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EARLY WINTER CHEMICAL CONTROL OF BITTERWEED
by
RODNEY C. CHAMBERS, B.S.
A THESIS
IN
RANGE SCIENCE
Submitted to the Graduate Faculty of Texas Tech University in
Partial Fulfillment of the Requirements for
the Degree of
MASTER OF SCIENCE
Approved
Dfe^ember,^971
AEU'U7S&
ACKNOWLEDGEMENTS
Most of all I would like to thank Dr. Henry A. Wright for his
guidance, suggestions, and patience throughout this study. My
appreciation also goes to Dr. Samuel Curl and Dr. Ronald Sosebee
for their constructive criticisms.
To Hal Noelke I wish to express my gratitude for his hospitality
and support in times of need. I would also like to thank my wife,
Gloria, for her encouragements and help in preparation of this manu
script.
n
TABLE OF CONTENTS
Page
ACKNOWLEDGEMENTS ii
LIST OF TABLES iv
LIST OF FIGURES v
I. INTRODUCTION 1
II. LITERATURE REVIEW 3
Description of Plant 3
Poisonous Properties 4
Distribution of Plant 5
Ecology of Bitterweed 8
Past Bitterweed Research 9
Herbicides 11
2,4-D Effects on Plants 12
III. METHODS AND PROCEDURES 15
Description of Area 15
Field Methods 15
IV. RESULTS 20
V. DISCUSSION 26
VI. SUMMARY AND CONCLUSIONS 32
LITERATURE CITED 33
APPENDIX 35
• • m
LIST OF TABLES
Table Page
1. Yield of bitterweed (lb/acre) in relation to season and treatment on May 20, 1971 21
2. Yield of grasses (lb/acre) in relation to season and treatment on May 20, 1971 22
3. Yield of desirable forbs (lb/acre) in relation to season and treatment on May 20, 1971 24
4. Yield of desirable forbs and grasses (lb/acre) in relation to season and treatment on May 20, 1971. . 25
TV
LIST OF FIGURES
Figure Page
1. Distribution of bitterweed in Texas (Sperry, 1949). . . 6
2. Areas heavily populated with bitterweed such as the one above are common on abused rangeland throughout the Edwards Plateau of Texas 7
3. Herbicidal treatments were applied with the above spray rig in this study 18
4. Effects of 2,4-D treatments on bitterweed production 27
5. An example of the 3 3/4 lb/acre treatment of 2,4-D is shown in the left front of the above photograph. Now that the influence of bitterweed has been removed, the revegetation process can begin 29
6. The above area had 20% foliar cover of vegetation other than bitterweed before a 2,4-D spray treatment removed the bitterweed competition. Grass production has greatly increased and will be highly competitive with bitterweed in the future. . . . 31
CHAPTER I
INTRODUCTION
Bitterweed (Hymenoxys odorata DC. Actinea odorata (DC.) Ktze.),
a native plant, was first recognized in Texas as a poisonous plant in
1922 (Sperry, 1949). Though much research has been done on bitterweed
since 1922, it still causes a 1 to 6% loss of the average sheep herd
annually in central Texas (Sultemeier, 1961). Most sheep suffer from
bitterweed poisoning in winter and early spring before green range
forage is available (Sperry, Dollahite, Morrow, and Hoffman, 1955).
Good range management is one answer to the bitterweed problem
(Sperry, 1949). Vance (1958) reports that when the soil is covered
with grass, the bitterweed will be eliminated. In the Vance article,
Leo Merrill stated, "Wherever there is bare soil, there will be a
bitterweed problem. Grass cover is the only practical control, and
it can be done by a rest period from grazing." However, control of
bitterweed by reduced stocking rates alone may be a wery slow process.
Therefore, many ranches are still heavily infested with bitterweed
despite these recommendations.
Spraying with 2,4-D during the maximum growing period of bitter
weed, usually April, gives effective control of bitterweed (Sperry,
1949). Spraying with 2,4-D in early spring is the suggested bitterweed
control practice at the current time (Sperry, et al., 1955). However,
numerous animals may be lost during the winter if bitterweed is not
eliminated until spring. For this reason it is difficult for a rancher
1
to economically justify a spring spray program.
Wright (1970) found that spraying with 2,4-D can be as effective
in winter as in early spring, though he did not determine the optimum
rate of application. Winter applications are advantageous in that they
may reduce animal losses for the year of application. Bitterweed
competition with grasses may also be reduced during the insuing spring.
This study was conducted to test various rates of 2,4-D for winter
treatment of bitterweed. Atrazine and dicamba were also tested for
effectiveness against this weed.
CHAPTER II
LITERATURE REVIEW
Description of Plant
Bitterweed is an annual in the Helenium tribe of the Compositae
family (Hardy, Cory, Schmidt, and Dameron, 1931). It may be found in
moist areas and attain a height of about 50 cm. Usually it is branched
at the base with the branches being leafy and terminating in small
heads. Stems are usually purplish at the base. The leaves are
alternate, once to thrice parted into thread-like segments, not rigid,
glandular dotted throughout, and glaucous-wooly under the surface.
The number of infloresences per plant varies with the branching
of the plant and its environmental growing conditions. Cory (1951)
counted as many as 3000 heads on one plant and Sperry (1950) reported
from 47 to 66 achenes in each head. Thus a single plant could produce
as many as 150,000 seeds in a single growing season. However, at this
time the viability of bitterweed is not known.
Ray-flowers are bright yellow and the corolla is ligulate.
