .l . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Materials and Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Conclusions . . . . . . - . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Literature Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Metric-English Equivalents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Appendix: Scientific Names of Plants
Mentioned in Text . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . back cover

KEYWORDS: Poisonous plants/range management/woolly locoweed/Earle loco/purple loco/Texas loco/
2,4-D/dicamba/3,G-dichloropicolinic acid/picloraln/tebuthiuron/trichlopyr/pelleted herbicides.

Woolly Locoweed and Forage Response
to Herbicides in West Texas
M. R. Freeman, D. N. Ueckert, and J. T. Nelson*

Summary

Applications of 2,4-D1 in fall, winter or spring did not
satisfactorily control woolly locoweedz in the Davis
Mountain area of the Trans Pecos of west Texas. Pic-
loram at 0.2 t0 1.1 kilograms per hectare (kg/ha) alone or
in mixtures with other herbicides, applied in fall or
winter, usually controlled woolly locoweed for a year or
longer. Woolly locoweed was usually killed within 120
days following application of foliar sprays containing
picloram, whereas control usually was not manifested for
almost a year following application of picloram pellets.
Woolly locoweed was effectively controlled in the fall
with foliar sprays of picloram + triclopyr ester (1:1) at
0.6 kg/ha; picloram at 1.1 kg/ha; 3,6-dichloropicolinic
acid + triclopyr (1:1) at 0.6 kg/ha; 2,4,5-T + picloram
(1:1) at 0.6 to 1.1 kg/ha; picloram + dicamba (1:1) at 0.6
kg/ha; dicamba a_t 1.1 kg/ha; 3,6-dichloropicolinic acid +
dicamba (1:1) at 0.6 kg/ha; and 2,4-D + dicamba (3:1) at
1.1 kg/ha. Control of woolly locoweed during fall was
generally superior to that in winter or spring, for most
herbicides evaluated, presumably because of more
favorable temperatures for herbicide absorption and
translocation, and because of the earlier phenological
stages of the weeds. Control of woolly locoweed where
densities averaged 1.4 to 1.9 weeds per square meter
(m2) did not increase production of desirable forages.

Introduction

Ranch firms in the Big Bend region of the Texas Trans
Pecos have suffered serious livestock losses for decades
as the result of poisoning by poisonous plants including
woolly locoweed (Astragalus mollissimus Torr. var. ear-
lei [Rydb] Tidestr.), also referred to as Earle loco,
purple loco, and Texas loco. Woolly locoweed, a robust
perennial of only a few years duration, occurs in the
Trans Pecos resource area of Texas and in Chihuahua
and Coahuila, Mexico (Norris, 1951; Sperry et al., 1964;
Correll and Johnston, 1970).

Cattle, horses, and sheep are the primary livestock
affected by woolly locoweed (Norris, 1951; Dollahite,
1965; Nielsen, 1978). The toxic principles in woolly
locoweed have not been determined, but it has been

‘Chemical names of herbicides mentioned in text are given in Table 2.

2Scientific names of plants mentioned in text are given in the appendix.

suggested that there are several toxins (James, 1972).
Livestock consume woolly locoweed primarily during
fall, winter, and spring. Animals usually will not con-
sume lethal quantities of woolly locoweed or other toxic
species of Astragalus if high quality forage is available
(James and Johnson, 1976; Krueger and Sharp, 1978;
Sperry et al., 1964). Sperry et al. (1964) indicated that
proper supplemental feeding reduced the amount of
woolly locoweed consumed by livestock. However,
James et al . (1968) reported from Utah that sheep addict-
ed to Green River milkvetch consumed the weeds in
preference to cottonseed meal, mineral supplement, or
alfalfa hay.

The present recommendation for controlling woolly
locoweed in west Texas is application of foliar sprays of
esters of 2,4-D at 1.1~ kg/ha during October through
April (Sperry et al., 1964). However, this treatment is
not commonly used, because it has resulted in erratic
control (Norris 1951). Research on herbicidal control of
various species of Astragalus as well as Oxytropis has
been reported by Alley (1973), Cronin and Williams
(1964), Norris (1951), Sperry (1951), and Williams
(1970). Foliar sprays of dicamba, 2,4-D, or 2,4-D +
dicamba (3:1) at 2.2 kg/ha or picloram at 0.3 kg/ha have
killed 90 percent or more of the Lambert crazyweed
treated in Wyoming studies (Alley, 1976). James et al.
(1980) reported that locoweeds can be controlled by
spraying actively growing or budding plants with 2,4-D
ester at 2.2 to 3.4 kg/ha; dicamba at 1.1 kg/ha; or
picloram at 1.1 kg/ha. This study was initiated in 1978 to
evaluate several herbicides applied at different seasons
for consistent and extended control of woolly locoweed
in west Texas and to determine the response of as-
sociated forage species to control of woolly locoweed.

Materials and Methods

Description of the Study Areas

The major portion of this study was conducted on the
Morrow-McIntyre Ranch, 19 kilometers (km) west of
Alpine, Texas. Two experiments were conducted on the
Billy and Tommy Weston Ranch, 6 km southeast of Ft.
Davis, Texas. Elevations of both study areas are about
1586 meters (m) and average annual precipitation is 40
centimeters (cm). About two-thirds of the annual pre-
cipitation occurs from June through September. Both

*Respectively, research associate, Texas Agricultural Experiment Station, San Angelo; professor, Texas Agricultural
Experiment Station, San Angelo; and assistant professor, Sul Ross State University, Alpine, Texas (Department of

Range Animal Science).

ranches are cow-calf operations normally stocked year-
long at 1 animal unit per 10 hectares (1 A.U./10 ha).

