Prophylactic effect of L-cysteine to acute and subclinical nitrate toxicity in sheep

Four rumen fistulated Suffolk wethers were allocated to a 4×4 Latin square designed experiment. High nitrate (1.5 g NaNO3 kg−0.75 body weight), high nitrate with high L-cysteine (0.55 g sulphur equivalent kg−0.75 body weight), low nitrate (0.75 g NaNO3 kg−0.75 body weight), and low nitrate with low L-cysteine (0.275 g sulphur equivalent kg−0.75 body weight) were administered into the rumen through fistulae as a single dose after a morning meal. Gaseous exchanges were monitored by an open circuit respiratory system using a hood over the animalʹs head. High or low L-cysteine remarkably decreased nitrite production from ruminal reduction of high or low nitrate. Consequently, methaemoglobin formation was suppressed by L-cysteine in both levels of nitrate. Oxygen consumption, carbon dioxide production and metabolic rate were depressed as methaemoglobin was formed. L-cysteine suppressed the pulmonary dysfunction induced by methaemoglobin. L-cysteine equivalent to 60% of the upper allowance of dietary sulphur appeared to be useful as a prophylactic for acute poisoning of nitrate. Thus, dosage of L-cysteine can be adjusted to correspond with the nitrate content in feeds.