Flower petals are wedge-shaped, three lobed at the apex, and are 0.7 cm
long. The full flower head is 2 cm wide, fully one-third of which is
due to the diameter of the disk. The one seeded fruit (achene) is
broadest upward, densely hairy, and has a pappus of several awned or
sharply pointed, yery thin, cleft scales. A prominent characteristic
of the plant is its aroma and its bitter taste (Sperry, 1949; Hardy
et al., 1931).
Poisonous Properties
Symptoms of bitterweed poisoning vary according to the condition
of the sheep and the percentage of bitterweed in their diet rather than
the quantity of bitterweed eaten (Sperry, 1949). Fatal results may
follow intake of the green plant if a sheep consumes as much as 1.3%
of its body weight in bitterweed (Boughton and Hardy, 1937; Hardy et
al., 1931). According to Schmutz, Freeman, and Reed (1968) the
poisonous principle of bitterweed is still unknown. The symptoms are
principally salivation, nausea, vomiting, depression, and weakness.
In acute cases the pulse is distinctly faster and weaker and the body
temperature is higher than before feeding on bitterweed (Sperry, 1949).
Sheep can succumb to the poisonous effects of bitterweed within
a comparatively short time (Clawson, 1931). There is a complete loss
of appetite, cessation of rumination, depression, indication of
abdominal pain, bloat, frequently a light froth at the mouth, and many
times a discharge from the nose that is stained a deep green with plant
coloring matter. Animals have been found with abdominal pain indi
cated by the animal standing with its back arched and indisposed to
move or it moves only slowly.
Hardy et al., (1931) observed that there was a froth at the mouth
or a greenish discharge from the nose in 8 out of 17 experimental cases
at Texas A&M University. Vomiting was observed for 1 of 17 cases in
this study. In most animals they found the pulse to be greatly ac
celerated, but body temperatures were normal or subnormal. Accelerated
respiration was not recorded.
Distribution of Plant
Sperry (1949) reported that bitterweed has been found in almost
every county in Texas west of the 99th meridian (Fig. 1). Its range
extends into western Oklahoma and eastern and southern New Mexico.
Bitterweed is found a little above sea level to an altitude of 4000 ft
(Clawson, 1931). It is not found in the mountains, and it does not
occur in areas of high humidity.
Bitterweed in Texas is mainly a problem in the western portion of
the Edwards Plateau which is approximately 10 million acres. The areas
of heaviest infestation are in about 15 counties; Coke, Crockett,
Edwards, Irion, Pecos, Reagan, Runnels, Schelicher, Sterling, Sutton,
Taylor, Terrell, Tom Green, Upton, and Val Verde. Bitterweed does not
occur uniformly over this area, but seems to be present in more or
less abundance in well defined but irregular areas (Hardy, et al.,
1931).
Within the areas of greatest infestation, floods have been re
sponsible for much of the spread of bitterweed (Sperry, 1949).
Drainage areas, lake beds, draws, and flooded sites are the usual
places of infestation. It is common along the roadways, trails, bed
grounds, watering places, and headquarters. Perennial vegetation is
often killed out in these sites by standing water or by overgrazing
and trampling; then the bitterweed takes over (Fig. 2).
Bitterweed usually occurs on areas with heavy soils such as clays
or clay types. Bottomlands and lowland areas become rapidly infested
under heavy grazing pressure or abuse. If there is an available seed
source, bitterweed can grow on almost any soil that has a lack of
t</S^Scattered growth and localized infestation
^^3Severe poisoning and heavy losses
Fig. 1. Distribution of bitterweed in Texas (Sperry, 1949)
E O CD
Qi S-fC
(U > o
JO ro
• I/) CO X (U
<U h-s= O M-
<U J C -•->
to <T3
JCZ o 3
o C3 rtJ (U
.4-> 03 r— Ou
to CO - o
-a (U
S-(0 •^
cu -o 2 s-<u
LU
<U -M -c: -•-> •n" J D
J C
+-> -M rJ o
••-> j u r • p -
s -a
en 13 o S-
0) sz -i-> (O r— Z3 D . O
4->
-o c «3
r— a . o) >>
r— •r->
CD C rtJ %-
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Qi to rs
to .Q fO O) s-
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CM
0 1
fO
E O
8
plant cover. It is not unusual to find ranches, previously in excel
lent condition, now occupied by bitterweed (Jones, Hill, and Bond,
1932).
The recognition of bitterweed and its increase in abundance
parallels the increased stocking rates on the rangelands of west Texas,
From 1913 to 1923 the number of cattle increased from 650,000 to 1
million (Jones, et al., 1932). This large increase in livestock
numbers placed severe grazing pressure on the vegetation. Desirable
grazing plants of the rangelands gradually became overgrazed and
species of lesser grazing quality increased. By 1928 losses from
bitterweed poisoning had become an alarming problem for ranchers in
the area (Sperry, 1949; Hardy, et al., 1931).
Ecology of Bitterweed
Seedlings or older plants may.be found at almost any time of
the year, usually in late fall, but most growth takes place from early
winter to midsummer. If climatic conditions are favorable, growth
may start as early as October. Most bitterweed plants germinate in
November and December but a few new plants may be found throughout
the year (Sultemeier, 1961).
Under favorable conditions bitterweed grows rapidly and reaches
a height of 30 to 35 cm. This dense growth, occupying land in the
spring, greatly reduces the growth of grass and continually deter
iorates the grass sod (Hardy, et al., 1931). Sultemeier (1961)
observed that dense, dry, mature bitterweed will continue to suppress
growth of grass as late in the season as August or September.
