Poisonous plants: What’s the risk to livestock?

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poison hemlock
Poison hemlock. Photo credit: UGA/Bugwood.org

It seems to me that raising livestock as well as farming in general, is often about risk management. Every year I get questions from livestock owners regarding poisonous plants; either for identification or for information on how to manage around a known poisonous plant.

Management is key

Avoiding livestock health problems due to ingestion of poisonous species is a matter of being able to identify potentially poisonous plants, understanding risk factors and taking some proactive management steps.

What are the risk factors for livestock ingestion of poisonous plants? Here is my list:

  • overgrazed pastures
  • drought conditions
  • summer storms
  • well-intentioned neighbors

Common poison species

Some common tree and weed species rated as poisonous species that pose a risk to grazing livestock includes: wild black cherry, red maple, and Ohio buckeye, as tree species and poison hemlock, buttercup/cressleaf goundsel, milkweed, nightshade, horse nettle and star of Bethlehem as weed species.

This is not an inclusive list, but it does include some of the common species associated with pastures. Now let s match up species with risk factors and provide a little more information about the toxicity of each species.

Weed control

One of the general principles of forage weed control, whether in a pasture or hayfield situation, is that 90 to 95 percent of weed control is provided by competition from the forage crop. Proper soil pH and soil fertility are important components to allow forages to grow competitively and thrive.

Another principle is to avoid overgrazing. This management practice will promote a denser pasture sward that will help to keep weeds from invading a pasture, including poisonous species.

Paddock management

Inspect mismanaged and overgrazed pasture paddocks for potential poisonous plants. Recognize that when desirable pasture forage is in short supply, the risk of livestock grazing a poisonous plant if present, is greater. In the spring, buttercup and cressleaf groundsel are both weeds that may appear.

These are weeds that are rated as low to moderate toxicity. They are also low in palatability so generally livestock will avoid ingesting these weeds. Cressleaf groundsel is a weed that is becoming increasingly more common. The compounds responsible for toxicity, pyrrolizidine alkaloids, are not destroyed in the haymaking process, and so inclusion of this weed in significant quantities in dried hay may pose a bigger threat to livestock than in a grazing situation.

Sheep management

Sheep are considered to be more resistant to the pyrrolizidine alkaloids than cattle or horses and reportedly have been used in some areas to control the plant provided the infestation is not too heavy.

Milkweed and nightshade are poisonous plants that are both considered unpalatable to livestock and generally only eaten if there is a shortage of good quality forage. Both of these plants can retain their toxic properties in hay. Cardiac glycosides are the toxic compounds in milkweed and alkaloidal glycosides are responsible for the toxicity of nightshade.

Low toxicity

Milkweed is rated as low toxicity while nightshade is rated as a moderate to highly toxic plant. Both of these plants are toxic to all classes of livestock, but in the case of milkweed, sheep are considered as most at risk from ingestion and poisoning.

Poison hemlock is a biennial plant that is appearing with greater frequency in both pastures and hayfields throughout Ohio. This plant is reminiscent of wild carrot with its finely divided leaves, but its key identification characteristic is the purple blotches or spots on a smooth stem.

Plant ingestion

All parts of the plant are poisonous and all livestock species can be affected if any part of the plant is ingested. The principle toxic compounds in poison hemlock are alkaloids coniine and gamma-conicine. It does not take much to get a lethal dose. In horses 4 to 5 pounds ingested can be lethal, 1-2 pounds in cattle and a mere 4-8 ounces in sheep. Toxicity is not diminished in either the ensiling process or the hay making and drying process so stored feed made from fields where poison hemlock is present is a livestock health concern.

Star of Bethlehem is a perennial, bulbed plant that may remind you of wild garlic or onion, but it does not have the odor. It has star shaped flowers and was originally introduced as a garden ornamental. All parts of this plant are toxic, and especially the bulbs. The toxic components are cardiac glycosides and all grazing livestock are susceptible.

Herbicide options

The bad news is this plant is capable of taking over areas of a pasture paddock and whereas the other poisonous weed species previously mentioned have some herbicide options for control, Star of Bethlehem has limited herbicide options that are only moderately successful in controlling it.

In drought years there are more cases of livestock ingesting poisonous plants. In part this is due to the slow pasture growth that results in livestock not having enough desirable forage available. They may eat plants, including poisonous plants that they otherwise would not eat. Be very diligent about inspecting pasture paddocks for potentially poisonous plants during drought conditions.

Nature’s path

Summer storms and the accompanying downed limbs and trees can present another risk of livestock ingesting poisonous plants, particularly if some of those downed limbs or trees are from wild black cherry, red maple or our state tree, the buckeye.

With regard to red maple only horses are known to be affected. Horse owners need to be aware that red maple leaves are rated as highly toxic to horses, ingestion of as little as 3 pounds can be a lethal dose. Within 48 hours of ingestion, chemicals in the leaves will cause a destruction of red blood cells.

Leave ingestion

The Ohio buckeye as well as a related species, the horse chestnut, are rated as a moderately to highly toxic. Ingestion of the leaves can affect the gastrointestinal and nervous systems of all grazing livestock.

When small amounts are eaten symptoms may be limited to the gastrointestinal system and include excessive salivation, abdominal pain and diarrhea. When larger amounts are consumed neurological symptoms including trembling, staggering and breathing difficulty may be expressed. The issue with wild black cherry trees is that freshly wilted leaves can contain high levels of hydrogen cyanide (HCN) compounds. Young leaves generally have a higher HCN potential than older leaves so as we go through the growing season, HCN levels in wilted leaves decline.

Susceptible to poisoning

However, ruminant animals are very susceptible to poisoning from HCN. Research indicates that the lethal dose for sheep/cattle can be as little 0.46 grams to 1.82 grams of plant material per pound of body weight. To give this some perspective, there are 454 grams in one pound, so this is a small amount. For a 1200 pound cow, consuming 1.2 to 4.8 pounds of wilted black cherry leaves could be a lethal dose.

Signs of HCN toxicity can occur quickly, as soon as 15 to 20 minutes after ingestion. Typical signs are distress, followed by severe weakness to the point where the animal is barely able to stand, or even to the point of collapsing. Animals will exhibit rapid and labored respiration. If they have collapsed there may be kicking/paddling of the legs and/or kicking associated with seizure like symptoms. This entire sequence can progress in 10 to 15 minutes with a high dosage of HCN and up to 45 to 60 minutes with a lower dose.

Recovery is possible

References state that if the animal does not die in the first hour, there is a good chance for recovery. A final risk factor is the well intentioned neighbor. The most common example of a good intention causing a tragedy is when trimmings from a yew plant are dumped into a livestock pasture as a treat for the animals.

Yew plant

Livestock will readily consume the trimmings but yew is a highly toxic plant and death can occur very quickly following ingestion. It has been reported that one mouthful is enough to kill a cow or horse.

There are some good websites available to learn more about poisonous plants and that can help with identification. Some that I use include: http://research.vet.upenn.edu/poisonousplants from the University of Pennsylvania Veterinary School and www.ansci.cornell.edu/plants/index.html from Cornell University Animal Science Department.

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1 COMMENT

  1. Mr. Roy,

    You’ve done an excellent job here! Just wish you had pictures of the various poisonous plants.
    Thank you so much for all your efforts!

    BG Hirt

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