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Noxious Weeds

Good pasture management can go a long way in preventing horses from becoming poisoned by toxic plants.

“Through good pasture management, recognizing toxic plants and understanding the effects of toxins on animals, plant poisoning can be largely avoided,” according to Tony Knight, who works with the College of Veterinary Medicine at Colorado State University (CSU).

“Plants contain a variety of toxic compounds that help to deter herbivores and insects from eating them,” he continued. “A classic example of this is milkweed that contains a milky sap that is an irritant and therefore distasteful. It is also poisonous.”

Plant poisonings not only causes death loss but can also cause economic and reproductive losses.

Producers also incur additional expenses associated with spraying herbicide to control these plants, he said.

Species susceptibility

Each species of animal has different susceptibility levels to plant poisoning, Knight explained.

“Sheep and goats have more carbonic acid in their saliva, so they can consume more poisonous plants. Horses have an entirely different digestive system than cattle, so they can consume feed with nitrates,” he said.

Some plants, like locoweed, are poisonous because they have developed a mutually beneficial relationship with specific fungi that, when growing in the plant, produce a toxic alkaloid poisonous to horses and livestock, Knight said.

“There are numerous native range plants that are potentially poisonous to livestock, but rarely is an animal poisoned by eating a few mouthfuls of these plants,” he explained. “Livestock poisoning occurs most often when rangeland is overgrazed, and animals are forced to eat whatever is available. Feeding hay that is full of weeds can have similar repercussions. Make sure you know what is put up in hay.”

Plants causing sudden or acute death will do so within a few hours of the onset of clinical signs. Most of the time, animals are found dead before physical symptoms appear.

Native plants causing sudden death are water hemlock, some species of milkweed, death camas, larkspur and choke cherry.

Water hemlock

“Poison hemlock is an invasive weed that, if eaten before it matures, can also cause fatalities,” Knight stated.

Water hemlock is a perennial that grows in wet or marshy environments. It can grow four to six feet by maturity. Water hemlock has hollow, compartmentalized stems with leaves that are compound, alternate and pinnate with leaflets five to eight centimeters long and one to two centimeters wide. It has many small, white flowers.

The tuberous roots produce a highly toxic, pungent, yellowish fluid when cut, Knight explained.

A lethal dose of water hemlock roots is less than 0.5 percent body weight.

“The potential for water hemlock poisoning is high in horses and livestock grazing pastures that have marshy areas or where animals have access to stream or river banks where the plants tend to grow,” Knight explained. “Animals find water hemlock palatable, especially the new leaves in the spring.”

Sorghum hay

Consuming sorghum hay also has the potential to be toxic for some animals.

“Although horses are rarely fatally poisoned by plants containing cyanogenic compounds, horses, cattle and sheep can develop a syndrome of posterior ataxia and urinary incontinence after consuming sorghum hay for a period of weeks,” Knight said.

Poisoning causes degeneration of the white matter of the spinal cord, hind leg ataxia, tail paralysis and urinary bladder paralysis leading to incontinence. The degenerative changes in the nerves are irreversible, and fetal limb deformities have been reported in foals and calves whose dams are fed sorghums during pregnancy.


Knight also encouraged producers to watch out for milkweeds. A few species of milkweed can be poisonous to livestock, horses and domestic fowl, especially when other forages are scarce or milkweeds are incorporated into hay.

Milkweeds contain a variety of toxins that affect the heart, digestive system or the nervous system. The highest concentration of cardenolides is found in milky sap, but all parts of the plant are toxic. Milkweed remains toxic when dried, so animals consuming the dried plants in hay are at risk.

As little as one kilogram of green milkweed plant material is lethal to an adult horse. Clinical signs usually start about 12 hours after consumption and consist of depression and labored, slow respiration. Horses may show colic and diarrhea.

Death Camas

There are 13 species of Death Camas in North America that are poisonous to livestock, horses and alpacas. All parts of the plant are toxic, especially new growth and bulbs.

Death results from respiratory paralysis.

Most poisonings occur in the spring because Death Camas shoots appear before grasses and other plants and are considered particularly palatable to livestock.


Horses should be kept away from locoweed, added Knight.

At least 2,000 species of locoweed exist in western North America. However, few are toxic.