During the winter, toxicity of bitterweed increases with maturity
and under drought conditions (Schmutz, et al., 1968). With favorable
moisture conditions, a new crop of bitterweed may appear late in the
spring and bloom quite freely. However, plants occuring at this time
will usually not cause sheep loss (Hardy, et al., 1931). This is
probably due to the fact that sheep will not eat bitterweed if other
green forage is available.
When grasses and desirable forbs are dominant in an area there
is no bitterweed problem (Sperry, 1949). On the other hand where
bitterweed is a problem, grasses seem to recover wery slowly, if at
all. In these areas some type of chemical control will enhance the
recovery of grasses.
Past Bitterweed Research
There have been many attempts- to control bitterweed since it
first was recognized as a poisonous plant problem around 1930. Hand
pulling was first tried but it was tedious, expensive, and often had
to be repeated every year (Sultemeier, 1961). Due to the errotic
germination of bitterweed, some plants had to be pulled throughout
the year. Therefore, this control method could only be practically
applied on a small scale.
For years many ranchers in west Texas tried to kill the weed by
concentrated grazing with sheep on the plant while it was young
(Green, 1971). This did not work because as soon as early symptoms
of poisoning occurred, the sheep had to be removed from the pasture.
Sick sheep were then placed in areas free of bitterweed with good
10
forage to regain their health. It was often necessary to place the
sick animals in feed lots which was an expensive practice.
Mechanical mowing is a recommended control practice for many
pasture weeds (Klingman, 1961). However, it is ineffective on plants
with leaves close to the ground; thus, short bitterweed plants would
still be able to grow and produce seed. Moreover, rough terrain and
brush infestation on rangelands in west Texas often make access with
a power mower impossible.
Biological control with insects gave some success but no insect
has given a large enough percent kill to be effective. Jones et al.
(1932) found 10 species of insects feeding on bitterweed. Of these
insects a species of weevil did the most damage but it only destroyed
about 20% of the seed heads.
Based on research from 1949-1958, Sperry and Sultemeier (1965)
stated that the most effective control of bitterweed is with herbicidal
control using an ester formulation of 2,4-D applied during the spring
or rapid growth stage. Sperry (1959) reported kills of up to 100%
from applications of one pound of 2,4-D applied per acre. However,
this time of application allows the weed to utilize winter moisture
as well as being a poisonous plant hazard during it's most dangerous
period of the year.
In a recent pilot study by Wright (1970) control of bitterweed
was attempted by burning and spraying with 2,4-D. Burning was not
successful in a complete kill of bitterweed. The 2,4-D amine salt
formulation applied in January and February resulted in a 99% kill
n of bitterweed four months after application of the treatments. Based
on these results it appears that bitterweed can be sprayed before the
plants have germinated or when they are very young. This would solve
the problem of removing the bitterweed before it becomes a poisonous
plant problem or uses winter moisture.
Herbicides
One positive attribute of the herbicide 2,4-D is that it is one
of the oldest and most tested chemicals on the market today. Hoffman,
Fisher, and Haas (1970) have shown that 2,4-D will give effective
control of most herbaceous weeds found on rangelands of Texas.
There are essentially two formulations of 2,4-D, the amine salt
and the ester. Esters of 2,4-D are generally considered more toxic
to plants than the amine salts (Crafts and Robbins, 1962). This
greater toxicity is probably due to their compatability with the
cuticle and leaf waxes which the esters can penetrate more readily.
According to Wright (1970), the amine salts of 2,4-D appear to have
a greater pre-emergent effect on bitterweed than esters. This is
probably true because of the greater persistence of the amine salts
in plants and soil (Muzik, 1970).
Crafts and Robbins (1962) conducted research which showed that
2,4-D is readily leached from the soil. This indicates that there
is little danger of accumulation in the soil in toxic quantities.
However, proper attention must be paid to the rate, season of appli
cation, and to subsequent management of the treated areas.
Another important factor in choosing a weed control practice is
12
the cost of treatment. Helena Chemical-Southwest price listings
(Appendix A) show 2,4-D to be the cheapest chemical on the market
that could possibly control bitterweed.
Dicamba, another chemical, is a chlorobenzoic acid with herb
icidal actions similar to those of 2,4-D (Muzik, 1970). It is a
substituted benzoic acid which can be formulated, like 2,4-D acids,
into esters or amine salts. Dicamba is applied as an aqueous solu
tion of an amine salt. Fryer and Evans (1968) state that for general
weed control, it is often mixed with 2,4-D or other growth regulator
herbicides in order to broaden the spectrum of weed control. Dicamba
is very persistent in the soil which gives it a pre-emergent effect.
Atrazine may also have the potential to control bitterweed. It
affects photosynthesis following uptake by the root system (Fryer and
Evans, 1968). Atrazine may also enter a plant through the foliage.
Residues persist in the soil for considerable periods which is an
asset in long term weed control. In comparison to the other members
of the triazine group, atrazine is the most independent of soil
moisture for effective weed control. For this reason, it could be
used in fairly dry growing conditions. However, the manual on Weed Con
trol (1968) of the National Academy of Sciences mentions that herbicides
active in the soil, such as atrazine, may be toxic to seedlings of
perennial grasses.