Experiments at the Alpine study site were installed 0n
an Igneous Hill and Mountain range site typified by
Brewster clay loam soils (loamy-skeletal, mixed, thermic
family of Lithic Haplustolls) on 1 to 1.5 percent slopes.
Soils of the Brewster series are shallow to very shallow,
well-drained, noncalcareous stony soils underlain by
igneous bedrock. Permeability is moderate in these
soils, and runoff is rapid.

Experiments at the Fort Davis study site were in-
stalled on a Draw range site on Cageby loam soils (fine-
loamy, mixed, thermic family of Cumulic Haplustolls).
The Cageby series consists of deep, well-drained, non-
calcareous loamy soils formed in stratified alluvium.
Permeability is moderate in these soils, runoff is slow,
and these soils are occasionally flooded in summer.

Chemical and physical analyses of soils from the study
areas were conducted from six bulk samples taken from
0- to 8-cm, 9- to 41-cm, and 42- to 76-cm depths (Alpine
study site) or 0- to.30-cm and 31- to 61-cm depths (Fort
Davis study site). Soil analyses included texture by the
hydrometer method (Day, 1965), organic matter by the
Schollenberger method (Allison, 1965), and pH mea-
sured in 0.1 M CaClg (Peech, 1965).

Soils of the Igneous Hill and Mountain range site were
clay loams or sandy clay loams overlaying clay, clay loam
or loam subsoil (Table 1). Clay content of the surface 8
cm of soil averaged 24 percent (range 16.2 to 31.3
percent) and increased with depth to 35 percent (range
29 to 41.9 percent). Soil organic matter content averaged
3.2 percent in the upper 8 cm and decreased to 1.3
percent at the 42- to 76-cm depth. The soils were slightly
acidic (Table 1).

Soils of the Draw range site were clay loams to a depth
of 61 cm (Table 1). Clay content of the surface 30 cm of
soil averaged 28 percent and increased with depth to 31
percent. Soil organic matter content averaged 3.5 per-
cent in the upper 30 cm and decreased to 2.2 percent at

TABLE 1. GENERALIZED SOIL CHARACTERISTICS OF RANCE
SITES UTILIZED FOR EVALUATION OF VARIOUS HERBICIDES
FOR WOOLLY LOCOWEED CONTROL IN THE DAVIS MOUN-
TAINS, TEXAS

Depth (x532? TexturaI components (%)
(cm) (%) pH Sand Silt Clay Texture

Morrow-Mclntyre Ranch Winter & Spring 1979 Experiments

0-8 4.1 6.5 24.9 43.8 31.3 clay Ioam

9-41 2.3 6.6 23.7 37.9 38.4 clay Ioam

42-76 1.4 6.8 25.1 32.9 41.9 clay
Morrow-Mcintyre Ranch Fall 1979 Experiment

0-8 2.2 6.6 55.1 28.7 16.2 sandy Ioam

9-41 1.6 6.9 43.3 31.7 24.9 Ioam

42-76 1.1 7.2 36.8 34.2 29.0 clay Ioam

Weston Ranch Winter 1978-79 Experiments
0-30 3.5 6.6 29.0 42.6 28.4 clay loam
31-61 2.2 6.9 27.2 42.1 30.7 clay Ioam

the 31- to 61-cm depth. The soils were slightly acidic
(Table 1).

Major plants on the Alpine study site were blue
grama, threeawns, woolly locoweed, garbancillo, cane
bluestem, Halls panicum, and wolftail. Major plants on
the Fort Davis study area were blue grama, sideoats
grama, cane bluestem, woolly locoweed, hook threeawn,
and inland saltgrass.

Herbicide Applications

Herbicide treatments were applied at the Alpine site
during winter, spring, and fall 1979 (Table 2) to 10-m by
20-m plots arranged in a completely randomized design
with three replications. Plots treated in winter and
spring 1979 were fenced to exclude livestock. Her-
bicides were applied on January 23, May 15, and
November 14, 1979. Herbicides including 5 and 10
percent active ingredient (a.i.) picloram pellets, 20 per-
cent a.i. tebuthiuron pellets (0.16- and 0.32-cm diame-
ter) and 5 percent a.i. dicamba granules were applied at
1.1 kg/ha (a.i.) to 10-m by 20-m plots arranged in a
completely randomized design with three replications
on December 5, 1978 or February 9, 1979 at the Fort
Davis site. _

Liquid herbicide formulations were applied in 140
liters per hectare (L/ha) of a 1:14 (vzv) diesel fuel-water
emulsion with 0.1 percent emulsifier from a tractor-
mounted, small-plot sprayer equipped with a 6-m boom,
except for the wettable powder formulation of
tebuthiuron, which was applied in water with 0.1 per-
cent surfactant. Pelleted herbicides were applied with a
hand spreader. Each herbicide treatment was applied at
1.1 kg/ha (a.i.) in all experiments, except that in the fall
1979 experiment at the Alpine study site herbicides
were also applied at selected lower rates (Table 2).

Soil temperature, relative humidity, air temperature,
wind speed, and cloud cover were recorded during and
subsequent to treatment applications. Soil water content
was determined by the gravimetric method (Gardner,
1965) by taking 25 randomly-selected soil samples from
0- to 8-cm, 8- to 15-cm and 15- to 30-cm depths.
Precipitation data were recorded after each significant
occurrence during the study on the Alpine study site.

