ABSTRACT
Confusion concerning the number of species is understandable, considering the relatively low degree of distinctness separating some and the tendency of some Senecio species to freely hybridize. Although toxic Senecios are poisonous to most animals and to man, they derive their greatest economic importance from the losses they cause to the cattle industry. Cattle that eat toxic Senecios may be affected either acutely or chronically. Most Senecio intoxications that cause economic loss are chronic and may take many months to develop. Senecios may directly affect the endothelium of certain liver blood vessels causing eventual vessel blockage and disintegration. The economic impact of toxic Senecios in range and pasture lands is difficult to accurately assess. In some regions, threadleaf groundsel plants produce a flush of new vegetative growth during any season following significant precipitation. Acute poisoning on threadleaf groundsel commonly occurs when cattle are crowded into pastures where the subshrub is abundant and availability of desirable forages is limited.