2,4-D Effects on Plants
The 2,4-D compounds are unique weed killers. They have several
herbicidal actions. They may have caustic effects on leaves or if
13
applied to the soil, 2,4-D may be absorbed by the roots of weeds and
cause their death. Work by Anderson and Wolf (1947) and Anderson and
Ahlgren (1947) showed that 2,4-D is effective as a pre-emergent treat
ment applied through the soil. This obviously indicates that 2,4-D
is absorbed by roots as well as by foliage. Another attribute of 2,4-D
is that two or more of these actions may be effective at the same time
(Crafts and Robbins, 1962).
Muzik (1970) reported that the increased brittleness in plants
following 2,4-D treatment was related to increased turgor pressure
in the cells. The twisting and curvature of the stems are due to dif
ferential turgidity and unequal rates of cell division and cell en
largement which is typical of auxin effects.
According to Crafts and Robbins (1962), 2,4-D moves within the
plant to the buds and young growing regions, resulting in a very
efficient control, even though the chemical is not thoroughly dis
tributed over the entire leaf surface of the weed. This means that
when 2,4-D is applied to leaves of plants that are actively growing,
it will be accumulated in meristematic regions where the chemical is
known to have its most drastic toxic action (Van Overbeek, 1947).
Both crop plants and weeds vary widely in their susceptibility
to 2,4-D sprays (Crafts and Robbins, 1962). In general, members of
the Gramineae family are resistant, while most broad-leaved plants are
susceptible. Grasses, however, are subject to 2,4-D toxicity and are
generally most susceptible at germination and gain tolerance as they
grow older (Muzik, 1970).
14
Application of 2,4-D to plants may cause several distinct re
sponses. First, it causes a twisting or bending of the stems and
leaves. This results from differential growth rates in petioles and
elongating regions of the stem. Second, it causes a thickening of
leaves and sometimes stems, accompanied by increase in turgor. Third,
and most important, there is a cessation of growth. This is followed
by characteristic browning and drying of stems and leaves, often
followed by decay of roots in the soil (Crafts and Robbins, 1962).
CHAPTER III
METHODS AND PROCEDURES
Description of Area
This study was conducted in Irion county on the Hal Noelke ranch
which is located 45 miles west of San Angelo and 11 miles east of
Barnhart. The vegetation on this ranch is typical of that found on
millions of acres of rangeland throughout the Edwards Plateau of Texas
(Thomas and Young, 1954). Perennial grasses consist of tobosa grass
(Hi 1 aria mutica), red grama (Bouteloua trifida), threeawns (Aristida
sp.), buffalograss (Buchloe dactyloides), and many other minor grasses.
Some of the problem plants for ranchers of the area include mesquite
(Prosopis glandulosa), bitterweed (Hymenoxys odorata), sachauista
(Nolina texana), and prickly pear (Opuntia lindheimeri).
The Noelke ranch has had a past history of heavy grazing by cat
tle, cheep, goats, and deer. Presently, the overall range condition
of the ranch is fair. In 1969 a four pasture deferred rotation system
was put into effect which has greatly improved the vigor and production
of the range.
Field Methods
The pasture in which the study plots were located was part of the
four pasture deferred rotation system. It was rested from January 15
through May 15. This minimized variation in the data due to livestock
influences.
The soils on this ranch are clay types or phases of clay types,
15
16
moderately deep, underlain with limestone or caliche. Topography of
the area is gently rolling. Although the general surface appearance
of these soils are similar, there are differences in surface relief
and moisture relationships. The plots in this study are located on
a bottomland site composed of the Nuvalde clay loam soil series.
According to Thomas and Hildreth (1957), the average annual rain
fall of San Angelo (east of the ranch) is 19.6 inches. The U. S.
Climatological Data report (1969) shows that the rainfall for Big
Lake (west of the ranch) is 18.7 inches.
Temperature, wind, and relative humidity v/ere recorded at the
time of applying the treatments (Appendix B). Six soil moisture
samples were also taken on each treatment date in the interval of 0-6
inches (Appendix C). There was no rainfall during the study from
December 20, 1970 to May 20, 1971.
A randomized block design with 4 blocks, 7 treatments, and 4
dates of application were used in this study. Dicamba was not applied
on December 20. Therefore, a total of 108 plots, 25 ft X 25 ft were
staked out in a typically infested bitterweed area on the ranch
(Appendix E). A 1 ft buffer zone was established around the outside
of each plot to help prevent overlap of chemical treatments.
On December 10, 1970, cover of perennial plants was estimated on
all plots (Appendix D). This was done using cover estimates of twenty
1 ft^ frames in each plot. Based on cover, the vegetation was grouped
into four blocks; 1) greater than 20% canopy cover of red grama and
threeawns (high), 2) 10% to 20% cover of red grama and threeawns
(medium), 3) less than 10% cover of red grama and threeawns (low).
17 and 4) a predominately tobosa grass community.
Six of the seven treatments were herbicidal sprays. The seventh
treatment was a control. Four rates of 2,4-D were applied; 1 1/4
lb/acre, 2 1/2 lb/acre, 3 3/4 lb/acre, and 5 lb/acre. Dicamba and
atrazine were applied at the recommended rates of 1/2 lb/acre of
dicamba and 1 1/2 lb/acre of atrazine.
The spray mixtures consisted of equal portions of chemical and
diesel in 3 gal of water. The diesel was added to the herbicides to
act as a carrier and dispersing agent. The herbicidal sprays were
applied with a specially built spray rig (Fig. 3). The spray rig is
described in appendix F.