Densities of live woolly locoweed were determined
before and at selected intervals after treatment by count-
ing live plants within a permanently marked, 18-m by
1.2-m, belt transect on a diagonal across each plot.
Herbicide effectiveness was based on percentage reduc-
tion in numbers of live woolly locoweed plants in the
belt transects. Phenology of woolly locoweed was
recorded throughout the study. The number of seeds
produced by individual woolly locoweed plants was de-
termined from 15 plants in the spring of 1980. Response
of herbaceous vegetation was determined on plots
treated in winter or spring 1979 on the Alpine study site.
Standing crop of herbaceous vegetation was harvested in
ten, 30- by 30-cm quadrats in each plot during Septem-
ber 1979. Herbage was separated into grasses and forbs,
ovendried for 48 hours and weighed.

Analysis of variance was applied to standing herbage

data. Weed control data were analyzed by analysis of co-
variance, using pretreatment densities as the covariate
(X). Where appropriate, Duncan’s Multiple Range Test
was used to determine differences among adjusted
means at P€0.05.

Results and Discussion

Woolly Locoweed Phenology

Woolly locoweed seedlings emerged from October
through December in both 1978 and 1979. Densities of
woolly locoweed plants on untreated rangeland at the
Alpine study site averaged 1.9 plants/mz in the winter of
1978-79, and 1.4 plants/mz in the winter of 1979-80. The
plants developed vegetatively through March, and flow-
ering began in April and continued into July. Seeds were
present on woolly locoweed plants from May to early
July. Mature woolly locoweed plants produced an aver-
age of 1,673 seeds each. Barneby (1964) reported that
locoweed seeds may remain viable in the soil for several
years until germination requirements are met. About 75
percent of the woolly locoweed population died during
July through September in 1979 and 1980.

Woolly locoweed is well adapted to the desert grass-
lands of the Davis Mountain area. The species produces
large numbers of seed that remain viable in the soil for
several years. Its seeds germinate and seedlings emerge
in fall when soil water content is high. The seedlings

grow deep taproots, develop vegetatively, and produce
flowers and seeds when the dominant grasses are dor-
mant due to low temperatures and/or low soil water
content.

Winter Treatments

Conditions were favorable for growth of woolly
locoweed when herbicide treatments were applied on
January 23, 1979 near Alpine. Over 5 cm of precipitation
had fallen within 60 days prior to treatment, and 3.3 cm
were received within 6O days after treatment (Table 3).
Soil water content to 3O cm deep averaged 14 percent.
About 9O percent of the woolly locoweed plants were
seedlings less than 5 cm tall. Relative humidity was 93
percent, air temperature was O degrees centigrade (° C),
and soil temperature at 2.5 cm deep was 2° C at time of
herbicide application.

At 60 days after treatment, foliar sprays of dicamba at
1.1 kg/ha (a.i.) had killed 99 percent of the woolly
locoweed. F oliar sprays of 2,4-D + dicamba (3:1); 2,4-D
+ picloram (4:1); and 2,4,5-T + picloram (1:1) at the
same rates had killed 80, 76, and 7O percent of the
weeds, respectively (Table 4). All other herbicide treat-
ments controlled less than 70 percent of the woolly
locoweed. Foliar sprays of 2,4-D ester at 1.1 kg/ha
reduced densities of the species by 53 percent, not
significantly different from control achieved with foliar
sprays of dicamba.

TABLE 2. HERBICIDAL TREATMENTS EVALUATED FOR WOOLLY LOCOWEED CONTROL

Rates
Common name(s) Chemical name(s) Formulation(s) (kg/ha a.i.)
2,4-D (2,4-dichlorophenoxy)acetic acid 2-ethylhexyl ester 1.1
2,4-D + dicamba (3:1) (2,4-dichlorophenoxy)acetic acid + dimethylamine salts 1.1
3-6-dichloro-Q-anisic acid
2,4-D + picloram (4:1) (2,4-dichlor0phenoxy)acetic acid + triisopropanolamine salts 0.7, 1.1
4-amino-3,5,6-trichloropicolinic acid
dicamba 3,6-dichloro-Q-anisic acid dimethylamine salt 1.1
5% a.i. pellets 1.1
3,6-dichl0r0picolinic 3,6-dichloropicolonic acid monoethanolamine salt 0.3, 0.6
acid
3,6-dichloropicolonic 3,6-dichloropicolinic acid + monoethanolamine salt + 0.6
acid + dicamba (1 :1) 3,6-dlChlOI'O-Q-8|1lSlC acid dimethylamine salt
3,6-dichloropic0linic 3,6-dichloropicolinic acid + monoethanolamine salt + 0.6
acid + triclopyr [(3,5,6-trichloro-2-2pyridinyl)oxy] ethylene glycol buytl
(1 :1) acetic acid ether ester
picloram 4-amino-3,5,6-trichIoropicolinic potassium salt 0.3, 0.6, 1.1
acid 10% a.i. pellet 1.1
5% a.i. pellet 1.1
picloram + dicamba 4-amino-3,5,6-trichloropicolinic potassium salt + 0.6
(1:1) acid + 3,6-dichloro-_Q-anisic acid dimethylamine salt
picloram + 4-amino-3,5,6-trichloropicolinic potassium salt + 0.6
triclopyr (1:1) acid + [(3,5,6-trichloro-2-pyridinyl) ethylene glycol butyl
oxy]acetic acid ether ester
2,4,5-T + picloram (2,4,5-trichlorophenoxy)acetic acid triethylamine salt + 0.3, 0.6, 1.1

(1 :1) + 4-amino-3,5,6-trichloropicolinic
acid

N-[5-(1,1-dimethylethyl)-1,3,4-
thiadiazol-2-yl]-_lj,_l:l’-dimethylurea

tebuthiuron

diethylamine salt

80% a.i. wettable powder 1.1
20% a.i. pellets (0.32-cm
diameter) 1.1
20% a.i.pellets (0.16-cm
diameter 1.1

At 120 days after treatment, woolly locoweed was
completely controlled on plots treated with foliar sprays
of dicamba (Table 4). More than 85 percent of the weeds
were controlled on plots treated with sprays of 2,4-D +
picloram (4:1) or 2,4,5-T + picloram (1:1). However,
reduction in woolly locoweed densities from dicamba
sprays was not significantly greater than that attained
with 2,4-D (52 percent). Neither 2,4-D, the 1O percent
(a.i.) formulation of pelleted picloram, nor any of the
formulations of tebuthiuron significantly reduced woolly
locoweed densities compared to untreated rangeland
(Table 4).

