A number of plants can be toxic to cattle, and one of the most common problems occurs when cattle eat forages that have accumulated high levels of nitrate. While most plants contain some nitrate, certain species including forage sorghum, sudangrass, pearl millet and weeds such as pigweed and kochia are more likely to accumulate dangerous amounts.

As plants take up nitrate from the soil, it is converted to protein building blocks — but this process requires energy from sunlight, adequate water and favorable temperature. Nitrates accumulate in plants during periods of moderate drought because the roots continually absorb nitrate from the soil, but poor growing conditions inhibit its conversion to protein building blocks in the plant. 

During a severe drought, although lack of moisture prevents nitrate absorption by plant roots, the problem occurs when following a drought-ending rain — the roots rapidly absorb nitrate. The nitrate then accumulates to high levels, and requires seven to 14 days before the nitrates will be metabolized back to low levels. In addition, conditions such as hail, light frost or plant disease can damage plant leaves to the extent that nitrate conversion is inhibited and nitrates accumulate in the plant.

Nitrate in cattle diets is converted by rumen bacteria to nitrite and then to usable nutrients. But if cattle take in too much at once, they can’t process it fast enough, and nitrite rises to dangerous levels. This disrupts oxygen transport in the blood, so cattle show signs of lack of oxygen to the brain and muscles, leading to depressed behavior, weakness, muscle tremors and possibly death. 

Signs of nitrate poisoning can appear within one to six hours after cattle eat affected forage. Pregnant cows may abort a few days after exposure, even if they showed no other signs of illness.

When using forages that can possibly have high nitrate levels, it is best to test them before feeding. Nitrate levels are measured in parts per million (ppm) on a dry-matter basis. Forages with less than 3,000 ppm nitrate are considered safe. Between 3,000 and 6,000 ppm, the forage may be acceptable for healthy animals but should be limited in pregnant or stressed cattle. Forages in the 6,000-to-9,000 ppm range are risky and should not be the only source of feed. 

If you must use high-nitrate forage, blend it with safe forages and introduce it gradually. Anything more than 9,000 ppm is considered dangerous and can easily result in death.

Managing the risk from nitrate-accumulating plants begins with awareness and testing. Delay harvest of stressed sorghum and sudan forages for at least a week after growing conditions improve, giving the plant time to reduce nitrate levels. Raise the cutter bar to avoid harvesting the lower stalk, where nitrates are most concentrated. 

Silage can lower nitrate content by up to 60%, but even silage should be tested before feeding. If turning cattle onto forage that may be dangerous, feeding hay prior to turnout can slow grazing intake and reduce risk.

If nitrate poisoning is suspected, contact a veterinarian immediately. Treatment must be given intravenously early in the disease process to be effective. However, prevention through appropriate management is your best defense. Knowing when forages are risky and using testing, feeding strategies and timely harvest decisions can prevent nitrate problems before they start.  

Editor’s note: Robert L. Larson is a professor of production medicine and executive director of Veterinary Medicine Continuing Education at Kansas State University in Manhattan, Kan.