One application of each of the seven treatments was applied in
each of the 4 blocks. With the exception of December 20, a total of
28 plots were sprayed on each date of application. The spray dates
were December 20, January 10, January 30, and February 20. Thus,
treatments from early winter through late winter were studied.
On May 20, 1971, twenty 2.4 ft^ frames were clipped on each plot.
The weight data was all converted from green weight to dry weight by
taking a 100 gram moisture sample of each major species, oven drying
it, and then calculating a percent dry weight. The weight data was
used to compare the production among treatments.
Due to lack of winter rainfall in 1971, there was a sparse annual
bitterweed crop. Control plots averaged 120 lb/acre of bitterweed.
Under average rainfall conditions, based on previous years of study,
five to ten times this amount could have been expected. This low
rainfall also resulted in low yields of other vegetation in this study.
18
Fig. 3. Herbicidal treatments were applied with the above spray rig in this study.
19
The weight data was plotted and found to fit a Poisson distribution
Therefore, it was necessary to normalize the data before it could be
analyzed using the standard analysis of variance techniques. The data
was transformed by the square root of X + 1. A split plot factorial
was used because the dates of treatments could not be randomized.
Duncan's multiple range test was used to compare means.
CHAPTER IV
RESULTS
Bitterweed was controlled to some extent using all the herbicides
in this study (Table 1). The 3 3/4 lb/acre and 5 lb/acre 2,4-D treat
ments and the atrazine treatment were especially effective in con
trolling bitterweed. Over all seasons they reduced the yield of
bitterweed 88%, The 1 1/4 lb/acre and 2 1/2 lb/acre 2,4-D treatments
and the dicamba treatment were less effective in control of bitter
weed. However, on December 20 the 2 1/2 lb/acre treatment of 2,4-D
was as effective as the higher rates of 2,4-D.
Time of application also had a marked effect on the response of
bitterweed to 2,4-D and atrazine treatments. The earlier in the
season that 2,4-D was applied, the more effective it was in reducing
yields of bitterweed. By contrast, the earlier that atrazine was
applied, the less effective the treatment.
The December 20 and January 10 applications of 2,4-D were similar
and differed significantly from the January 30 and February 20 dates.
The 2,4-D treatments on the first two dates yielded better kills of
bitterweed than the last two dates. This substantiates the fact that
early winter 2,4-D treatments are more effective than late winter
treatments for bitterweed control.
Grass yields (Table 2) were decreased only by dicamba and the
January 30 atrazine treatment. However, over all seasons the effect
20
21
Table 1. Yield of bitterweed (lb/acre) in relation to season and treatment on May 20, 1971.
Dates
Dec. 20
Jan. 10
Jan. 30
Feb. 20
Ave.
1 1/4
25
28
78
137
67ad
2,4-D treatments 2 1/2
6
52
145
77
70ad
3 3/4
6
6
7
36
14be
5
0
6
9
48
16be
Ave.
9f
23f
60g
75g
i
Ati razine
52
19
2
1
19b
Dicamba
57
76
69
67a
Control
146
139
125
109
130c
'Means within a row (column) followed by the same letter are not significantly (P<.05) different.
22
Table 2. Yield of grasses (lb/acre) in relation to season and treatment on May 20, 1971.
2,4-D treatments Dates 1 1/4 2 1/2 3 3/4 5 Ave. Atrazine Dicamba Control
Dec. 20 315 342 359
Jan. 10 313 387 229
Jan. 30 293 270 264
Feb. 20 282 150 267
Ave. 301a 287ab 280ab 271ab
222
352
268
242
271ab
310
320
274
235
270
308
932
236
226b
124^
282
197
201c
271
288
270
221
262ab
'Means within a row followed by the same letter are not significantly (P<.05) different.
^Means differ significantly (P<.05) from other treatments within the same row.
23
of atrazine on grass production was not significant; production was
slightly lower than on control plots. Grass yields were not reduced
by any of the 2,4-D treatments.
Desirable forbs (Table 3) were greatly reduced by the atrazine
and dicamba treatments on all dates of application. The 5 lb/acre
2,4-D treatment also reduced forb yields, but to a lesser extent than
the atrazine and dicamba treatments. Lower rates of 2,4-D, 1 1/4 lb/
acre, 2 1/2 lb/acre, and 3 3/4 lb/acre, did not reduce yields of forbs.
Desirable forb and grass data were combined in Table 4 to show
the total yield of desirable vegetation. Atrazine and dicamba treat
ments reduced total yields. Total yields on all 2,4-D treatments were
similar to control areas. Therefore, 2,4-D did not reduce the total
yield of desirable vegetation.
24
Table 3. Yield of desirable forbs (lb/acre) in relation to season and treatment on May 20, 1971.
2,4-D treatments Dates 1 1/4 2 1/2 3 3/4 5 Ave. Atrazine Dicamba Control
Dec. 20 30 46 22 9 27
Jan. 10 36 75 71 28 52
Jan. 30 72 49 20 22 41
Feb. 20 46 45 54 72 54
Ave. 46a 54a 42a 33b
^Means within a row followed by the same letter are not significantly (P<.05) different.
45
3
.5
1
13c
5
25
14
15c
64
28
78
58
57a
25
Table 4. Yield of desirable forbs and grasses (lb/acre) in relation to season and treatment on May 20, 1971.
Dates 1
Dec. 20
Jan. 10
Jan. 30
Feb. 20
Ave.
/ •
1/4
345
348
364
329
347a
!,4-D treatments 2 1/2
388
439
319
195
341a
3 3/4
380
300
284
320
321a
5 Av
230
379
289
314
301a
e.
