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Beware of Contaminated Hay and Grazing Cattle Around Poisonous Plants

by Wyoming Livestock Roundup

The winter of 2023 was a tough one for cattle and producers in Wyoming. In several parts of the state, winter came early, stayed late and brought colder temperatures with above average snowfall. 

Due to the long winter, many cattle producers were forced to feed extra hay for longer periods of time than expected. Many Wyoming ranchers were short on hay resources and were forced to utilize additional hay to get through the winter. 

Demand for hay led to a shortage of hay resources in many parts of the state. Some ranchers purchased hay from other places and/or fed hay that wasn’t ideal to maintain their herds. As a result, several cases of cattle poisoning have been reported.

As we move into the summer months, grazing around toxic plants is still a concern. Below are some things to watch out for and considerations for minimizing losses due to toxic hay and toxic plants.

Testing hay for nitrates

A very common route of poisoning for cattle fed hay is nitrate poisoning. Acute nitrate poisoning occurs when ruminants consume high nitrate forages during a short period of time.

Ruminant species convert nitrate to nitrite during rumination. The nitrite then binds to the red blood cells and inhibits oxygen, leading to cyanosis and sudden death. 

Even in cases where poisoning isn’t enough to cause sudden death, chronic poisoning can result in abortions and/or decreased performance.

Some plants accumulate nitrate more than others. 

Crops like oats, barley, corn and wheat are known for accumulating high amounts of nitrates during certain times of the year, especially when they are harvested following stressful periods such as drought, high temperatures or low relative humidity. 

Also, overfertilization can contrite to higher nitrate accumulations in some forages. Certain weed species which often contaminate hay like Canada Thistle and Bindweed are also known nitrate accumulators. 

Regardless, to safely feed forages known to contain nitrates, a forage analysis should always be conducted prior to feeding. If the forage analysis detects high levels of nitrates, it may still be possible to feed the hay, but it should first be diluted with low nitrate hay. 

Testing is inexpensive and a University of Wyoming Extension educator can help analyze nitrate test results and offer feeding and mixing suggestions. 

Contaminated hay and lanceleaf sage

Hauling in hay from other places can be risky due to the possibility of contamination of poisonous plants. In fact, a 1992 study concluded the economic value of losses due to poisonous plants cost the livestock industry over $340 million dollars per year, adjusted to exceed $600 million in today’s dollars. 

Contamination and poisoning can occur with a wide range of native and non-native plant species found in the area where hay is harvested. 

One specific case of poisoning occurred in Fremont County, caused by hay contaminated with Salvia reflexa, commonly referred to as lanceleaf sage.  

A Crowheart herd sustained significant, sudden death losses due to contaminated hay being brought into the area and fed in January. Necropsies were conducted by the Wyoming State Vet Lab on multiple dead cows from this herd. All animals showed signs of acute liver disease. 

Pathologists identified hepatoxic compounds – Salviarin and Rhyacophiline, which are found in lanceleaf sage – in both the contaminated hay samples and rumen contents of the cows.

Lanceleaf sage is an annual species native to most of North America. Although not especially common in Wyoming, it is a county declared weed in Park County. It is also found in several locations throughout Montana and Canada. 

It’s been known to contaminate alfalfa fields, especially along field edges and irrigation ditch banks.

Grazing around larkspur and other poisonous plants

As things begin to warm up in Wyoming and cattle are being turned out to their summer ranges, it is also important to be aware of some poisonous plants they might encounter. 

Many toxic plants emerge early in the growing season and/or persist in range environments which have been overgrazed or misused. 

Being able to identify high densities of toxic plants on the range land is important and should be considered every year before turning out cattle in a pasture.

Larkspur is an especially common rangeland plant in Wyoming and is one of the most toxic threats for grazing cattle. Larkspur poisoning has been known to cause up to 15 percent of all cattle deaths in Western North America and contributes to millions of dollars in economic losses. 

The various forms of larkspur contain alkaloids which can cause sudden death or significant decreases to mobility, organ function and overall performance. 

Larkspur toxicity and palatability can vary throughout the year, but in general, cattle must consume one-half to three percent of their body weight to be affected.

Toxicity can occur anytime, but timed grazing in areas with larkspur can be an effective method of avoiding toxicity. 

Grazing cattle during the early vegetative stage when palatability is low can work if other vegetation is available. However, alkaloid levels are highest in the spring and low larkspur can still be palatable in early stages.  

Grazing later in the growing season during the pod stage is usually when alkaloid levels are lowest and can be a good time to graze pastures with larkspur. The riskiest window is between the flowering and bud stages when palatability and toxicity levels are both relatively high. 

Multi-species grazing and herbicides are additional management strategies which could be helpful.

Final thoughts

Feeding hay to cattle and grazing around poisonous plants is an important issue to be aware of in Wyoming. It’s crucial to test hay and verify, when possible, hay hasn’t been contaminated before feeding it to a cow herd in large quantities. 

The best way to avoid toxicity in grazing animals is to responsibly manage grazing intensity and regularly inspect the rangeland and pastures for toxic plants. 

Chance Marshall and Barton Stam are UW Extension educators for Fremont County and Hot Springs County, respectively. They can be reached by visiting

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