“A few species of locoweed have long been recognized as important poisonous plants affecting horses and livestock, causing more overall economic losses to the livestock industry than any other group of plants,” Knight stated.

Horses are particularly susceptible to locoweed poisoning.

“Animals find locoweed palatable and will graze the green plants readily in the spring before grasses emerge from dormancy,” he said.

Horses exhibit the most distinctive signs of locoism, characterized by changes in normal behavior including marked depression, drowsiness, blindness, abnormal gaits, marked difficulty in walking and backing-up. Abnormal attempts at chewing food and spastic jaw movement may also be observed, Knight said.

Affected horses also show unpredictable behavior, such as rearing on their hind legs and falling over backwards, along with sudden periods of sudden and extreme excitement.

Riding loco-affected horses is dangerous.

If clinical signs are recognized early and the horse is removed from locoweed, clinical improvement will occur, but it may take several months for abnormal behavior to resolve completely, Knight noted.

Horses should not be allowed to graze locoweed in the spring when other forages are unavailable. Since locoweed is palatable and nutritious, animals will readily consume it.

Locoweed can be controlled with herbicides, but dried locoweed stems remain toxic at the end of the growing season, Knight explained.

Overgrazing or overstocking pastures will increase the potential for locoweed poisoning.

Gayle Smith is a correspondent for the Wyoming Livestock Roundup. Send comments on this article to This email address is being protected from spambots. You need JavaScript enabled to view it..

Worland – With many cattle and sheep on the rangelands grazing in Wyoming, UW Extension Specialist Barton Stam encouraged producers to be wary of poisonous plants on the range. 

“We have to be cautious of poisonous plants,” he said.

Stam spoke during the 2014 WESTI Ag Days, held in Worland on Feb. 4-5.

In sagebrush and on rocky hillsides, Stam noted that death camas is prevalent and deadly.

“Death camas will kill just about anything – including humans, horses and cows,” he said. “Sheep are most likely to be affected because it is one of the first green plants to come up.”

The plant resembles wild onions, though the bulbous root doesn’t smell like onion. 

Another deadly plant is hallogeton, which produces sodium oxalate.

“Sodium oxalate is what causes the problems in animal metabolism,” Stam said. “Sheep are heavily impacted, even though a high calcium diet will give a higher tolerance.”

Because calcium binds to oxalate, the high calcium concentration of rangeland plants is helpful.

“In a dry lot, however, where there is nothing else to eat and the sheep get hungry, we can see problems,” Stam commented. “If sheep are hauled and dumped into loading pen or we are trailing down borrow pits, we can have problems.”

Hallogeton grows well in disturbed areas and should be watched closely.

Lupine is a native plant that is common in many areas.

“If a pregnant cow eats lupine in the 40 to 70 days of gestation range, she may have a deformed calf,” Stam said. “It results in crooked calf syndrome and cleft palates.”

While lupine can be a valuable nitrogen-fixing plant that can improve soils, it must be managed. 

In many cases, producers also have to deal with larkspur poisoning. 

“Larkspur is responsible for more cattle poisonings than any other poisonous plants,” Stam said. 

Several varieties of larkspur can be found across Wyoming, and Stam noted that on the Big Horn Mountains, in particular, tall larkspur provides a struggle for producers.

“The thing with larkspur is that there is a toxicity window,” he said. “Throughout the year, the toxicity of larkspur goes down while the palatability increases. There is a little window where it is just poisonous enough and just tasty enough that is causes death in cattle.”

Sheep have a higher tolerance for larkspur, he continued, suggesting that running sheep through Forest Service leases and mountain pastures first may help producers avoid the toxicity window.

“Larkspur can also be controlled through spraying,” Stam noted. “It is a native plant, though, and is tough to deal with.”

“A lot of our poisonous plants can be easily sprayed and killed,” Stam said. “One of the problems we have, however, is that by the time we see symptoms, it is too late.”

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

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..

Laramie − Controlling invasive weeds with the mentality of “just kill the weeds” can do more harm than good, according to Daniel Tekiela, University of Wyoming invasive plants specialist. 

Tekiela explains he began his career at the University of Wyoming at a time when people were excited about the use of bioherbicides as a solution to the growing issue of the highly invasive cheatgrass. However, when he looked into literature for the products, there was no research regarding the use of bioherbicides for cheatgrass control. 