336
366
314
290
Atrazine
315
310
912
237
238b
Dicamba
1292
306
211
215b
Control
335
316
348
378
319a
^Means within a row followed by the same letter are not significantly (P4:.05) different.
2Means differ significantly (P<.05) from other treatments within the same row.
CHAPTER V
DISCUSSION
Based on this study 2,4-D is the best herbicide for control of
bitterweed. Atrazine and dicamba can be ruled out because they reduce
the yield of desirable forbs. Also, dicamba reduced the production
of desirable grasses. The effectiveness of 2,4-D treatments in
killing bitterweed is illustrated in Fig. 4.
The earlier that 2,4-D is applied, the lower the rate needed
for excellent control. The 2 1/2 lb/acre rate on December 20 was
more effective than any other 2,4-D treatment applied on February 20.
From the standpoint of economics, early spraying (December 20) of
2,4-D at the rate of 2 1/2 lb/acre appears to be the best treatment.
The surest and most effective kills of bitterweed will be ob
tained using a 3 3/4 lb/acre or a 5 lb/acre rate of 2,4-D. These
two rates produced excellent results when sprayed on December 20,
January 10, or January 30. The 5 lb/acre rate is not as desirable
as the 3 3/4 lb/acre rate because of reduced forb yields.
The analysis of the dates of application of 2,4-D clearly
indicate one point. All 2,4-D treatments were most efficient in kill
ing bitterweed when applied early in the winter on December 20 or
January 10. It appears that the date 2,4-D is applied is more im
portant than the rate used.
26
27
150--
140"
^ 2,4-D treatments in lb/acre
70-.
60-
s-o CO
^ 50.
OJ CD
^40
E a 2 1/2 CZ] 3 3/4 E ^ 5
CO
30-.
20 -
10-.
0
%
^
0 •I / r,
I
1 I
',
/
/
;̂ /
i i /
12/20/70 1/10/71 1/30/71 2/20/71 Date of Application
Fig. 4. Effects of 2,4-D treatments on bitterweed production.
28
None of the chemical treatments, except dicamba, reduced grass
yields significantly. However, atrazine, dicamba, and the 5 lb/acre
rate of 2,4-D reduced yields of desirable forbs. The lower rates of
2,4-D, 1 1/4 to 3 3/4 lb/acre, did not lower production of desirable
forbs. Thus, it appears that for maximum growth of desirable grasses
and forbs, and for maximum damage to bitterweed, the 2 1/2 lb/acre
rate of 2,4-D applied during December or the 3 3/4 lb/acre 2,4-D treat
ment applied any time before February 1 are the optimum treatments.
Bitterweed is an annual which establishes itself during the winter
when there is no competition from perennial plants. As a result of
this attribute, most bitterweed areas are usually very sparsely
populated with other growing species of vegetation. A stand of bitter
weed already occupying an area greatly inhibits the spring growth of
perennial plants. Slow spring growth may greatly reduce production
of perennial plants in hot dry summers. This is probably the reason
that perennials have great difficulty in regaining dominance in bitter
weed infested areas, and also why bitterweed areas remain as a poi
sonous plant problem to sheep on many ranches today.
When the influence of bitterweed is removed (Fig. 5) other plants
will enter the area and make efficient use of the newly available
nutrients, and moisture for growth. In a good growing season, more
desirable vegetation will make rapid progress at recovering in the
newly opened areas.
In a year without winter moisture, such as the 1970-71 winter,
growth of a late bitterweed crop will be inhibited by perennial
29
OJ
-M
C M-• 1 —
C ^ o .c to
to • 1 —
Q 1
O
• E
• 1 —
cr (U X2 U E CU 13
Cfl c
• 1 —
(L) "sf ^
w% CM
«4-
•»->
.4-> CO
O JC
+-> C <u E
-M n3 <U
4->
E fd o to lO <u o o s-Q-
E O
• 1 —
-f-> «3
S +-> O
2 :
• S - ^
•t-i
(U S-o CO
• s - s ^
O -fO
<U cn <U > <U S-
OJ S- XT Ol-l-> o
+ J «% o -a JO J C
r—
*d-• ^ s ^
CO
CO
OJ
<v O- >
O) > o CO
<U
o E <u s-E <u CD
x : - c X I -M .4->
4 - ^ -o (U
r—•
to CO
o x : 4-) - O E
CL CD E S-CO H -X OJ
c •a:
• LO
• CD
OJ <u :5 S-cu
4-> -l-> Cf-<U
*"
.4-> • r -X5
30
vegetation. Control plots in this study with greater than 20% cover
of grass only produced an average of 33 lb/acre of bitterweed compared
to 252 lb/acre on areas with less than 10% cover. Bitterweed is not
able to germinate and grow on an equally competitive basis with
perennial grasses and forbs (Fig. 6).
It is not likely that bitterweed can ever be removed from areas
by spraying alone. So many seeds have accumulated in the soil over
the years, that stopping seed production for one year may have little
effect on the size of the following year's bitterweed crop. At this
time, however, we do not know the long-term effect of the chemicals
used in this study on bitterweed.
The feasibility of herbicidal control of bitterweed is based
upon the idea of removing bitterweed competition so that more
desirable plant species are allowed to gain dominance. Therefore,
proper grazing management is essential following spray treatments
for long term pasture improvements.
