Date of this Version
Each year various regions of western range country are affected by weather and other environmental conditions that stress growing plants, causing them to accumulate nitrates. Of 183 forage samples analyzed for nitrates at the Wyoming State Chemical Lab from July 1 to December 31, 1989, 27% had potassium nitrate levels of 1.5% or greater. At these levels, when ingested by livestock and particularly cattle, the possibility of nitrate toxicoses are highly probable. In this paper, an attempt will be made to review the literature in order to give the producer a better understanding of potential problems and precautions necessary to utilize high nitrate forages.
Under normal growing conditions, stems and leaves of plants convert nitrate to protein about as fast as it is absorbed through the roots. However, this process does fluctuate and under certain environmental conditions, the speed of this conversion process is inhibited. This reaction is dependent on adequate water, energy from sunlight and a temperature conducive to rapid chemical reactions. If any one of these factors is inadequate while soil temperature is reasonably warm, the root continues to absorb nitrogen at a similar rate while storing it unchanged in the stalk and lower leaves. When these conditions exist, nitrates have accumulated in the plant (1,2).
Under normal feeding conditions, nitrate consumed by cattle is converted to nitrite and then ammonia by bacteria in the rumen. The rate at which nitrate is converted to nitrite, exceeds the conversion rate of nitrite to ammonia. Therefore, when higher than normal levels of nitrate are consumed, an accumulation of nitrite may occur in the rumen. Nitrite will then be absorbed into the bloodstream. When this takes place, hemoglobin which normally transports oxygen, is converted to methemoglobin. Methemoglobin is unable to transport oxygen to body tissues. If the amount of nitrate consumed is great enough and the animal is not treated, it will die of anoxia or lack of oxygen. However, there is also some indication that sublethal levels of higher than normal nitrate ingestion may affect growth, reproduction, milk production, in addition to vitamin A and iodine status of the animal (2).
The occurrence of nitrate poisoning is difficult to predict because it is influenced by many circumstances. Nitrate levels change rapidly in the plant (1). In addition, there appear to be large differences in the levels of nitrate that individual cattle can tolerate.
Environmental factors that affect nitrate accumulation in the plant include fertilization practices, light intensity and drought. Generally, the higher the level of nitrogen (N) fertilizer, the greater the potential for nitrate accumulation in the plant (1). The potential also appears to be greater when N is furnished by nitrate fertilizers rather than ammonium sulfate or urea.
In western range areas, lack of adequate water often increases the accumulation of nitrates by various plants. Cloudy (low light intensity) and cool days may further compound this situation. Plant species differ markedly in their ability to accumulate nitrates. Cereal grain plants, especially oats and corn, tend to be accumulators. Ryegrass, sorghum and sugar-beet tops can also cause nitrate problems. Oat hay and cornstalks cause many nitrate concerns. In addition, certain wild grasses and weeds such as pigweed, kochia, carelessweed, lambsquarter, sunflower, bindweed and many others may accumulate nitrate when the plants are stressed. Under high levels of fertilization, forage species commonly used for pasture and hay production can even develop nitrate levels potentially hazardous to animals’ health. High levels of nitrogen fertilization have also been attributed to increasing levels of nitrogenous compounds in surface and ground water. Effects of nitrate from multiple sources are additive and both feed and water should be considered when evaluating a problem (3).