Don't Poison Your Cattle by Grazing Poisonous Plants

While it's not nice to fool Mother Nature, the opposite isn't much fun, either. If nature plays tricks with drought, too much rain or even too many clouds, your high-quality fodder can become a toxic nightmare that can poison some or all your cattle

Larry Stalcup

April 1, 2010

6 Min Read
Don't Poison Your Cattle by Grazing Poisonous Plants

While it's not nice to fool Mother Nature, the opposite isn't much fun, either.

If nature plays tricks with drought, too much rain or even too many clouds, your high-quality fodder can become a toxic nightmare that can poison some or all your cattle.

It's not a common occurrence, but poisonous nitrates or prussic acids can form in everything from bermuda or ryegrass to alfalfa, says Clay Wright, livestock consultant for the Samuel Roberts Noble Foundation in Ardmore, OK. The private, nonprofit foundation serves ranchers and other producers in a 47-county area across southern Oklahoma and northern Texas. Cattle nutrition is high on its list.

Wright says the physiology of plants can change and create toxins that can harm cattle.

“There are growth processes that go on in all plants, including bermudagrass, sudan, Haygrazer, ryegrass and crabgrass,” he says. “When the process is interrupted, danger can exist. Conditions like drought and freezes can shock the plant. Even extended cloudiness can cause toxins to accumulate in a plant. Sorghum-related forages are often the most susceptible to toxin accumulations.”

How toxins develop

Nitrogen absorbed by plant roots and transported through the stems is typically used by the plant about as fast as it's absorbed from the soil. However, anything that hinders plant growth can cause nitrate accumulation, mostly in plant stems.

“Nitrate poisoning occurs when excessive nitrate is consumed and converted to nitrite faster than the animal can use it,” Wright says. “Free nitrite in the rumen is readily absorbed into the bloodstream, where it destroys the blood's ability to absorb and carry oxygen. Drying or ensiling forages tend to have a small effect on nitrate levels. Hays and silage from high-risk plants may remain toxic.”

Prussic acid, according to Noble Foundation consultants, is usually attached to a larger sugar molecule, is part of the normal growth process in problem plants, and is usually not harmful to the animal. But problems occur when environmental conditions slow plant growth, causing the sugar molecules to accumulate in the plant, Wright says.

“Accumulation is mostly in younger leaves and new growth, with drought usually the cause of such poisoning,” he adds. When the plant wilts, its cells rupture and the prussic acid is freed from the sugar molecule.

“If consumed by grazing livestock, the free prussic acid is readily absorbed into the bloodstream, where it prohibits the animal's ability to take oxygen from the blood. Even in plants that aren't wilted, chewing and digestion in the rumen can release toxic levels of prussic acid, meaning this can be a problem anytime.”

Wright says a veterinarian should be consulted for diagnosis and treatment of livestock for nitrate and prussic acid poisoning.

If nitrate poisoning is suspected, multiple samples of pasture forage should be sent to a reputable lab for testing. “You'll need a good sample of bermuda, millet, sudan, alfalfa or other forage to make sure you obtain an accurate test reading,” Wright says. Labs that provide analysis for nutritional qualities usually can test for nitrate levels as well.

“One plant may have prussic acid or nitrate poisoning and another may not. So samples should be on a macro, not micro level. There are reputable labs available for testing samples in about every region of the country.”

Hay sampling

Don't limit hay sampling to just one bale. For example, if there are 100 bales, take samples from at least eight to 10, Wright says. He suggests using a push probe or “punch” to take samples of hay. The probe can retrieve samples from up to 18 in. inside the bale. Accumulated samples can be placed in a paper bag for use by a lab for testing. Testing runs about $10.

“If you're buying hay, even just a few bales, make sure it is safe for your cattle or horses,” he says. “A reputable hay producer will have a test analysis for you. In fact, a nitrate analysis should be part of a producer's best-management practices.” (Learn more about forage-testing labs at

Wright says he no longer recommends the “quick test” sampling method because it isn't accurate enough.

“Quick tests weren't meant to be quantitative. At best, they only indicate whether potentially dangerous levels of accumulation exist in a sample,” he says. “It doesn't determine a specific level and may or may not have been representative of a whole pasture or crop.”

For example, the quick test for nitrate involves splitting a stalk and applying a chemical mixture on the exposed inner surface. “The speed and degree the clear mixture turns blue is an ambiguous indication of nitrate content (the darker the color, the greater the accumulation),” Wright says.

“The quick test for prussic acid involves suspending a chemically treated strip of filter paper from a stopper in a test tube above chopped-up leaves of a suspect plant. Again, the speed and degree the strip turns brick red is an indication of possible prussic acid content. That's all these tests show.”

If you collect 20 plants at random across a pasture or crop and most or all react strongly and rapidly to the quick test, you can assume that a potential problem might exist, but you won't know the levels those reactions represent.

“The real dilemma is when most or all of those plants test negative. Due to differences in soil type, stage of growth or other conditions, there still could be pockets of toxicity across the field or pasture,” Wright says.

Killing the poison can be tricky, but there are ways of dealing with high-test forage. For forage infected with prussic acid, you can bale it and let it cure, which will release the toxic gas and reduce the elevated levels of acid.

But haying and baling nitrate-poisoned forage won't relieve the danger. “Not even a killing frost will rid sorghum sudan or other plants of nitrate accumulation,” Wright says, adding that producers should be cautious of their final hay cutting.

Larry Stalcup is an Amarillo, TX-based freelance writer.

Johnsongrass — friend or foe?

Chan Glidewell, Noble Foundation livestock consultant, reminds producers that johnsongrass can be a cattle killer, as well as an excellent forage.

Glidewell says johnsongrass is listed among the 10 most noxious weeds in the world. “It can accumulate nitrates during the summer if exposed to several dry, cloudy days in a row. It can also produce prussic acid after stressful conditions such as drought, freezing weather or exposure to a herbicide that kills grasses,” he says.

If your johnsongrass is subjected to any of these conditions, keep cattle away for about a week to allow the prussic acid to dissipate, he advises.

However, when it's safe for cattle, “as far as nutritive value is concerned, johnsongrass is tough to beat,” he adds. One Noble Foundation study found johnsongrass had 11.6% crude protein (CP), compared with 11.4% CP for bermuda. It also had 58% total digestible nutrients, compared with 59.8% for bermuda. (For more on johnsongrass nutritional qualities, go to

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