Field Guide to Plants Poisonous to Livestock - Western U.S.

Facts About Plant Poisoning

The statistics are there to show that the threat of plant poisoning is real. According to the U.S. Department of Agriculture, 1% of cattle and over 3% of sheep in the western U.S. are killed each year by eating poisonous plants. The death toll for other plant-eating livestock--goats, horses, llamas, alpacas, and pigs--is less well quantified, but all of these animals are known to be susceptible to poisoning by certain plants.

Death is not the only significant consequence of plant poisoning. Animals eating less than lethal doses may suffer temporary acute or chronic pain and illness, abortions, decreased productivity or birth defects. Some poisons taint milk of affected animals. A few cause types of damage that render animals unable to perform the type of work for which we intend them. A notorious example is the debilitating effect of "locoweeds" on horses. Again, while not inclusive of all livestock, poisonous plant expert Lynn F. James of the USDA Poisonous Plant Research Laboratory in Logan, Utah, reported to the Third International Symposium on Poisonous Plants a purely economic loss to the western cattle and sheep industry of nearly $340 million in 1989.

Since there are few known antidotes for plant toxins and what therapies there are can be expensive--especially if numbers of animals are involved--prevention is by far the most prudent course of action. But how does one go about preventing grazing and browsing animals, all of which spend a large number of their waking hours eating, from ingesting poisonous plants?

There is no simple answer to that question. The related toxicology is a highly complex and evolving science. But the effective use of good information about poisonous plants and animal response to them is a good place to start. Additionally, good pasture and herd management is founded on assuring that safe, nutritious forage is available in adequate supply.

For complicated reasons of plant and animal evolution, well-nourished herbivores will generally avoid eating most toxic plants, but as many livestock owners know, good animal husbandry takes more than leaving it up to animals to do the right thing. There are, indeed, some poisonous plants to which livestock have been found to be attracted. A few toxic plants appear to be aversive initially, but if animals once eat them--usually because they find themselves without other choices--they may become habituated. Some safe plants become toxic when sprayed with herbicides or fertilized. They may simultaneously become more palatable. Weather stresses and other natural conditions may convert the non-toxic to toxic. Animals learn some of their aversion to poisonous plants from other animals, particularly their mothers. If taken to a new plant environment, they will lack experiential knowledge. Furthermore, the relative health of the animal (or if a ruminant, of the microflora in its rumen) figures into how well it can handle some toxic chemicals in the plants it consumes.

 Probably most importantly, if put into a situation where only poisonous plants are available or make up a large percentage of available forage, livestock will eat them. That may happen because some poisonous plants green up earlier in the spring than most other forage. They may also get new growth after fall rains or early snowfalls when other plants are scarce. Others are drought resistant and may be the only food left under arid conditions. Some of the most significant livestock losses due to plant poisoning were caused by unloading and trailing hungry animals through areas heavily infested with toxic plants such as Holgetan. Other cases have resulted from humans tying horses or pack llamas to toxic shrubs or trees. And beyond the matter of grazing fresh plants, many poisonous plants grow in hay and grain fields and become mixed in feed. Some lose their toxicity over time or with drying; others do not.

Since poisonings may still occur even with diligent efforts at prevention, other essential knowledge includes recognizing the physiological or behavioral signs of poisoning. While it is critical to seek the advice and intervention of a veterinarian when treatment is needed, livestock owners can benefits from understanding something of the care and treatment options in response to particular poison types. For example, knowing that animals poisoned by a plant containing cardiac glycosides must be kept quiet and unstressed may make the difference between life and death.

The focus of the Field Guide is basic information that livestock owners need to have readily available. The book's targeted, straightforward approach should make solid pasture and animal management decisions easier, as well as allow livestock owners to take their animals to pasture and out on the trail wisely, but without undue concern.

(Excerpted from Field Guide to Plants Poisonous to Livestock: Western U.S.)