Amino acids truly digestible in the small intestine. Factors of variation in ruminants and consequences on milk protein secretion

Abstract

The production of proteins (milk, meat, wool, hair) of ruminants can be limited by insufficient contributions of certain amino acids (AA) so called limiting AA. Contrary to what is generally suggested, the composition of the amino acids digested by ruminants is not constant. The microbial protein amount and its relatively constant AA composition buffer the variations of the composition in AA of the intestinal contents. However this composition varies according to the composition in AA of the ration and its content in the rumen undegradeable proteins. The importance of the variations of the AA composition of the digestive contents on the production of milk proteins was studied by increasing postruminal amounts of an amino acid. The postruminal supply of lysine or leucine increases the milk protein content up to 4 g/kg. The maximum increase is only 2 g/kg with methionine, histidine, phenylalanine and threonine. In all cases, the increase in the protein content is related to casein content. In early- and mid-lactation histidine and threonine also increase the volume of produced milk in a dose-related way.