Nitrate Toxicity: A Forage-Related Animal Disorder
Nitrate is common in most forage plants. Under most conditions, it is the primary form of plant available nitrogen (N) in the soils. Nitrification, a process by which ammonium forms of N are converted into nitrate forms, occurs rapidly in the soils. As a result, when N is applied to the soils as ammonium forms, such as urea, forage plants will most likely take N in the form of nitrate.
Negative impacts, such as animal death, may occur when animals consume large amounts of nitrate in forage. The death occurs because of nitrate toxicity. The toxicity is common when ruminants ingest nitrate in excess of the ability of rumen microbes (microorganisms living in rumen) to convert the nitrite intermediate to ammonia. Rumen microbes need ammonia for their amino acid needs for protein production. However, if nitrite utilization by the rumen microbes is less than nitrite produced, some nitrite is absorbed by the bloodstream. Hence, it restricts the ability of bloodstream to transport oxygen to the body tissue. The symptoms of nitrate poisoning include rapid breathing, muscle tremors, incoordination of movements, diarrhea and frequent urination. Blood of affected animal becomes chocolate brown color.
Nitrate toxicity is most likely to occur in grasses when they receive regular N fertilization. Warm-season annuals such as corn, sorghum and pearlmillet may have high potential of accumulating high nitrate concentration as they normally receive high rate of N fertilizer. Weather conditions have great influence on nitrate accumulation in plant tissues. For example, drought, frost, plant diseases and cloudy weather can contribute to the maintenance of high levels of nitrate in the plants. Because these stressed conditions slow or prevent normal nitrate metabolism in the plants and cause nitrate concentration to be maintained high levels for long time. Also, there is a possibility of nitrate toxicity in situations when drought stressed crops not suitable for grains are grazed or harvested for feeding livestock.
Nitrate concentration varies depending on the components and age of the plants. Usually stems are high in nitrate concentration, especially the lower one-third of the stem. Younger plants or younger tillers on old plants contain the highest levels nitrate concentration. Greenchopped forage can also cause nitrate toxicity because nitrite can be produced within the chopped forage before consumption by animals.
Nitrate concentration is normally stable during storage of dry hay. Microbial activity associated with silage fermentation may reduce nitrate levels by more than one-half. However, the low moisture silage (below 50% moisture) that results under drought conditions can lead to reduced microbial activity and to the occurrence of high levels of nitrate.
Forage with small amounts of nitrate is not harmful. However, caution should be taken, especially for pregnant animals, when forages with higher nitrate are used to feed animals. In general, forages with up to 0.44% nitrate in the dry matter are considered safe to feed for all classes of cattle. If nitrate poisoning is suspected, animals should be moved to some other forage sources. Feeding with an emergency supplement such as corn will be useful. Because corn can hasten microbial utilization of nitrate thus reducing nitrite concentration in the rumen.
To determine whether a significant risk exists for harming livestock, a quick field test for forage nitrate concentration can be done using Nitrate Test Kits. However, for accuracy and authentic determination, laboratory analyses for nitrate testing are recommended before feeding animals.
Anowar Islam is an assistant professor and the University of Wyoming Cooperative Extension Service Forage Agroecologist in the Department of Plant Sciences in the College of Agriculture and Natural Resources. He can be reached at (307) 766-4151 or mislam@uwyo.edu.
