Nitrate Poisoning in Cattle
Nitrate poisoning in cattle is a noninfectious conditions that can kill livestock. Although uncommon in normal years, poisoning occurs when cattle eat forages stressed from severe environmental conditions such as drought. The stress disrupts normal plant growth and may cause the plants to accumulate too much nitrate. Knowing the causes, symptoms and treatments for this disease can help producers prevent losses. Sampling and testing can indicate when forages pose a danger to livestock.
Nitrate is present to some degree in all forages, and technically, nitrate poisoning is better described as nitrite poisoning. When livestock consume forages, nitrate is normally converted in the rumen from: nitrate to nitrite to ammonia to amino acid to protein. When forages have an unusually high concentration of nitrate, the animal cannot complete the conversion and nitrite accumulates. Nitrite is absorbed into the bloodstream directly through the rumen wall and converts hemoglobin (the oxygen carrying molecule) in the blood to methemoglobin, which cannot carry oxygen. The blood turns a chocolate brown color rather than the usual bright red. An animal dying from nitrate (nitrite) poisoning actually dies from asphyxiation, or a lack of oxygen. Nitrate poisoning usually does not occur rapidly, but over time, depending on how high the nitrate level in the forage.
Nitrate poisoning can occur when: Forage consumed contains high levels of nitrate; The diet changes rapidly or suddenly; Parasitism or other conditions causing anemia; Livestock consume supplements of urea or high-protein feeds along with forage containing moderate levels of nitrate; and/or Livestock directly consume nitrite.
Acute nitrate toxicity symptoms generally include death, blue mucous membranes (lack of oxygen), fast breathing, high pulse rate, weakness, uneasiness, excessive salivation, frequent urination and dilated and 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 verify the cause of death.
Plants need nitrogen for growth and development. However, when drought prevents them from converting the nitrogen they absorb into new growth, nitrate levels may rise. Many kinds of plants can accumulate nitrate. Plants in the sorghum family — Johnson grass, sudan grass, forage sorghum and sorghum hybrids — are generally implicated first. “Oat-hay” poisoning is caused by high nitrate levels. Corn, small grains, careless weed or pigweed, sunflower and leafy vegetables that are highly fertilized can accumulate toxic levels. Turning cattle into holding pens or corrals full of manure with careless weeds or grasses can result in immediate poisoning. Nitrates do not accumulate when there is normal rainfall or irrigation. Under those conditions, nitrate nitrogen absorbed by roots and moved into the plant is rapidly transformed into plant proteins. However, under dry conditions, plant roots continue to absorb small amounts of nitrogen, but the plant has too little water to keep growing. Nitrate accumulates and is stored in lower leaves and stems, ready for the plant to mobilize and use when rapid growth resumes. Nitrate levels can change from day to day and even from morning until evening. Small grains can accumulate toxic levels of nitrate on overcast days. In cloudy periods, plants continue to absorb nitrogen from the soil but lack the photosynthetic activity to convert the nitrogen into proteins.
Producers can take steps to help prevent nitrate poisoning: Never turn hungry animals into possibly high nitrate forages. During drought, producers sometimes “turn onto” temporary forages to help animals in poor condition. The combination of poor body condition, high nitrate levels in the forage and high consumption can be deadly. Turning one old cow into a new field to observe is not an effective test for nitrates, because cattle tend to bite the tops of plants first, where the concentration is lowest. As cattle are forced to eat the lower plant parts, poisoning could occur later when it is not suspected. Have hay tested before feeding if you suspect that it is high in nitrate. Nitrate levels remain constant in hay. If hay is high in nitrate, feed carefully with an energy supplement or in combination with low protein forages, or other hay low in nitrates. Never feed high-nitrate hay free choice.
Texas Agricultural Extension