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Extension Agent: Toxicities during drought

June 21, 2011 at 1:21 a.m.

STEPS TO PREVENT LOSSES FROM NITRATENever turn hungry animals into possibly high-nitrate forages.

Don't test the forage/pasture with one old cow. She'll eat the tops of plants first, where the concentration is lowest, leaving the most toxic.

Test grass or hay before baling or feeding if you suspect high nitrate. Nitrate levels remain constant in hay.

If hay nitrate is high, feed carefully with an energy supplement, or with low protein forages, or other hay low in nitrates. Never feed high-nitrate, hay-free choice.

Generally, it takes three to four days of plant growth after a good rain for nitrate levels to be reduced.

PREVENT LOSSES FROM PRUSSIC ACIDDo not graze any of the cyanogenic-accumulating plants (sorghums) that have been subject to drought or injury, unless tested.

Defer grazing of herbicide or frost damaged grasses until well recovered from injury, cut for hay, or plant tissue has been allowed to dry.

Do not graze sorghum family plants until they are 2- to 3-feet tall.

Graze second-growth sorghums with caution if growing conditions are poor.

Remove livestock from feed or poor-growing pasture if an animal is found dead suddenly.

Prevent animals from grazing wilted plants, or those with young tillers.

Generally, it takes two weeks of plant growth, after a good rain, for prussic acid levels to be safe to graze.

Test forage before grazing if you suspect prussic acid. Prussic acid levels dissipate once forage is dry enough to bale for hay.

By Joe Janak

Several calls have come in related to possible toxicities from harvesting grain sorghum or corn stubble for hay and other general forage toxicities because of the current drought. Besides the toxicities addressed below, ranchers need to also be checking their range and pastures for toxic weeds and plants as livestock are forced to graze atypical plants.

During a drought, if pastures have been heavily fertilized with nitrogen, there is an added chance that nitrate poisoning may occur. Certain plants, such as pigweed, may accumulate excessive nitrates, even if not fertilized. In high nitrate forages, levels build as nitrite, resulting in reduced oxygen in the blood causing asphyxiation, or a lack of oxygen and death within four hours of consumption.

Plants in the sorghum family (grain sorghum stubble), Johnson grass, sudan grass and forage sorghums are generally implicated first. Corn, small grains, carelessweed or pigweed, sunflower and highly fertilized, leafy vegetables can accumulate toxic levels. Pens or corrals with carelessweeds or grasses fertilized with just the manure can result in immediate poisoning. Highly fertilized bermudagrass pastures can also accumulate toxic nitrate levels. Nitrate accumulates and is stored in lower leaves and stems, so typically the lower leaves and stems may have higher concentrations. Cutting, drying and baling hay will not reduce nitrate levels.

Nitrate toxicity symptoms generally include death, blue mucous membranes (lack of oxygen), fast breathing, high pulse rate, weakness, staggering, uneasiness, excessive salivation, frequent urination, and dilated, bloodshot eyes. Animals treated with methylene blue may recover. But by the time an animal goes down, it is often too late to treat and rescue. A veterinarian should be called to assist with treatment, if needed, or verify the cause of death. In general, all ruminants can safely eat forages that contain up to 1 percent nitrates on a dry weight basis. Monogastrics (horses, mules and pigs) are less sensitive to nitrate intoxication.


Another problem that can occur during or immediately after a drought is prussic acid poisoning - also called hydrocyanic acid or cyanide poisoning. It is one of the most toxic and rapid-acting of any common poison. Cyogenic compounds can develop in plants that are stressed. In the rumen, these compounds are converted to cyanide, which can kill livestock. Livestock can show symptoms of intoxication within five minutes of eating plants with the poison, and may die within 15 minutes. Salivation and labored breathing occur first, followed by muscular tremors, uncoordinated movements, bloating, convulsions and death from respiratory failure.

It can accumulate in plants in the sorghum family, such as Johnson grass, sudan grass, forage sorghums and grain sorghum. It is also found in bahia, corn, cocklebur, white clover and other minor plants, but seldom at toxic levels. One problem with prussic acid is that it tends to come-and-go in the plant. It may be present for a short time and then dissipate. It appears to occur when plants are injured by herbicides, frost or drought. High concentrations of prussic acid may also be associated with rapid cell division or rapid growth, such as shortly after a rain or irrigation on previously drought-stressed fields, or warm weather after a cool period. Under good growing conditions, toxic concentrations can also form in young, rapid-growing plants.

On the positive side, prussic acid dissipates from plants that are properly cured for hay. However, in hay baled early at high moisture or plants chopped for immediate feeding, the prussic acid may not have had a chance to dissipate.


The Texas Veterinary Medical Diagnostic Laboratory tests grass and hay for nitrates for $6 per sample: 1 pound dry hay, or 5 pounds green grass mailed in a paper sack and box. Prussic acid affected forages tests cost $5 per sample. Collect prussic acid samples and ship immediately in an air tight jar. Visit their website,, for forms or call for more information.

The Texas AgriLife Extension Soil, Water and Forage Testing Laboratory also tests for nitrate for $5 per sample. Visit their website,, for collection instructions and forms.

Joe Janak is a Victoria County extension agent.



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