Pastures often contain weeds that are potentially dangerous to livestock. The toxic compounds in plants are usually a defense mechanism against predation and have a distinct, unpleasant odor or a bitter taste and are not preferentially grazed. Consumption of unpalatable plants will increase under some circumstances, primarily if other forage is not available. Understanding the dangers and various management strategies to control toxic plants will reduce the risk to your livestock.
Many plants have characteristic that can cause injury to grazing animals. Some grasses can be palatable when young, but can cause injury to the nose, eyes, mouth and ears of grazing animals when plants mature with long awns.
Cardiac glycosides are the most common toxin affecting cardiovascular health. Generally all parts of the plant are highly toxic and lethal if eaten in small quantities. However, animals typically will not readily eat these unpalatable plants, unless no other forage is available.
This group of plants contain cyanogenic glycosides that are converted to hydrogen cyanide or prussic acid when the plant cells are damaged. Chronic poisoning over time causes loss of nerve function while acute poisoning causes death.
This group of plants contain pyrrolizidine alkaloids which are the most common cause of liver damage, but also can cause kidney damage, heart failure, cancer and photosensitization. Animals typically will not readily eat plants with pyrrolizidine alkaloids, unless no other forage is available.