Nitrates and Nitrites are found in soils, forage feeds and as contaminants of water from streams, dams, and bores. Nitrates are converted to the more toxic Nitrites by bacterial action. Certain plants may concentrate Nitrates in stalks or leaves especially in moist or acid soils or when frosted. Nitrogen fertilizers are a potential source of poisoning of livestock. Ruminants are more susceptible to poisoning by Nitrates and Nitrites than horses where toxicity is comparatively rare.
The mechanism oof action is that first Nitrates are converted to Nitrites by gut bacteria. Nitrites, following absorbtion, react with the Iron in Haemoglobin converting it from the ferrous to the ferric form causing haemoglobin to convert to methemoglobin which cannot transport oxygen. Clinical signs are those of oxygen deprivation with rapid breathing and pulse, exercise intolerance, discoloured bluish mucus membranes and chocolate brown fresh blood. Veterinary treatment and removal from the nitrate source are required.
Broodmares. Another toxic effect of Nitrates is through interference with Iodine uptake by the Thyroid gland and Nitrate is regarded as a potential endocrine disruptor because of this. Mares grazing on high Nitrate pasture has been linked to dysmaturity in foals, consistent with clinical signs of hypothyroidism. Theoretical safe levels for pasture Nitrate have been suggested at maximum 2.0g/Kg Dry Matter though levels well above this are commonly recorded on stud farms in Australia without apparent ill effect. Drinking water Nitrate levels should not exceed 400mg/Liter and for Nitrite 30mg/Liter, the latter being 10-15 times more toxic.