49624 NZ Grasslands Association.pmd

Although plant roots take up nitrogen (N) both as ammonium (NH 4 ) and nitrate (NO 3 ) ions, under most soil conditions uptake of NO 3 dominates. Once absorbed by the plant roots, the NO 3 is reduced to NH 4 , which is subsequently assimilated into organic compounds. However, when the ra te of uptake exceeds the rate of NO 3 reduction, accumulation of NO 3 in plants occurs. Ruminant animals with high NO 3 levels in their diets accumulate nitrite (NO 2 ). 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 NO 3 content. The most common causes of high NO 3 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 NO 3 accumulating plant species, such as annual weeds. In this review paper, the processes of uptake and assimilation of N by plants, factors affecting NO 3 accumulation in plants, and the treatment and prevention of nitrate poisoning in grazing animals are discussed.