By 1 year after treatment all picloram treatments and
herbicide mixtures containing picloram had reduced
woolly locoweed densities by at least 84 percent (Table
4). Reduction in live woolly locoweed density averaged
83 percent where dicamba had been applied. All her-
bicide treatments reduced densities of the species com-
pared to untreated rangeland, but none of the treat-

TABLE 3. MONTHLY RAINFALL DURING THE PERIOD IN WHICH
VARIOUS HERBICIDES WERE BEING EVALUATED FOR WOOLLY
LOCOWEED CONTROL ON THE MORROW-MCINTYRE RANCH
NEAR ALPINE, TEXAS

Rainfall by year (cm)

55-year

Month 1979 1980 average
January 1.0 1.5 1.9
February 2.3 0.5 1.0
March t1 2.0 0.9
April 1.5 1.1 1.2
May 2.0 2.6 3.0
lune 4.4 2.4 6.6
July 10.1 0.1 6.9
August 10.2 6.7 6.4
September 3.0 17.5 5.7
October 0 4.2 3.4
November 0 2.6 1.3
December 0 1.9 1.3

Annual Total 34.5 43.1 39.6

‘t = trace

ments provided significantly better control than 2,4-D
(69 percent).

At 420 days after herbicide application, all picloram
treatments or herbicide mixtures containing picloram,
except the 10 percent (a. i.) pellet formulation, controlled
85 percent or more of the woolly locoweed (Table 2). All
treatments except O.16-cm tebuthiuron pellets, signifi-
cantly reduced woolly locoweed densities compared to
untreated rangeland. Control achieved with 5 percent
(a.i.) picloram pellets (96 percent) was not significantly
better than that achieved with 2,4-D ester (65 percent)
(Table 4).

At 126 days after application on December 5, 1978
near Fort Davis, tebuthiuron pellets at 1.1 kg/ha had
controlled 81 to 96 percent of the woolly locoweed
population (Table 5). Population densities were not af-
fected by dicamba granules or 1O percent a.i. picloram
pellets. By 190 days after treatment, emergence of new
populations of woolly locoweed seedlings had nullified
the early control achieved by tebuthiuron pellets. At 1
year after treatment, all of the original woolly locoweed
plants on these unfenced plots had been consumed by
cattle and only a few seedlings about 5 cm in height were
present. Analysis of variance on seedling density data
indicated no significant differences in seedling density
among the treatments or untreated rangeland (p<0.05)
(data not shown). Cattle losses were heavy in this pasture
in the winter of 1979. Thirty-five calves died in the
winter of 1979 due to woolly locoweed toxicity (Billy
Weston, personal communication).

At 103 days after application on February 9, 1979 near
Fort Davis, none of the pelleted herbicides had effec-
tively controlled woolly locoweed (Table 5). At 1 year
after treatment all of the mature woolly locoweed had
been consumed by cattle. Analysis of variance of new
seedling density data indicated no significant differences
among the herbicide treatments or untreated rangeland
(P<0.05) (data not shown).

TABLE 4. PERCENTAGE REDUCTION IN WOOLLY LOCOWEED DENSITIES AT 60,120, 365 AND 420 DAYS FOLLOWING APPLICATION OF
11 HERBICIDE TREATMENTS AT 1.1 kg/ha ON JANUARY 23, 1979 NEAR ALPINE, TEXAS

Days after treatment‘

Herbicide Formulation 60 I 120 365 420
None — 0 a 0 ab 0 a O a
2,4-D Liquid 53 b-e 52 b-e 69 bc 65 b
2,4-D + dicamba (3:1) Liquid 80 de 58 c-e 56 bc 50 b
Dicamba Liquid 99 e 100 e 83 bc 76 b
2,4-D + picloram (4:1) Liquid 76 de 85 de * 94 c 94 b
Picloram Liquid 69 c-e 74 c-e 87 bc 85 b
Picloram 5% pellets 64 c-e 58 c-e 95 c 96 b
Picloram 10% pellets 33 a-d 23 a-c 85 bc 75 b
2,4,5-T + picloram (1:1) Liquid 70 c-e 86 de 84 bc 90 b
Tebuthiuron Wettable powder 55 b-e 39 a-d 66 bc 71 b
Tebuthiuron 20% pelletsz 17 ab -15 a 48 bc 56 b
Tebuthiuron 20% pellets3 29 a-c 26 a-c 43 b 45 ab

‘Means within a column followed by similar lower case letters are not significantly different at P<0.05.

20.32-cm diameter.
30.16-cm diameter.

