Sheep primarily but also cattle when forage availability is poor.
Hymenoxys odorata - Bitterweed
Several toxic sesquiterpene lactones - hymenoxon, hymenoxynin anongst others, are present in all parts of the plant. Highest concentrations are in the new plant growth.
Siliar toxic lactones are found in the sneezeweeds of the genus Helenium.
Comprising some 30 species, plants are either annuals or perennials with erect, branching stems up to 2 feet tall. Leaves are simple, alternate, primarily basal. Flowers are in heads with yellow ray flowers that are 3-lobed. The disc flowers are yellow. The seeds have hairs (pappus) that aid in in wind distribution.
The most important toxic species are:
Hymenoxys odorata - bitterweed
H. richardsonii - Colorado rubberweed, Pingue
Initially sheep are depressed, off-feed, and have decreased rumen activity with bloat. Teeth grinding and a 'hunched-up' appearance are indicative of abdominal pain. Some will show regurgitation of rumen contents that may appear as dried rumen conents around the lips and nostrils. At necropsy there is defuse congestion and hemorrhage of the abomasum and intestinal tract. Cattle eating large quantities of the rubber weed may die acutely, but if eaten in lesser quatity over a period of weeks, signs of poisoning are similar to those observed in sheep.
Muscle tremors progressing to weakness and recumbency
Remove sheep from the source of the bitterweed andd administer supportive treatment including oral water and electrolytes. Some evidence suggest the beneficial effects of methionine in diets with soybean meal. Feeding onions that are high in cysteine may also have benefit especially if fed prior to or during the grazing of bitterweed.
Regurgitation of rumen contents can lead to inhalation pneumonia. In such cases there is coughing, rapid respirations and death often occurs from the inhalation pneumonia. Lung congestion, consolidation, and hemorrhages are present at necropsy.
Prevention and Contol
Bitterweeds can be effectively controlled with 2,4-D, picloram, clopyralid, or metsulfuron applied from fall to early spring before flowering when the plant is actively growing. Follow label directions for application rates and precautions for handling herbicides.