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Weed & Pest

Toxic plants are still problematic in late summer, early fall

Written by Emilee Gibb

Often overlooked and considered simply a problem in the spring, livestock producers should be aware of toxic plants in the late summer and early fall, says University of Wyoming (UW) Agriculture and Horticulture Extension Educator Brian Sebade.

“In the spring, we have things like larkspur that causes pretty sudden death for a lot of our cattle,” explains Sebade, “A lot of the fall plants have symptoms that might not pop up right away or livestock just look like they’re not doing quite as well.”

Contributing factors

Many factors can attribute to animals being more prone to eat toxic plants in the fall, says Sebade.

He notes that oftentimes, toxic plants are the only plants that are still green, making them look more desirable to animals.

“Most of the time, we have animals that are really good at picking at stuff around the toxic plants, but sometimes, if it’s the only green thing around, they’re more willing to try it out,” explains Sebade.

Toxic plants can also be a concern if a producer is moving animals to an unfamiliar pasture or if producers purchase livestock from a different area.

“If a rancher is moving animals from a pasture that they’re familiar with to a pasture that they’re not used to – or just animals that the producer has brought in from a completely different area and moved them in – sometimes producers can have some issues. If livestock don’t know what the toxic plant is, they might try it out,” says Sebade.

In some cases, moving animals quickly through an area can result in intoxications as the animals only graze on the toxic plants.

“Moving through, they’ll chomp on it because they’re hungry. Then, they won’t eat anything else, and they get a high dosage,” says Sebade. “Most of these are toxic, but as long as they eat other things, they’ll likely be okay. Toxicity is based on the animal's bodyweight.”

Sebade suggests having adequate water and minerals available to animals to self-regulate their needs. Good grazing management is also crucial in reducing the incidence of intoxications.

“Try to move things around so animals aren’t always stuck at the water even though that’s a little bit tougher in August and September because it’s hot and dry,” continues Sebade.

Different species

The species that producers should be conscious of varies depending on the moisture of the site.

In wet areas that have moisture present in the top layer of soil and in the subsoil, Sebade’s top concerns are water hemlock, arrow grass and horsetail.

“Water hemlock is native to Wyoming, actually, and there’s poison hemlock, which is introduced. We actually had a lot of water hemlock that was along a lot of our riparian areas,” says Sebade. “Again, that’s kind of green, and it can kind of get mixed in with other palatable stuff.”

In soils with subsoil moisture that does not reach the surface, Sebade recommends producers be aware of hounds tongue, poison hemlock, hemp dogbane, chokecherry and Russian knapweed.

Sebade explains that chokecherry is not normally extremely toxic, but stress can increase the concentration of toxins in the plant.

“Most of the time it’s not a big deal, but if plants get damaged or they’re drought stressed, they tend to accumulate more of the toxin. It’s kind of like grapes where if we stress the plant out, we get more sugars in the grape. That’s what happens with chokecherry,” he says.

Producers should be conscious of bracken fern, tall larkspur and orange sneezeweed in higher elevation sites with some subsoil moisture, he says.

Toxic plants that favor dry or upland sites with little to no moisture in the subsoil are greasewood, halogeton and nightshades.

“Some of our upland sites might have halogeton, which can sometimes be pretty toxic. It’s greener later in the year making it look more palatable,” explains Sebade.

Spring versus fall

The plant species that are present during the fall are different than in the spring, says Sebade. As such, clinical manifestation of intoxication can present differently.

“The signs of many of the spring plants are going to be a little bit different than what we might have in the fall,” he says.

The type of toxin in the plant is the primary factor in how the plant will affect the animal. Most of the spring toxic plants, as well as some fall plants contain a more potent toxin.

“We have cyanide-type poisoning with something like chokecherry, poison hemlock or water hemlock. That’s a pretty quick deal,” says Sebade.

Alternatively, many fall toxic plants contain a less severe toxin, comments Sebade.

“Hounds tongue, on the other hand, builds up over time, and sometimes the symptoms don’t pop up right away,” he notes.

In either season, Sebade notes that intoxication is not considered a herd disease but rather affects individuals of herd.

“It gets a little difficult, and that’s probably why toxic plants are just kind of a pain. They don’t ever kill an entire herd. It’s usually just a few animals and sometimes the livestock just just look kind of stressed out,” concludes Sebade.

Emilee Gibb is editor of Wyoming Livestock Roundup and can be reached at This email address is being protected from spambots. You need JavaScript enabled to view it..