4

TABLE 5. PERCENTAGE REDUCTION IN WOOLLY LOCOWEED
DENSITIES AFTER APPLICATION OF FIVE PELLETED HERBICIDES
AT1.1 kg/ha ON DECEMBER 5,1978 AND FEBRUARY 9, 1979 NEAR
FORT DAVIS, TEXAS

Treatment dates

December 5, 1978 February 9, 1979
Days after treatment

Herbicide Formulation 126 190 103
None — 0 a1 0 a 0 b
Dicamba 5% granules 58 ab 51 a 13 b
Picloram 5% pellets 68 b 58 a —55 ab
Picloram 10% pellets 41 ab 40 a —121 a
Tebuthiuron 20% pelletsz 96 b 68 a 5 b
Tebuthiuron 20% pellets3 81 b 32 a 8 b

‘Means within a column followed by similar lower case letters are not
significantly different at P<0.05.

20.32-cm diameter.
30.16-cm diameter.

Spring Treatments

Growing conditions and plant phenology were less
than optimal for herbicidal control of woolly locoweed
when treatments were applied on May 15,1979. Only
1.7 cm of precipitation had fallen within 6O days prior to
herbicide application (Table 3). Soil water content av-
eraged 4.8 percent, air temperature was 25° C, soil
temperature 2.5 cm deep was 29° C, and relative humid-
ity was 17 percent at time of herbicide application. Most
of the woolly locoweed plants were mature and flower-
ing. More than 14 cm of precipitation were received
within 60 days after treatments were applied.

F oliar sprays of 2,4,5-T + picloram (1:1) at 1.1 kg/ha
controlled 81 percent of the woolly locoweed at 3O days
after treatment (Table 6). Control with 2,4,5-T + pic-
loram (1:1) was significantly greater than that achieved
with foliar sprays of 2,4-D (14 percent) at the same rate.
Control on plots treated with all other herbicides was 61
percent or lower.

At 240 days after treatment, foliar sprays of 2,4,5-T +
picloram (1:1) and picloram at 1.1 kg/ha rates reduced
woolly locoweed density by 83 percent compared to

adjacent untreated rangeland (Table 6). F oliar sprays of
2,4-D + picloram (4:1) and 5 percent picloram pellets at
1.1 kg/ha had controlled 75 percent and 78 percent of the
weed population, respectively. The other treatments
had not effectively controlled woolly locoweed. Howev-
er, control attained with 2,4-D foliar spray (37 percent)
was not significantly different than that attained with
foliar sprays of picloram or 2,4,5-T + picloram (1:1) (83
percent).

At 1 year following herbicide applications in May
1979, woolly locoweed densities were reduced by 82, 84
and 89 percent on plots treated with 5 percent picloram
pellets, foliar sprays of 2,4,5-T + picloram (1:1) and
foliar sprays of picloram at 1.1 kg/ha respectively, com-
pared to untreated rangeland (Table 6). Foliar sprays of
2,4-D + picloram (4:1) controlled 79 percent of the
woolly locoweed plants whereas all other herbicide
treatments killed 37 percent or less. Control with 2,4-D
foliar sprays (34 percent) was not significantly different
from control attained with picloram foliar spray (89
percent).

Fall Treatments

Treatments were applied on November 14, 1979. The
woolly locoweed population consisted of assorted age
classes, but all plants were vigorous and in vegetative
growth stages. Soil water content averaged 4.9 percent,
air temperature was 16° C, soil temperature at 2.5 cm
was 20° C, and relative humidity was 23 percent at time
of treatment. Precipitation in the 60-day period prior to
treatment totalled 3 cm and only 1.5 cm were received
for 6O days after treatment (Table 3).

At 6O days following November 1979 treatments, only
foliar sprays of picloram or 2,4,5-T + picloram at 1.1
kg/ha had controlled more than 72 percent of the woolly
locoweed (Table 7). None of the other herbicides had
effectively controlled woolly locoweed.

At 120 days after treatment, foliar sprays of picloram
+ triclopyr ester (1:1) at 0.6 kg/ha had completely
controlled woolly locoweed (Table 7). Picloram sprays at
1.1 kg/ha had killed 96 percent of the weeds. Herbicide
treatments controlling 85 to 93 percent of the woolly

TABLE 6. PERCENTAGE REDUCTION IN WOOLLY LOCOWEED DENSITIES AT 30, 240 AND 365 DAYS FOLLOWING APPLICATION OF 11
HERBICIDE TREATMENTS AT 1.1 kg/ha ON MAY 15, 1979 NEAR ALPINE, TEXAS1

Herbicide Formulation 30 240 365

None -_ O a-c 0 a-c 0 a-d
2,4-[) Liquid 14 bc 37 a-c 34 a-d
2,44) + dicamba(3:1) Liquid 5 a-c —41 a ~47 a

Dicamba Liquid 26 b-d 39 a-c 37 a-d
2,4-D + picloram (4:1) Liquid 50 b-d 75 b0 79 Cd
Pidoram LlqLlld 61 Cd 83 C 89 d

Picloram 5% pellets 45 b-d 78 c 82 cd
Picloram 10% pellets ~15 ab 64 bc 68 b-d
2,4,5-T + picloram (1:1) Liquid 81 d 83 C 84 Cd
Tebuthiuron Wettable powder 1 a-C —14 8b "13 ab
Tebuthiuron 20% pelletsz —53 a —13 ab —6 a-c
Tebuthiuron 20% pellets3 26 b-d 19 a-c 16 a-d

‘Means within a column followed by similar lower case letters are not significantly different at P<0.05.

20.32-cm diameter.
30.16-cm diameter.

