A review of factors affecting and prevention of pasture-induced nitrate toxicity in grazing animals

Abstract

Although plant roots take up nitrogen (N) both as ammonium (NH4 +) and nitrate (NO3 -) ions, under most soil conditions uptake of NO3 - dominates. Once absorbed by the plant roots, the NO3 - is reduced to NH4 +, which is subsequently assimilated into organic compounds. However, when the rate of uptake exceeds the rate of NO3 - reduction, accumulation of NO3 - in plants occurs. Ruminant animals with high NO3 - levels in their diets accumulate nitrite (NO2 -). Nitrite is absorbed into the blood and combines with hemoglobin to form methemoglobin. This condition is known as nitrate poisoning (i.e., methemoglobinaemea). Nitrate poisoning occurs when animals eat forage material with high NO3 - content. The most common causes of high NO3 - content in forage tissue include: (i) high application of N fertilizers; (ii) drought conditions; (iii) damage to plant tissue (such as defoliation as a result of herbicide application); (iv) low light intensity, which reduce photosynthetic activity; and (v) presence of NO3 - accumulating plant species, such as annual weeds. In this review paper, the processes of uptake and assimilation of N by plants, factors affecting NO3 - accumulation in plants, and the treatment and prevention of nitrate poisoning in grazing animals are discussed. Keywords: forage crops, methemoglobinaemea, methylene blue, N fer tilizer, nitrate accumulation, nitrate poisoning, nitrate reductase