PROTEIN NUTRITION OF DAIRY-COWS RECEIVING GRASS-SILAGE DIETS - THE EFFECTS OF POST-RUMINAL SUPPLEMENTS OF PROTEINS AND AMINO-ACIDS

Milk production and food intake responses to abomasal infusion of proteins were examined in dairy cows receiving a basal diet of high-quality grass silage ad libitum plus a barley-soya supplement at 5 kg day-1. Four cows received each of the following five treatments: (i) basal diet, (ii) basal diet plus 230 g casein day-1 infused into the abomasum, (iii) basal diet plus 273 g soya-protein isolate (SPI) day-1 infused into the abomasum, (iv) basal diet plus 273 g SPI day-1 plus methionine (MT) and tryptophan infused into the abomasum (SPI + MT), and (v) basal diet plus 273 g SPI day-1 plus a mixture of essential amino acids (AAs) infused into the abomasum (SPI+AA). The SPI+MT and SPI+AA were designed to be equivalent to the casein treatment with regard to the supply of methionine and tryptophan and all essential AAs, respectively. The cows were allocated to four columns of a 5 x 5 latin square design with 7-day periods. All infusions increased (P < 0.01) the yields of milk and milk protein relative to the basal treatment, the values being 20.2, 22.4, 21.5, 21-9 and 21.7 kg day-1 for milk yield and 626, 725, 673, 689 and 691 g day-1 for protein yield for the basal, casein, SPI, SPI + MT and SPI + AA treatments, respectively. Casein infusion produced a higher (P < 0.05) yield of milk protein than all other treatments. It is concluded that the nutritional value of a protein supplied post-ruminally cannot be assessed solely in terms of its AA composition. The possible roles of factors such as the rate of absorption of AAs and the release of biologically active peptides during digestion are discussed.