TABLE 7. PERCENTAGE REDUCTION IN WOOLLY LOCOWEED DENSITIES AT 60, 120 AND 180 DAYS FOLLOWING APPLICATION OF 18
LIQUID HERBICIDE TREATMENTS ON NOVEMBER 14, 1979 NEAR ALPINE, TEXAS

Days after treatment‘

Rate

Herbicides (kg/ha) 60 120 180

None — O ab 0 a 0 a

2,4-D 1.1 35 b-e 60 c-e 62 cd
2,4-D + dicamba (3:1) 1.1 56 c-f 85 d-g 86 de
2,4-D + picloram (4:1) 1.1 57 c-f 76 d-g 75 c-e
2,4-D + picloram (4:1) 0.7 41 c-f 57 b-d 53 bc
Dicamba 1.1 64 d-f 88 e-g 87 de
3,6-dichloropicolinic acid 0.6 24 a-c 76 d-g 78 c-e
3,6-dichloropicolonic acid 0.3 26 a-d 67 d-f 65 cd
3,6-dichloropicolonic acid + triclopyr (1:1) 0.6 52 c-f 90 e-g 92 de
3,6-dichloropicolinic acid + dicamba (1:1) 0.6 34 b-d 86 d-g 83 c-e
Picloram 1.1 72 ef 96 fg 93 de
Picloram 0.6 32 b-d 68 d-f 71 c-e
Picloram 0.3 -6 a 29 b 31 b

Picloram + triclopyr (1:1) 0.6 55 c-f 100 g 100 e

Picloram + dicamba (1:1) 0.6 39 c-e 89 e-g 89 de
2,4,5-T + picloram (1:1) 1.1 78 f 93 fg 89 de
2,4,5-T + picloram (1:1) 0.6 64 d-f 87 d-g 87 de
2,4,5-T + picloram (1:1) 0.3 41 c-f 78 d-g 79 C-e
Tebuthiuron (w.p.)2 1.1 36 b-e 34 bc 32 b

‘Means within a column followed by similar lower case letters are not significantly different at P<0.05.

ZWettabIe powder formulation.

locoweeds included 3,6-dichloropicolinic acid + dicam-
ba (1:1); 2,4,5-T + picloram (1:1); picloram + dicamba
(1:1); and 3,6-dichloropicolinic acid + triclopyr (1:1) at
0.6 kg/ha rates, as well as 2,4-D + dicamba (3:1);
dicamba; and 2,4,5-T + picIoram (1;1) at 1.1 kg/ha. At
120 days following November 1979 treatments, sprays of
picloram + triclopyr (1:1) at 0.6 kg/ha; picloram; and
2,4,5-T + picloram (1:1) at 1.1 kg/ha controlled signifi-
cantly more woolly locoweed than foliar sprays of 2,4-D
at 1.1 kg/ha (Table 7).

At 180 days following fall 1979 treatments, foliar
sprays of picloram + triclopyr ester (1:1) at 0.6 kg/ha
completely controlled woolly locoweed (Table 7). Foliar
sprays of dicamba; 2,4,5-T + picloram (1:1); and pic-
loram at 1.1 kg/ha rates, as well as sprays of 2,4,5-T +
picIoram (1:1); picloram + dicamba (1:1); and 3,6-
dichloropicolinic acid + triclopyr (1:1) at 0.6 kg/ha rates
controlled 87 to 93 percent of the woolly locoweed.
Herbicide treatments that reduced densities of the
species by 75 to 86 percent at 180 days post-treatment

included 2,4-D + picloram (4:1) and 2,4-D + dicamba
(3:1) at 1.1 kg/ha; 3,6-dichloropicolinic acid and 3,6-
dichloropicolinic acid + dicamba (1:1) at 0.6 kg/ha; and
2,4,5-T + picloram (1:1) at 0.3 kg/ha. Only picloram +
triclopyr (1:1) at 0.6 kg/ha reduced woolly locoweed
densities significantly more than foliar sprays of 2,4-D at
1.1 kg/ha (100 vs 62 percent reduction) at 180 days after

treatment.

Season x Treatment Interaction

Among the seven herbicide treatments that were ap-
plied in all three seasons, significant season x treatment
interactions were identified for foliar sprays of 2,4-D +
dicamba (3:1); dicamba;_ and tebuthiuron at 1.1 kg/ha
rates (Table 8) (P<0.05). Fall and winter applications of
2,4-D + dicamba controlled significantly more Woolly
locoweed than spring applications, and fall applications
controlled significantly more woolly locoweed than win-
ter applications at 1-year post-treatment. A similar pat-
tern was observed for dicamba foliar sprays (Table 8).

TABLE 8. MEAN PERCENT REDUCTION OF WOOLLY LOCOWEED AT 60 DAYS AND 365 DAYS FOLLOWING WINTER, SPRING OR FALL
APPLICATIONS OF 7 LIQUID HERBICIDE TREATMENTS NEAR ALPINE, TEXAS

60 days 365 days
Rate
Herbicides (kg/ha) Winter Spring Fall Winter Spring Fall
2,4-D 1.1 53 a‘ 14 a 35 a 69 a 34 a 62 a
2,4-D + dicamba (3:1) 1.1 80 b 5 a 56 b 56 b —47 a 86 c
Dicamba 1.1 99 b 26 a 64 b 83 ab 37 a 87 b
2,4-D + picloram (4:1) 1.1 76 a 50 a 57 a 94 a 79 a 75 a
Picloram 1.1 69 a 61 a 72 a 87 a 89 a 93 a
Tebuthiuron (w.p.)2 1.1 55 b 1 a 36 ab 66 b —18 a 32 b
2,4,5-T + picloram (1:1) 1.1 70 a 81 a 78 a 84 a 84 a 89 a

‘Means within a row, for each evaluation date, followed by similar lower case letters are not significantly different at P<0.05.

