Bitterweed is a yellow-flowered annual weed varying in height from a few inches to 2 feet. It is erect and branches from the base. It is a bitter-tasting plant found mainly in the semiarid rangelands of the Southwest.
Where and When It Grows Though bitterweed occurs from southern Kansas to Mexico and from central Texas to California, it is a problem of magnitude principally in Texas. The critical time for livestock is generally in winter and early spring when animals may graze the plant because of a shortage of other green forage. Maximum plant growth occurs in spring and early summer, but seedlings may be found at any time of the year, depending on moisture and temperature conditions.
Bitterweed poisoning is a major problem of sheep in the Southwest; cattle are occasionally poisoned. Bitterweed is a member of the sunflower family and is closely related to Colorado rubberweed (pingue) both in appearance and in its effects on sheep. Pingue is a perennial, wheras bitterweed is an annual. Bitterweed has an extremely bitter taste and is unpalatable to livestock; thus it is generally a problem only when there is a shortage of other green forage. Toxicity increases with plant maturity. The toxin is a sesquiterpene lactone.
Signs and Lesions of Poisoning
Loss of appetite and weight
Weakness and irregular gait
Vomiting and coughing (lips and muzzle are usually stained green with vomitus)
Wasting and eventual death
Head may be elevated
Congestion of liver, lungs, and abomasum
Inflammation and hemorrhage of abomasum and duodenum and sometimes the rest of the gastrointestinal (GI) tract; arched back is indicative of abdominal pain
Affected animals may lag behind the herd
Lambs may move with a stiff gait
How It Affects Livestock As with many other poisonous plants, the tolerance of animals to bitterweed depends on many factors. Acute poisonings may result in death if large amounts are eaten rapidly. Under normal conditions, this will not occur. Chronic poisoning is common, with sheep developing a wasting disease. Under natural grazing conditions, signs of poisoning occurs 2 to 4 weeks after the sheep start grazing the plant. The animal will recover if removed from access to the plant at the first signs of poisoning. Generally speaking, about 1 percent of the animal's body weight of consumed plant can be lethal.
Bitterweed is a powerful irritant to the digestive tract; thus, poisoned animals may lose their appetite, the rumen may stop functioning, and animals may show signs of pain. They lose weight rapidly and may lag behind the flock. Animals may not die until some time after removal from the plant. Post-mortem examinations of animals show lung, heart, and kidney changes and irritation of the GI tract.
How to Reduce Losses There is no treatment for bitterweed poisoning. Recent research has shown that sheep fed an activated charcoal/protein supplement were less affected by bitterweed than unsupplemented sheep. Animals not severely poisoned may be taken from infested pastures, and they may recover within a few days if given good feed and water. Ranchers with bitterweed-infested pastures should be especially aware of what their sheep are eating, particularly in winter and spring months, and should observe the flock frequently for early signs of poisoning.
Bitterweed poisoning has been prevented by moderate stocking in a four-pasture deferred rotation grazing system or light continuous stocking with sheep, cattle, and goats.
Season control can be obtained from 2,4-D (1 lb ae/acre), picloram (0.5 lb ai/acre), clopyralid (0.25 lb ai/acre), or metsulfuron (1.25 oz product/acre) applied from autumn to early spring before flowering when the plant is actively growing.