Determination of the first-limiting amino acid for milk production in dairy cows consuming a diet of grass silage and a cereal-based supplement containing feather meal

Lactating dairy cows consuming a diet of grass silage and a cereal-based su pplement containing feather meal were given intravenous infusions of amino acids to determine the first-limiting amino acid for milk production, methi onine having been shown to be not-limiting in a previous experiment. The th ree infusion treatments were a mixture of methionine, lysine, histidine and tryptophan (4AA); the mixture without lysine (-Lys); and the mixture witho ut histidine (-His). The 4AA treatment markedly increased the yield of milk protein by about 18% and this response was not diminished by omission of l ysine. However, exclusion of histidine produced no response over basal, con firming histidine as the first-limiting amino acid. In a second experiment, lactating cows receiving a similar basal diet were used to examine the eff ects on milk production of progressively substituting avian blood meal (ric h in histidine and poor in methionine) for part of the feather meal. Blood meal was substituted for 0, 0.10, 0.20 and 0.40 of the feather meal in the supplement. The yield of milk protein was increased by about 15% by the fir st level of inclusion of blood meal, but there was no further response beyo nd the first level of inclusion. The results of the feeding trial confirm t hat dietary addition of protein rich in histidine and of low ruminal degrad ability substantially increased milk production with this basal diet, altho ugh it should be noted that the calculated supply of all the essential amin o acids were also increased, by varying degrees, by substitution of blood m eal. The results of the two experiments are discussed in relation to the li kely importance of histidine as a limiting amino acid in dairy cows consumi ng diets typical of those used in practice. (C) 1999 Society of Chemical In dustry.