2Wettable powder formulation.

6

Winter application of foliar sprays of tebuthiuron at 1.1
kg/ha controlled significantly more woolly locoweed than
spring applications, and there was a trend of better
control in winter compared to fall.

Fall applications of foliar sprays of picloram and 2,4,5-
T + picloram (1:1) at 1.1 kg/ha rates tended to control
more woolly locoweed than either winter or spring
applications, although differences were not significant
(Table 8). Fall and winter applications of 2,4-D at 1.1
kg/ha tended to be more effective than spring applica-
tions, but the differences were not statistically signifi-
cant. -

Forage Response

None of the herbicide treatments applied in January
or May 1979 affected production of desirable grasses or
forbs compared to untreated rangeland (data not shown).
Standing herbage crop averaged 1,565 kg/ha in the
ungrazed exclosure in the fall of 1979. Woolly locoweed
apparently does not compete with desirable forage
species at the low weed densities encountered in this
study.

Conclusions

Control of woolly locoweed was better following fall
treatments with most herbicides, compared to winter or
spring treatments, presumably because seedlings were
emerging and actively growing in the fall. In the Davis
Mountain area conditions for rapid vegetative growth of
woolly locoweed are normally more favorable during fall
than during winter or spring, because of the summer-fall
rainfall pattern. Herbicidal control improved in the fall
of 1979, when soil water content averaged less than 5
percent and air temperature was 16° C at time of her-
bicide application, compared to that achieved in the
winter of 1979 when soil water content averaged 14
percent and air temperature was 0° C. This suggests that
the ‘susceptibility 0f woolly locoweed to several her-
bicides may be reduced at low temperatures, even when
soil water contents are optimal for plant growth. Ueckert
et al. (1980) reported similar responses for bitterweed.
Rates of absorption and translocation of foliar-applied
herbicides increase with increased air temperatures, so
less herbicide may be required at moderate tempera-
tures (i.e. 16 to 20° C) than at lower temperatures (i.e. 0°
C) for absorption and translocation of lethal amounts of
herbicide (Klingman and Ashton, 1975; Leopold and
Kriedemann, 1975; Ashton and Crafts, 1973).

Winter applications of most herbicides tended to con-
trol more woolly locoweed than spring applications,
probably because growing conditions were more favor-
able and plants were younger when treated. The suscep-
tibility of most herbaceous plants to herbicides usually
decreases with advancing maturity and as growing condi-
tions become less favorable.

Foliar sprays of esters of 2,4-D at 1.1 kg/ha did not
satisfactorily control woolly locoweed. F oliar sprays of
picloram or herbicide combinations containing picloram
at 0.2 to 1.1 kg/ha effectively controlled woolly locoweed
for a year or longer following winter or fall applications.
However, picloram pellets did not consistently control

woolly locoweed, and 10 percent a.i. picloram pellets
were less effective than 5 percent a.i. pellets, presum-
ably due to uneven distribution. Herbicidal control of
woolly locoweed with foliar sprays of picloram was usual-
ly manifested within 120 days or less, compared to 1 year
for pelleted formulations of picloram. F oliar sprays of
dicamba at 1.1 kg/ha completely controlled woolly
locoweed by 60 to 120 days after application during a
cold winter. However, short-term control following
spring or fall applications of dicamba was poor to fair.
Long-term (365 days i) control of woolly locoweed was
generally best on rangeland treated with either picloram
at 1.1. kg/ha or herbicide mixtures containing picloram
at 0.2 to 0.6 kg/ha, apparently because of the persistence
of picloram in the soil (Crafts, 1975; Bovey and Scifres,
1971; Scifres et al., 1971).

Tebuthiuron, applied as foliar sprays or as 20 percent
a.i. pellets, did not effectively control woolly locoweed.
Herbicide mixtures of 2,4-D + dicamba (3:1) at 1.1
kg/ha, applied in the fall, killed 85 percent of the weeds
by 120 days after treatment. However, winter and spring
treatments did not effectively control the weed.

F oliar sprays of 3,6-dichloropicolinic acid applied in
fall at 0.3 to 0.6 kg/ha did not effectively control woolly
locoweed. However, fall application of foliar sprays of
3,6-dichloropicolinic acid + triclopyr (1:1) or 3,6-
dichloropicolinic acid + dicamba (1:1) at 0.6 kg/ha killed
over 86 percent of the weed population by 120 days after
treatment. F oliar sprays of picloram + triclopyr (1:1)
and picloram + dicamba (1:1), applied in the fall at 0.6
kg/ha controlled 100 and 89 percent of the weed within
120 days after treatment, respectively. Fall application
of foliar sprays of 2,4,5-T + picloram (1:1) at 0.6 kg/ha
were as effective for woolly locoweed control as the 1.1
kg/ha rate.

Production of desirable forage was not affected by the
herbicide treatments evaluated, presumably because the
low densities of woolly locoweed (1.4 to 1.9/m2) did not
compete with desirable forage plants.

Literature Cited

Alley, H. P. 1976. Herbicide evaluations for control of Lambert crazy-
weed. In Research in weed science. Univ. Wyoming Agr. Exp. Sta.
Res. I. 91-R. p. 80-81.

Allison, L. E. 1965. Organic carbon. In C. A. Black (Ed) Methods of
soil analysis. (Part II). Amer. Soc. Agron., Madison, Wis. p. 1367-
1378.

Ashton, F. M. and A. S. Crafts. 1973. Mode of Action of Herbicides.
John Wiley and Sons, New York. 504 pp.

