Mature sheep were fed a maintenance ration of timothy hay and protein supplement and 1, 2, 4 and 5% of urea or potassium nitrate (KNO3) or combinations of these compounds. The effects of the additions on rumen fluid ammonia nitrogen and pH, whole blood ammonia nitrogen, plasma urea nitrogen, total hemoglobin, methemoglobin, total serum protein, the percentage of serum albumin, alpha, beta and gamma globulins and thyroid activity were measured. Sheep consumed 4% of KNO3 when it was mixed into the ration and the animals were gradually adjusted to increasing levels. Nitrate was rapidly reduced to ammonia in the rumen without an apparent need for adaptation. No signs of toxicity were observed at any level of KNO3. Sheep receiving a high roughage maintenance ration consumed 4% and efficiently utilized 1 or 2% urea. Blood ammonia levels did not reach levels associated with toxicity at any level of urea used. The plasma urea level seemed to reflect the overall change in ammonia production in the rumen.
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Carver et al. (1974) studied this question.