ABSTRACT
Bitterweed is a much-branched annual plant with a slender tap root. Mature plants vary in height from about 7 to 60 cm, depending on environmental conditions. Populations of bitterweed containing the ancestral number of chromosomes occur in a gypsiferous highland area in northcentral Mexico, and the species has apparently spread from this area. The area with heaviest infestations of bitterweed in western Texas is also the area with the greatest concentration of sheep and, consequently, the area where most sheep poisoning problems occur. The amount of bitterweed necessary to produce signs of poisoning varies greatly among individual sheep. Isolation of sesquiterpene lactones and lactone glycosides from Hymenoxys species lead to the suggestion that a sesquiterpene lactone might be the toxic principle in bitterweed. Bitterweed control with herbicide sprays depends on air and soil temperatures, soil moisture, plant phenology, and herbicide type. Bitterweed poisoning can be reduced or eliminated in some areas by managing grazing through rotation and proper stocking.
