Landers, Jr., R. Q.

Ueckert, Darrell

English

Texas A&M University

Texas Agricultural Extension Service
..........-.----...~- P~,P& H~Utt/ P~,P&------------'
MANAGING BITTERWEED TO REDUCE
SHEEP LOSSES
R. Q. Landers, Jr. and Darrell N. Ueckert*
Bitterweed poisoning of sheep remains a serious'
problem in some areas of Texas (illustrated by
shaded area in figure 1). Herbicide use and livestock
management can reduce animal losses.
Figure I. Bittenoeed (Hymenoxys odorata) problem area in Texas.
Bitterweed
An annual, cool-season plant of the sunflower
(composite) family, bitterweed (Hymenoxys odorata
DC) grows in dense stands on the Edwards Plateau
and Trans-Pecos areas of Texas. Individual plants
are tightly rounded clusters ranging in height from a
few inches to 2 feet (figure 2). Numerous bright
yellow ray and disk flowers appear from early spring
to summer. Dozens, often hundreds of flower heads,
each producing 50 to 75 seeds, may grow on each
plant. Plants germinate in late fall and winter,
especially during wetter-than-average years on
limestone-derived soils and on mechanically
disturbed or overgrazed areas. Leaves are finely
divided. The plant tastes bitter and has a strong
aromatic odor when crushed.
* Extension range specialist and professor, Texas Agricultural
Experiment Station, respectively, The Texas A&M University
System.
36.9
Figure 2. Mature plant of bitterweed in early summer showing
finely divided leaves and flower heads with toothed rays.
Signs of Poisoning
Sheep with acute or chronic poisoning stop
eating, salivate, vomit, become depressed and weak
and lie down most of the time. A green salivary
discharge and stain around the muzzle is common.
Affected animals often lag behind the flock and may
stand with backs arched.
Ingestion of fresh bitterweed equivalent to
approximately 1.3 percent of an animal's weight was
lethal in controlled feeding studies; but, only one-
half this amount is required when bitterweed
develops under drought conditions. Recent field
studies suggest that sheep grazing under natural
conditions can tolerate much more than 1.3 percent
of their body weight of bitterweed. If removed from a
source of bitterweed at the onset of poisoning, sheep
may recover in 7 to 10 days. Cattle and goats also are
susceptible but rarely are actually poisoned. The
toxic principal is a sesquiterpene lactone called
hymenoxon.
Texas Agricultural Extension Service. The Texas A&M University System • College Station, Texas
Control by Management
The following list includes management techni-
ques to use in controlling bitterweed poisoning.
• Rotation and proper grazing levels
• Flash grazing of bitterweed at critical times
• Keeping animals out of bitterweed problem
areas
• Drylot feeding of animals during bitterweed
season
• Combinations of livestock
• Supplemental feeding
Bitterweed poisoning can be reduced or stopped
by managing grazing through rotation and proper
stocking. Pasture deferment, for example, increases
vigor of grasses and forbs that compete with
bitterweed.
Flash grazing by sheep, that is, turning many
heep into a pa ture with bitterweed for a few days
and removing them before poisoning, may reduce
bitterweed vigor and allow desirable plants to
predominate. Several ranchers have avoided toxicity
by shifting sheep from bitterweed-infested pastures
to bitterweed-free pastures in 7-day cycles.
Poisoned sheep u ually recover after 10 days on
alfalfa hay. Drylot feeding with alfalfa or peanut hay
may be a temporary alternative when bitterweed-free
pastures are not available.
Combining cattle, sheep and goats at moderate
stocking rates greatly reduces and may eliminate
losses compared with sheep alone on heavily stocked
pastures. A four-pasture deferred rotation system
grazed moderately with cattle, sheep and goats was
e tablished by the Texas A&M University Agricul-
tural Experiment Station at Sonora in 1949. Despite
heavy stands of bitterweed, no sheep have died from
poi oning on these rotation pastures for more than
30 years, whereas annual losses on pastures heavily
grazed with sheep alone have averaged 8 percent.
There were no 10 es on lightly grazed pastures
sto ked with cattle, sheep and goats. Similar systems
and stocking at the Texas Range Station at Barnhart
have reduced, but not eliminated, death losses from
bitterweed.
Ranchers have consistently observed that feeding
upplemental protein aggravates bitterweed pro-
blems. This observation is supported by the fact that
high protein diets are detrimental in situations
where there is impairment of kidney and liver
functions. which occurs in bitterweed poisoning.
The problem would be even more severe if a portion
of the protein in the supplement were derived from a
nonprotein nitrogen source, such as urea. Because of
this, use supplemental feed providing energy and
moderate levels of natural protein before the time
sheep start eating bitterweed. Definitely do not give
supplemental feeds containing urea to heep
exposed to bitterweed, and high protein supple-
ments should probably not be fed to sheep already
poisoned on bitterweed.
Control with Herbicides
To control bitterweed with herbicides, con ider
the three items listed below.
• Spray before flowering
• Spray before dense stands are established
• Consider temperature and soil moisture
Bitterweed control with herbicide sprays depend
on air and oil temperatures, soil moisture, tage of
plant growth and herbicide type. Control before
flowering has been succe ful with the ester or amine
formulation of 2,4-D at rates of 1 pound acid
equivalent per acre when temperature and oil
moisture favor bitterweed growth. For thi
traditional method air temperatures should be above
72° F. with 20 to 25 percent soil moisture in clay
soils. When air temperatures are less than 60° F. or
when three-fourths of the plants are blooming,
control bitterweed with 2,4-D plus dicamba (3:1),
picloram, 2,4-D plus picloram (4:1) or 2,4,5-T plus
picloram. Forbs important in livestock diets,
including tallow weed, filaree, nuttall milkvetch
and croton, are also controlled or suppressed by th.e
herbicides.
Integrated Control
Use herbicides to prevent localized infestation of
bitterweed from spreading and to maintain
bitterweed-free pastures where animals can be h ld
during critical bitterweed periods.
Manually clearing small patches and i olated
plants is useful, although expensive on a large cale.
Removing plants may be necessary for many year
before the viable seed reserve in the soil is depleted.
Avoid disturbing soil with machinery or exce sive
hoof action of grazing animals. Avoid spreading
seeds into bitterweed-free areas on tires and
undercarriages of vehicles or equipment.
Because bitterweed grows from seed during cool,
moist periods, it does not compete succes fully
within good stands of vigorous perennial plants
under proper grazing management. Long-term
control depends on the vigor of desirable range
grasses and forbs encouraged through deferment and
proper stocking rates with combinations of
livestock.
Acknowledgment
The author greatly acknowledge the assistance of C.]. cifres, Leo B. Merrill and M.C. Calhoun,
profe or, retired professor and associate professor, Texas Agricultural Experiment Station, CoIl ge
Station, Sonora and San Angelo, respectively, The Texas A&M University System.
educational programs conducted by the Texas Agricultural Extension Service senle people of all ages regardless of socioeconomic level.
race, color, sex, religion, handicap or national origin.
Issued in funherance of Cooperalive EXlension Work in Agricullure and Home Economics, AClS of Congress of May 8, 1914, as amended,
and June 30, 1914, in ooperalion wilh lhe Uniled Slale Depanmenl of Agricullure. Zerle L. Carpenter, Direclor:, Texas Agricullural
Exlen ion ervice, The Texas A&M UniversilY Syslem.
2M-4-85, Revision R 3-3


Managing Bitterweed to Reduce Sheep Losses.

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Managing Bitterweed to Reduce Sheep Losses.

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