Poisonous plants are among the important causes of economic loss to the livestock industry and should be considered when evaluating illness and decreased productivity ( see Table: Poisonous Range Plants of Temperate North America Poisonous Range Plants of Temperate North America ). Poisonous plants can affect animals in many ways, including death, chronic illness and debilitation, decreased weight gain, abortion, birth defects, increased parturition interval, and photosensitization. In addition to these more obvious losses, other considerations include loss of forage, additional fencing, increased labor and management costs, and frequently interference with proper harvesting of forage.
Most poisonous range plants fall into two general categories: those that are indigenous to a range and increase with heavy grazing, and those that invade after overgrazing or disturbance of the land. Among those not in these categories are certain locoweeds and larkspurs, both of which form part of the normal range plant community. Poisonous plants can be found in most plant communities and should be considered in most grazing situations.
Livestock poisoning by plants often can be traced to problems of management or range condition, rather than simply to the presence of poisonous plants. Usually, animals are poisoned because hunger or other conditions cause them to graze abnormally. Overgrazing, trucking, trailing, corralling, or introducing animals onto a new range tend to induce hunger or change behavior, and poisoning may occur.
Not all poisonous plants are unpalatable, and they are not restricted to overgrazed ranges and pastures. Furthermore, poisonous plants do not always kill or otherwise harm animals when consumed; the dose determines toxicity. Many plants can be either useful forage or toxic. For example, plants such as lupine and greasewood may be part of an animal’s diet, and the animal is poisoned only when it consumes too much of the plant too fast. To prevent poisoning, it is important to understand the factors involved when a useful forage becomes a poisonous plant.
Definitive diagnosis of suspect plant poisonings is difficult. It is important to be familiar with the poisonous plants growing in the specific area and the conditions under which livestock may be poisoned. A tentative diagnosis is possible if the following information is available: 1) any local soil deficiencies or excesses (which may complicate plant toxicities or simply confuse as to cause of a syndrome), 2) the syndromes associated with each of the poisonous plants in the area, 3) the time of year during which each is most likely to cause problems, 4) the detailed history of the animal(s) over the last 6–8 mo, and 5) any change of management or environmental condition that may cause an animal to change its diet or grazing habits (in some cases, eg, locoism, this may be all that is required in addition to identification of the plant involved). Identification of the plant is important, whatever its stage of growth, and is especially useful if it can be identified in the stomach contents of the poisoned animal. Chemical analysis of toxicants often is not useful. Metabolic profiles are useful for some toxicities, and in some, the necropsy lesions are distinctive ( see Table: Poisonous Range Plants of Temperate North America Poisonous Range Plants of Temperate North America ).