Bitterweed is a major problem of sheep throughout the Southwest. Cattle are rarely poisoned. Bitterweed is a member of the sunflower family and is closely related to pingue both in appearance and in its effects on sheep. One of the major differences is that pingue is a perennial, whereas bitterweed is an annual. It is generally a problem only when there is a shortage of other green forage.
Toxicity increases with maturity or when the plant is drought stressed.
Though bitterweed can be found through much of the southwest, it causes livestock problems principally in western Texas and eastern New Mexico. Livestock are generally poisoned in the winter and early spring when animals may graze the plant because of a shortage of other forage. Maximum plant growth occurs in early spring and early summer, but seedlings may be found at any time of the year, depending on moisture and temperature conditions. As with many other poisonous plants, the tolerance of animals to bitterweed depends on many factors. Acute cases may result in death if large amounts are eaten rapidly.