“I can’t recommend a product if there is no scientific evidence to back it,” Tekiela comments. “My job is to recommend solutions to land managers based on proven information.” 

With no existing information, Tekiela set out to determine whether or not bioherbicides could effectively manage cheatgrass populations in various locations in Wyoming. 

Using bioherbicides

“The thing we have to understand about using bioherbicides is that we are introducing living bacteria to the soil,” Tekiela notes. “The idea is the bacteria enters the soil and damages the roots of the plant.” 

Tekiela explains because bacteria are living, the conditions at application need to be ideal to maximize the efficacy. 

“It needs to be cool, wet and overcast,” according to Tekiela. “The bacteria are known to do better in cool weather, wet conditions that allow for better incorporation into the soil and the sun can damage the bacteria.” 

After applying the bioherbicide at various rates and at various locations, Tekiela found it was not effective in controlling cheatgrass populations. 

“One plot I tested had nearly perfect conditions, and when it was all said and done, the product still had no effect on the cheatgrass,” says Tekiela. 


“There is significant evidence suggesting that preventing invasions is more economically sound than waiting for it to become a problem,” Tekiela comments. 

He blames the lack of prevention efforts, in most cases, on simple human nature. 

“We as humans are really good at solving issues happening in the moment,” says Tekiela,“but it’s hard for most people to think about an issue before it actually presents itself.” 

Tekiela explains prevention is the best tool we have in controlling invasive plants. 

“Once an invasion becomes large enough, the only option is herbicide use,” according to Tekiela. “If we are prepared for the possibility of an invasion, we can explore other options, such as introducing perennials to compete with the cheatgrass.”

Tekiela explains, to be proactive, land managers must consider the activities that take place on their property that could introduce invasive seeds to the area. 

“Construction equipment, livestock, migrating wildlife and vehicles can all introduce invasive seeds to the land,” says Tekiela. 

“Even though it can be difficult to know when an invasion will occur, being prepared and proactive can prevent the plants from becoming a problem,” he adds.


“One of the critical aspects to consider in weed management is the seedbank,” says Tekiela. “When we approach weed management, especially with cheatgrass, we want to target the seedbank because cheatgrass is an annual plant that can survive multiple years.”

Tekiela comments there have been recent studies in Colorado that suggest the seedbank of cheatgrass lasts approximately three to four years before it’s depleted. 

“While this study is very exciting, the thing we have to take into consideration is how the environment affects a plant and its seedbank,” says Tekiela. 

Tekiela notes, there is a major difference in environments between Colorado and Wyoming. 

“The change in weather between Colorado and Wyoming seems almost instant when we cross the border,” says Tekiela. “When we cross into Colorado, the wind seems to stop, and the temperature rises.”

Tekiela compares Wyoming to a refrigerator in that it is drier, colder and preserves things longer.

“Unfortunately, we don’t have a good idea how long seedbanks last in this environment,” says Tekiela. “It’s important to keep weeds under close observation because missing just one year can put us back at square zero as far as weed management.”

The big picture 

Tekiela comments, in the case landowners find themselves in a jam with invasives, they must look at the big picture. 

“Having a ‘just kill the weeds’ mentality causes much more harm than good,” says Tekiela. 

As an Extension consultant, Tekiela says he experiences many people only seeking a product to kill the weeds instead of looking at the bigger picture of their entire operation. 

Tekiela recommends  landowners reach out to Extension for help with weed management issues and consider the big picture. 

“Every invasion is different, and there is no one-size-fits-all solution,” says Tekiela. “We have to look at the challenges and goals of the specific situation to curate an effective plan for weed management.” 

Callie Hanson is the assistant editor of the Wyoming Livestock Roundup. Send comments on this article to This email address is being protected from spambots. You need JavaScript enabled to view it..

Invasive weed species are a burgeoning problem for Wyomingites – both in croplands and on the range. In particularly cheatgrass, ventenata, medusahead wildrye, palmer amaranth and glyphosate-resistant kochia should be of concern for Wyomingites. 

While some species, like cheatgrass and kochia, are more prevalent across the state, other invasive species, including ventenata, medusahead and palmer amaranth, are new threats that producers should watch out for. 


At the end of May or beginning of June, if producers see a dark purple weed begin to dry down across their landscape, often cheatgrass comes to mind. 

