Drought Conditions Can Cause Nitrate Poisoning in Cattle and Horse

Ann Swinker PH. D., Extension Horse Specialist

Nitrate poisoning can occur in cattle and horses grazing pastures or eating hays that have accumulated high levels of nitrates during growth. Nitrates accumulate in plants when excessive rates of fertilizer have been applied or when plants have been drought stressed. Nitrate levels tend to be higher in the lower one-third of the plant and accumulate more at night and on cloudy days. Some species of plants that are known to accumulate nitrates include Johnsongrass, sorghum, sweetclover, bromegrass, orchardgrass, lambsquarter, oat hay, rape, barley, wheat and corn. Ensiling forages suspected of having high nitrate levels usually reduces the chances for problems. However, hay continues to be dangerous as the accumulated nitrates do not decrease over time.

Low levels of nitrates can cause abortion without any other symptoms. Severely affected animals develop muscle tremors, lose coordination and become weak. Moving these animals will initiate difficult breathing commonly followed by collapse and death. Nitrate poisoning is often confused with prussic acid poisoning but is distinguished by a marked difference in blood color of affected animals. Animals poisoned by nitrates will have chocolate brown blood while those poisoned by prussic acid will have bright red blood. Treatment by a veterinarian can be effective if initiated early. Prevention includes mixing affected forages with normal forages to dilute the nitrate levels. Raising the cutter bar 10-12 inches to avoid cutting the lower one-third of the plant and not cutting drought-stressed forages for several days after a rain also helps avoid problems. If high nitrate levels are suspected, samples of the forage should be submitted to a competent laboratory for analysis before the forage is fed.

So what are some of the toxic levels of Nitrate in feed for livestock? The level of nitrate that causes toxicity in ruminants varies depending on rate of intake, diet, acclimation to nitrate and nutritional status. As a rule, forage containing less than 5,000 ppm NO3 on a dry matter basis is safe. Forage containing 5,000 to 10,000 ppm NO3 is considered potentially toxic when provided as the only feed. Forage containing over 10,000 ppm NO3 is considered dangerous but often can be fed safely after proper dilution with other feeds ( 1998, Colorado State University Cooperative Extension Fact Sheet). Generally, horses can tolerate a higher amount of nitrate levels in feed than cattle. However, sick, young or older horse may not be able to tolerate high levels. Also, check the nitrate content of water when a nitrate problem is suspected. Routinely testing any forage suspected of being excess in nitrates.

Nitrate poisoning can be a problem for livestock producers during a drought. Drought, excessive soil nitrogen, shade, frost, certain herbicides, acid soils, low growing temperatures and nutrient deficiencies can contribute to high nitrate levels in plants. Stems usually have higher nitrate content than leaves.

So what can you do? Use good management practices to avoid poisoning. Fertility programs consistent with plant needs and growing conditions minimize the problem. Test potentially dangerous forage before feeding. Hay containing high nitrates can be diluted with other feeds to be fed safely.