31
Wt to (/)
-a CO <u $-<U CD
JZ S -
+-> E :5
•r— •!— QJ X 3 4-> >
E -M -f-> ro <U ' r -
0 - - M E <U O O
JO
i -
x: -o -M OJ o cu
CL E o o
E s- x : O <U CD
-»-> - M x : (O 'r— +-> X) <u (U X) CD <U o; x ; I— > -i-J •—
o <u > o E CD >
o o
+-> S- E ro CU
•.- E I— +J O ro
Lf- CD S-
CD CM >s,
ro - o S-ro Q-
x : to
ro Q Qi I %- <i-ro *>
CM O) > ro O
X I CD ro i-
O CU ci -
x : Qi y— -Q
-a E ro
• o CU c/) ro CU S -o E •
• r - Qi s-
>. r3 »— - P
ro M-CU s- cu C D x :
cn ro E
E - a O CU
•I— Qi ••-> 5 o s-rs CU
- o •<-> O -!-> S_ - r -Q-XJ
LO
CD •I— Li-
CHAPTER VI
SUMMARY AND CONCLUSIONS
Three herbicides, atrazine, dicamba, and 2,4-D, were tested in
this study for possible control of bitterweed. All herbicides used
gave good control of bitterweed. However, atrazine and dicamba can
not be considered as good control agents due to harmful effects on
desirable forbs. Dicamba was also detrimental to grass production.
Among the 2,4-D treatments, the 3 3/4 lb/acre rate was the most desir
able if applied between December 20 and January 30. The 2 1/2 lb/
acre rate of 2,4-D was also an effective treatment when applied on
December 20.
Early winter application of 2,4-D will provide three primary
benefits. First, it removes the threat of bitterweed poisoning to
livestock by killing the weed as soon as it emerges. Second, winter
moisture can be stored in the soil. Third, competition is removed
allowing earlier spring growth and more rapid invasion of grass into
the newly opened areas. Following application of treatments, it is
best to rest the pastures during the spring to allow maximum growing
conditions for natural recovery of range vegetation.
32
LITERATURE CITED
Anderson, J. C. and G. Ahlgren. 1947. Growing corn without cultivating. Down to Earth. 3(1):16.
Anderson, J. C. and D. E. Wolf. 1947. Pre-emergence control of weeds in corn with 2,4-D. Amer. Soc. Agron, J. 39:341-342.
Boughton, I. B. and W. T. Hardy. 1937. Toxicity of bitterweed (Actinea odorata) for sheep. Texas Agr. Expt. Sta. Bull. 552. 15 p.
Clawson, A. B. 1931. A preliminary report on the poisonous effects of bitter rubber weed (Actinea odorata) on sheep. J. Agr. Res.,
Cory, V. L. 1951. Increase of poison bitterweed (Hymenoxys odorata) on Texas rangelands. Field and Lab. 19:39-44.
Crafts, A. S. and W. W. Robbins. 1962. Weed control. McGraw-Hill Book Co., Inc., New York. 423 p.
Fryer, J. D. and S. A. Evans. 1968. Weed control handbook. Blackwell Sci. Publ., Oxford and Edinburgh. 494 p.
Green, B. K. 1971. The village horse doctor. Alfred A. Knopf, Inc., New York. 306 p.
Hardy, W. T., V. L. Cory, H. Schmidt, and W. H. Dameron. 1931. Bitterweed poisoning in sheep. Texas Agr. Bull. 433. 18 p.
Hoffman, G. 0., C. E. Fisher, and R. H. Haas. 1970. Weed control with chemicals. Texas Agr. Bull. 1029. 28 p.
Jones, S. E., W. H. Hill, and T. A. Bond. 1932. Control of the bitterweed plant poisonous to sheep in the Edwards Plateau region. Texas Agr. Bull. 464. 23 p.
Klingman, G. C. 1961. Weed control as a science. John Wiley & Sons, Inc., New York. 294 p.
Muzik, T. J. 1970. Weed biology and control. McGraw-Hill Book Co., New York. 322 p.
National Academy of Sciences, The Subcommittee on Weeds. 1968. Weed control. Vol II. Washington, D. C. Publ. 1597. 471 p.
33
34
Schmutz, E. M., B. N. Freeman, and R. E. Reed. 1968. Livestock poisoning plants of Arizona. The Univ. of Ariz. Press, Ariz. 176 p.
Sperry, 0. E. 1949. The control of bitterweed (Actinea odorata) on Texas ranges. J. Range Manage. 2(3):122-127.
Sperry, 0. E. 1950. The effect of 2,4-D on bitterweed seed formation and germination. Texas Agr. Expt. Sta. Progress Report 1279. 3 p.
Sperry, 0. E. 1959. Bitterweed control with air and ground equipment in 1958. Texas Agr. Aviation Conf. 1 p.
Sperry, 0. E., J. W. Dollahite, J. Morrow, and G. 0. Hoffman. 1955. Texas range plants poisonous to livestock. Texas Agr. Bull. 796. 21-24.
Sperry, 0. E. and G. W. Sultemeier. 1965. Bitterweed...its control in relation to soil moisture. Sheep and Goat Raiser. 45(3):14-17.
Sultemeier, G. W. 1961. Responses of bitterweed (Hymenoxys odorata) to 2,4-D in relation to soil moisture. M. S. Thesis. Texas A&M Univ., College Station, Texas. 68 p.
Thomas, G. W. and R. J. Hildreth. 1957. Farming and ranching risk as influenced by rainfall. Texas Agr. Bull. MP-216. 34 p.