“Cheatgrass is a nasty weed, but it could also be ventenata,” explains University of Wyoming (UW) Extension Educator Jeremiah Vardiman. “I have heard people who have ventenata wish they had cheatgrass instead.” 

While the species are very similar, Vardiman says ventenata is less palatable and provides very little spring grazing ability. 

“In the seedling stage, ventenata looks almost identical to cheatgrass. It is also a winter annual plant that germinates in the fall and produces thousands of seed heads in one year,” he explains. “Areas with ventenata grass drop forage production by up to 50 percent.”
To distinguish ventenata from cheatgrass, Vardiman says the seed head is key. 

The panicle of ventenata is also open and loose. Ventenata has a pyramidal panicle, meaning the petioles and seeds are distributed in a pyramid shape. 

Additionally, the awns of ventenata are bent and twisted.

“If producers are out on the landscape and they see what they think might be ventenata, contact Extension or Weed and Pest,” Vardiman emphasizes. “We have seen the plant in Sheridan and Johnson counties, and we want to do what we can to avoid its spread.” 

Weed specialists are concerned the plant may spread via livestock, in either wool, hair or the rumen. 

“So far we have seen ventenata mostly in rangelands, but all producers should be on the look-out for it,” he says.


Another species of concern is medusahead wildrye, which is also a major problem in several western states. 

“Medusahead wildrye has rigid, stiff awns, even when it’s green,” Vardiman describes. “The awns only get tougher as the plant matures.” 

The seed head has characteristic long awns, but Vardiman says medusahead is often confused with native plants.

“The palatability of medusahead is really terrible,” he says. “Even when it’s green, it isn’t desirable to livestock, so it could be a real problem if it gets onto our rangelands.”

Palmer amaranth

In cropland systems, Vardiman says a new concern is palmer amaranth. The species is similar to red-root pigweed, which is commonly found in the Big Horn Basin. 

While it hasn’t been found in Wyoming, Nebraska corn producers have seen infestations, and Vardiman adds, “This is a weed we definitely don’t want.” 

“Palmer amaranth is aggressive and fast-growing,” he says. “It can get taller than corn in certain cases, and it can get as big around as a person’s wrist.” 

The annual broadleaf weed looks very much like pigweed, and Vardiman comments, “We might think it is red-root pigweed on steroids.”

Palmer amaranth originated in the dry deserts of Mexico and the Southwest, and Vardiman notes it was introduced to the irrigated cropland of the Midwest through cotton meal, where it took off. 

“Palmer amaranth was used to only six inches of water or less, so when it saw 40 inches of water in the Midwest, it went gangbusters,” he says. “If palmer amaranth gets into a field, it has been known to break equipment, including cutter bars on combines and swathers.”

He adds, “Palmer amaranth has a robust, woody stalk as it matures, and it’s hard to get rid of.” 

To identify palmer amaranth, Vardiman says the plant has obligate leaves that are oval or egg-shaped and evenly dispersed. The leaves might have a v-shaped watermark as it matures. 

“The biggest indication for a sure-fire way to identify palmer amaranth is the petiole,” says Vardiman. “The petiole of the plant is longer than the leaf blade.” 

Additionally, the leaf tip comes to a hair-like point, and the stalk of the plant does not have any hairs on it. 

“If anyone sees palmer amaranth, they should turn it in to their local Weed and Pest or Extension office as soon as possible,” Vardiman emphasizes.


Glyphosate-resistant kochia is not new to Wyoming, and the plant has been documented in the Big Horn Basin. 

“We can identify glyphosate resistant kochia when we see strange patterns of kochia show up in our fields. We can see where the tumbleweed rolled through the field and distributed the seeds,” Vardiman explains. 

When glyphosate resistant plants are seen, Vardiman notes all the plants are approximately the same size, and they are not present in straight lines. 

“If we see straight lines in a field, that is likely not resistance,” he explains. “In those cases, it’s more likely we had sprayer skip.”
Vardiman emphasizes, “It’s important to keep all these plants in mind as we do weed management in our fields and on our land.”

Vardiman presented during 2019’s WESTI Ag Days, held Feb. 12-13 in Worland. 

Saige Albert is managing editor of the Wyoming Livestock Roundup. Send comments on this article to This email address is being protected from spambots. You need JavaScript enabled to view it..