Concentrations of amino acids and urea in the plasma of the preruminant calf and estimation of the amino acid requirements.

1. A study was made of factors affecting the plasma concentrations of free amino acids (PAA) and urea (PU) in calves receiving liquid diets. 2. When calves were given whole milk (approximately 0-05 kg/kg live weight) at 10.00 and 17.00 hours there were marked decreases in PAA and slight decreases in PU for about 3-4 h after the morning feed. Both PAA and PU thereafter increased slightly and then remained constant for the next 10 h. In subsequent comparative experiments samples were taken 3 h after the morning feed. The variation in plasma methionine, isoleucine, leucine, phenylalanine, tyrosine and total PAA was apparently greater between than within animals. 3. In calves transferred from a diet of whole milk to one consisting of diluted whole milk supplemented with appropriate nutrients, including amino acids, but deficient in methionine, PU increased markedly and plasma methionine decreased. 4. Two calves (50-60 kg live weight) given the latter diet supplemented to give an intake of more than 4-5 g L-methionine/d together with 0-3 g cystine/d showed marked increases in plasma methionine. This was considered to correspond with the point at which methionine requirements were met and that under these conditions the methionine requirement for these calves (mean value plus or minus SE) was 4-5 plus or minus 0-2 g/d. In the same two calves PU decreased markedly with increasing levels of methionine supplementation up to 3-9 g/d and the estimated methionine requirement (mean value plus or minus SE) was 3-9 plus or minus 0-4 g/d.