by: Matt Welborn, DVM, MPH, DACVPM
Louisiana State University School of Veterinary Medicine

Considerable losses to the livestock industry occur every year due to toxicoses (or poisonings) from poisonous plants. In western states, an estimated three to five percent of cattle, sheep, goats and horses on open range are affected. In other regions of the country, where improved pastures and supplemental forages are grown, the economic losses may not be as high, or at least not as well documented.

No dollar value exists (there are some estimates reported in the range of $500,000/year, Keeler, 1974), nor is there data showing frequency of events. But, certainly, poisonings occur and can be significant to the animal owner (as well as to the animal!).

Most livestock owners have toxic plants on their property, yet have never experienced any problems. The majority of people do not recognize common poisonous plants, nor if particular plants growing on their premises have been eaten.

Toxic plants may be found in hay, green chop, silage, seeds from "screenings" included in feed, and in trash dump sites. People sometimes toss leaves, branches, grass clippings, shrub clippings, etc. over fences to their own or their neighbor's animals, unintentionally intoxicating livestock.

Presence of toxic plants, even if there is evidence of consumption, may not be sufficient to confirm a diagnosis. Much depends on the dose consumed and other factors.

Absence of toxic plants in the pasture, hay, green chop, silage or stomach contents does not always eliminate from consideration poisoning caused by plants such as crotalaria, bracken fern, senecio, oak, locoweed and mycotoxins. Signs of illness these plants produce may not become apparent for several days, weeks or even months after the last exposure.

Factors affecting a plant's toxicity

Both the degree of toxicity of poisonous plants and the susceptibility of various species of animals and individuals within species are subject to many factors including the following:

1. Plant factors
The particular species of plant - Some plants in the same family possess different levels of toxicity.
Stage of growth - Some plants are more toxic when young, others when they are mature.
Parts of the plant - Seeds, leaves, roots, etc. may contain differing levels of toxic agents.
Condition - Plants may become more palatable when wilted or dying.
Plant defenses - Palatability, presence of thorns, etc. make plants more or less likely to be eaten.

2. Animal factors
Species of animal - Cattle may be more (or less) affected by a particular plant compared to another type of livestock such as goats or horses.
Age - Calves may be more susceptible to some plant toxins
Body condition - Hungry cattle and those in poor body condition may seek out plants they normally would avoid.
The amount of toxin ingested and how rapidly it is consumed play a significant role in whether an animal is sickened.
Feeding/grazing management The amount of exposure to toxic plants will vary in cattle on open range versus cattle on poorly maintained pastures versus cattle on improved pastures.

3. Environmental factors This may determine the availability of plants in a region.
Season - Many plants are not available during winter months (but may be present in hay).
Soil type - Some plants prefer acidic soil conditions, sandy or loamy soils, etc.
Weather - Some plants prefer dry conditions, whereas others thrive with higher moisture levels.
Shade vs. Sun - Certain plants prefer shade, others sunny conditions.

Locations of toxic plants include a variety of settings

Pastures, fence lines, ditches/banks, corrals, edge of woods, woods, gardens, swamps, along streams/ponds, ornamental plant in yards and homes, etc.

Potential losses due to poisonous plants

Weight loss, decreased production, abortions, reproductive effects, death, additional fencing, altered grazing programs, loss of forage, photosensitization, secondary human intoxication, veterinary costs, replacement costs, etc.

Treatment of plant toxicoses

For the majority of toxic plants, treatment is symptomatic and supportive. Very few specific antidotes are available to treat poisoned cattle. Some exceptions include nitrate and cyanide toxicity.

Preventing toxicoses/losses include but certainly are not limited to the following

Remove toxic plants. Utilize proper grazing management. Don't overcrowd animals. Livestock typically will not seek out poisonous plants, as many are unpalatable. Offer clean, abundant water. Be knowledgeable of toxic plants in your area. Don't allow cattle access to plants treated with herbicides as palatability may actually improve. Common sense tells us to limit the animal's access to and consumption of poisonous plants and should be utilized as a preventive for all of the following plants.

In conclusion

The following plants are not listed in any particular order of importance. They are a few among many the author feels are important to cattle and other livestock in the southeastern United States.

Taking steps to reduce the likelihood your cattle will be poisoned by noxious plants falls in line with routine practices. Maintain pastures in good condition by keeping weeds and undesirable plants to a minimum. This not only protects your cattle, but also allows the pasture to perform at a higher level of productivity. Additionally, keeping cattle in good body condition will decrease their desire to consume toxic plants.

Learn to identify plants in your region that are toxic and limit their growth through pasture management. Visit with your Extension specialist or veterinarian for additional information.

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