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.
Grazing management is a critical component to maintaining pastures free of poisonous weeds. Avoiding overgrazing will help maintain an abundance of desirable forage plants that are able to compete with weeds and reduce the risk of livestock being forced to eat poisonous plants because no other forage options are available. Grazing pressure should be reduced during dry periods as drought can increase consumption of poisonous plants if there is a decrease of other forage.
Plant growth-stages can influence the palatability and toxicity of certain plants, as can climate and time of year. Some plants, like those that accumulate nitrates, can increase in toxicity after rainfall or on cool, cloudy mornings and evenings. Some plants become more palatable, while remaining toxic, after a frost. Many toxic plants have specific growth stages or plant parts that are most toxic, such as tall larkspur that becomes most palatable and most toxic while it bolts and sets flowers. Understanding the conditions under which plants are most harmful and avoiding grazing pastures when plants are most toxic will greatly reduce the chances of livestock being harmed.
Poisonous weeds can also become more palatable after herbicide application. Care must be taken to manage weeds when livestock will not be adversely affected. Pastures should not be grazed for three weeks after applying herbicides when poisonous species are present in most cases. Broad spectrum herbicides can also have unintended effects on desirable plants if used incorrectly. Care must be taken when applying herbicides to minimize drift and over spraying. As with all weed control, competitive desirable plants should be seeded as quickly as possible following weed removal to prevent the reinfestation of the area with other weed species.
The best way to protect livestock from toxic weeds is to develop and implement a comprehensive weed control program integrating cultural, chemical, physical and biological weed management. Two particularly important control methods are mowing and herbicide use. Mowing will reduce the likelihood of seed development and dispersal and persistent spot spraying will eliminate particularly harmful weed populations.
Some plants absorb excess nitrates or oxalates from the soil and store them in plant tissues. Toxicity problems can occur in animals which feed on these plants.
Plants causing liver disease and photosensitization (sensitivity to sunlight) are often grouped together, as photosensitivity is often a secondary symptom of liver disease cause by poisonous plants.
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.
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.
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.
Plants in this group contain compounds that may irritate an animal's digestive tract, mouth or skin if consumed. These plants are not generally palatable and are typically avoided.
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.