Barneby, R. C. 1964. Atlas of North American Astragalus. New York
Botanical Garden, New York. 1188 pp. _
Bovey, R. W. and C. ]. Scifres. 1971. Residual characteristics of pic-
loram in grassland ecosystems. Texas Agr. Exp. Sta. Bull. 1111: 24

p.

Correll, D. S. and M. C. Iohnston. 1970. Manual of the Vascular
Plants of Texas. Texas Res. Found., Renner, Texas. 1881 pp.
Crafts. A. S. 1975. Modern Weed Control. University of California

Press. Berkeley, California. 440 pp.

Cronin, E. H. and M. C. Williams. 1964. Chemical control of timber
milkvetch and effects on associated vegetation. Weeds 12:177-179.

Day, P. R. 1965. Particle fractionation and particle size analysis. In
C. A. Black (Ed) Methods of soil analysis. (Part I). Amer. Soc.
Agron., Madison, Wis. p. 545-567.

Continued

7

Dollahite, I. W. 1965. Poisonous plants deal heavy blows to livestock.
Texas Agr. Prog. 11:5-7.

Cardner, W. H. 1965. Water content. In C. A. Black (Ed) Methods of
soil analysis. No. 9 Monogr. Ser. (Part I). Amer. Soc. Agron.,
Madison, Wis. p. 82-127.

Iames, L. F., K. L. Bennett, K. C. Parker, R. F. Keeler, W. Binns,
and B. Lindsay. 1968. Loco plant poisoning in sheep. I. Range
Manage. 21:360-365.

Iames, L. F. 1972. Syndromes of locoweed poisoning in livestock.
Clinical Toxicol. 5:567-573.

Iames, L. F. and A. E. Johnson. 1976. Some major plant toxicities of
the western United States. I. Range Manage. 29:356-363.

Iames L. F., R. F. Keeler, A. E. Iohnson, M. C. Williams, E. H.
Cronin and I. D. Olsen. 1980. Plants poisonous to livestock in the
western states. U.S. Dept. Agr., Agr. Information Bull. 415. 9O pp.

Klingman, C. C., and F. M. Ashton. 1975. Weed Science. Principles
and Practices. Iohn Wiley and Sons, New York. 431 pp.

Krueger, W. C. and L. A. Sharp. 1978. Management approaches to
reduce livestock losses from poisonous plants on rangeland. I.
Range Manage. 31:347-350.

Leopold, C. A., and P. E. Kriedemann. 1975. Plant Crowth and
Development (2nd Ed.) McCraw-Hill Book Co., New York.
545 pp.

Nielson, D. B. 1978. The economic impact of poisonous plants on the
range livestock industry in the 17 western states. I. Range Manage.
31:325-327.

Norris, I. I. 1951. The distribution and chemical control of species of
Senecio, Astragalus, and Baileya in the highlands range areas of
West Texas. Ph.D. Dissertation, Texas A&M University. 93 p.

Peech, M. 1965. Hydrogen ion activity. In C. A. Black (Ed) Methods
of soil analysis. (Part II). Amer. Soc. Agron., Madison, Wis. p. 914-
926. .

Scifres, C. ]., R. R. Hahn, I. Diaz-Colon, and M. C. Merkel. 1971.
Picloram persistence in semiarid rangeland soils and water. Weed
Sci. 19:381-384.

Sperry, O. E. 1951. Use of herbicides in the control of poisonous range
plants in Texas. Texas I. Sci. 2227-233.

Sperry, O. E., I. W. Dollahite, C. O. Hofiaman, and B. I. Camp. 1964.
Texas plants poisonous to livestock. Texas Agr. Exp. Sta. Bull.
1028. 57 p.

Ueckert, D. N., C. I. Scifres, S. C. Whisenant and I. L. Mutz. 1980.
Control of bitterweed with herbicides. I. Range Manage. 33:465-
469.

Williams, M. C. 1970. Detoxication of timber milkvetch by 2,4,5-T
and silvex. I. Range Manage. 23:400-402.

Metric Units —— English Equivalents

Metric Unit English Equivalents
Centimeter (cm) 0.394 inches
Hectare (ha) 2.47 acres

Kilogram (kg) 2.205 pounds
Kilogram per hectare (kg/ha) 0.983 pound per acre
Kilometer (km) 0.62 statute mile
Liter (L) 0.264 gallon

Meter (m) 3.28 feet

Square meter (m2) 10.758 square feet

(Degrees centigrade >< 1.8 + 32) I Degrees fahrenheit

Acknowledgments

The authors express their appreciation t0 the Houston
Livestock Show and Rodeo for financial support of this
research, to Keith Morrow, Billy Weston and Tommy
Weston for providing land for the research, to Dr.
Charles E. Gates for assistance with statistical analyses,
to Brad Lisenbee for assistance with computer pro-
gramming and to R. L. Potter and I. L. Petersen for soil
analyses. a

APPENDIX
Scientific Names of Plants Mentioned in Text

Common Name Scientific Name

Bitterweed H ymenoxys odorata

Blue grama Bouteloua gracilis

Cane bluestem Bothriochloa barbinodis
Garbancillo Astragalus wootonii

Green River milkvetch Astragalus pubentissimus
Halls panicum Panicum hallii

Hook threeawn Aristida hamulosa

Inland saltgrass Distichlis spicata var. stricta
Lambert crazyweed Oxytropis lambertii
Locoweed Astragalus spp.

Sideoats grama Bouteloua curtipendula
Threeawns Aristida spp.

Wolftail Lycurus phleoides

Woolly locoweed Astragalus mollissimus var. earlei

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, ethnic origin,

religion, sex, or age.
1.2M——6-82