Thomas, G. W. and V. A. Young. 1954. Relationship of soils, rainfall and grazing management to vegetation western Edwards Plateau of Texas. Texas Agr. Bull. 786. 22 p.
U. S. Department of Commerce. Environmental Sci. Service Admin. 1969. Climatological data. Washington, D. C. 74(2):429 p.
Van Overbeek, J. 1947. Use of synthetic hormones as weed killers in tropical agriculture. Econ. Bot. 1(4):446-459.
Wright, H. A. 1970. Control of bitterweed. Noxious Brush and Weed Control Research Highlights-1970. ICASALS Special Report No. 40: 18-19. Texas Tech Univ., Lubbock, Texas.
APPENDIX
A. Costs of herbicides.
B. Weather conditions at time of application of herbicides
C. Soil moisture at time of application of herbicides.
D. Foliar cover taken on December 10, 1970.
E. Map of plot locations.
F. Description of spray rig.
35
36
APPENDIX A: COSTS OF HERBICIDES
Herbicide Cost/gal Cost/lb active ingredient Cost of Rates/acre
2,4-D $ 2.75 $ .70 1 1/4 Ib/ac $ .98 4 lb acid equivilent/ 2 1/2 lb ac $1.75 gal
3 3/4 Ib/ac $2.63
5 Ib/ac $3.50 Atrazine $ 8.80 $2.45 1 1/2 Ib/ac $3.65 4 lb acid equivilent/ gal
Dicamba $27.50 $7.00 1/2 Ib/ac $3.50 4 lb acid equivilent/ gal
37
APPENDIX B: WEATHER CONDITIONS AT TIME OF APPLICATION OF HERBICIDES
Dates Sprayed
December 20
January 10
January 30
February 20
Relat ive Humidi
70%
17%
30%
28%
ity Temperature
440 F
700 F
70° F
680 F
Wind
0 mph
0 mph
0 mph
0 mph
38
APPENDIX C: SOIL MOISTURE AT TIME OF APPLICATION OF HERBICIDES
Dates Site I Site II Site III Average
December 20 *No samples taken on this date
January 10 4.95% 3.75% 3.64%
4.06% 2.97% 5.71% 4.18%
January 30 4.78::; 3.75% 5.57%
5.45% 3.60% 5.15% 4.71%
February 20 6.34% 4.56% 5.24%
5.88% 4.81% 5.67% 5.40%
39
APPENDIX D: FOLIAR COVER OF GRASSES TAKEN ON DECEMBER 10, 1970.
Tobosa Low Medium High Plot % Cover Plot % Cover Plot % Cover Plot % Cover
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
8.1
17.5
25.0
19.5
14.8
9.5
12.8
6.7
10.3
8.2
9.1
19.4
9.0
7.7
9.5
8.4
17.8
49.8
20.6
8.0
7.8
35
36
108
109
110
46
47
52
70
61
62
63
64
65
66
67
68
69
45
72
76
6.8
3.8
4.3
2.6
2.3
5.2
5.0
7.7
9.3
5.1
3.6
1.9
4.2
3.1
5.5
9.0
1.9
1.9
8.6
7.6
3.2
32
42
43
51
54
55
56
57
58
59
60
71
73
82
83
84
80
53
93
94
95
13.8
15.1
17.3
11.0
15.0
12.5
10.4
15.4
13.0
10.7
12.3
12.3
15.9
11.8
17.3
16.3
10.3
10.2
14.6
14.5
12.4
31
33
34
37
38
39
40
41
44
48
49
50
74
75
85
86
87
88
89
112
91
22.0
30.6
28.0
21.3
26.9
31.3
21.3
24.3
25.8
20.0
27.0
20.2
21.4
24.7
21.4
22.8
41.3
36.8
40.1
33.8
37.3
40
APPENDIX D: CONTINUED
Tobosa Low Medium High Plot % Cover Plot % Cover Plot % Cover Plot % Cover
22
23
24
25
26
27
28
29
30
11.1
10.9
8.9
6.1
9.4
10.0
10.4
18.3
14.5
77
78
79
81
92
102
103
7.1
7.1
8.2
5.1
8.5
5.2
8.0
96
100
101
104
90
111
18.8
17.5
11.0
10.4
19.0
16.4
97
98
99
105
106
107
19.9
27.9
28.6
22.4
22.5
23.5
41
APPENDIX E: MAP OF PLOT LOCATIONS
San Angelo
State Highway 137
U. S. Highway 277
Eldorado
Ozona Sonora
APPENDIX E: CONTINUED
42
Headquarters
SITE I
4 3
2
J
5
6
7
8
12
11
10
9
Ranch
Road
SITE II
APPENDIX E: CONTINUED
43
3: 3̂
31
32
3 r
to H 11
38
^9
41
40
42
43 mJi 1D8
SITE II
1C9
SITE III 79 80
APPENDIX E: CONTINUED
44
nch Road
45
APPENDIX F: DESCRIPTION OF SPRAY RIG
A spray rig was built with a boom 80 inches long with nozzels
spaced at 20-inch intervals. The nozzels used were Delavan FS4's
with 100 mesh screens. The boom is mounted on a cart with bicycle
wheels. Four 1 gal stainless steel cans were mounted on the spray
rig. A small style 6 nitrogen bottle and single stage regulator
were mounted in the center of the spray rig. The nitrogen was used
to keep steady pressure in the spray cans. This assures an even
and accurate discharge of chemical at all times. Nitrogen was used
because it was considered not to react with the chemicals used in
the study as some other gases might have.
