The effects of intravenous administration of amino acids and glucose on the milk production of dairy cows consuming diets based on grass silage

Two experiments, using intravenous infusion of nutrients, were carried out with the aim of separating milk production responses due to the provision o f amino acids as precursors of milk protein synthesis from those due to the provision of amino acids as glucose precursors. Diets were based on grass silage of restricted fermentation and barley-based supplements because it h as been suggested that these diets might provide insufficient glucose precu rsors to meet the needs of lactose synthesis. The silages used in the exper iments were of similar lactic acid contents [62 and 63 g kg(-1) dry matter (DM)] but of different water-soluble carbohydrate (WSC) contents (206 and 2 0 g kg(-1) DM in Experiments 1 and 2 respectively). In Experiment 1, four d airy cows were given the following treatments in a 4 x 4 Latin square arran gement with periods of 10 d: (1) basal diet (Basal), (2) Basal plus jugular infusion of 182 g d(-1) of amino acids simulating casein (TAA), (3) Basal plus 101 g d(-1) of essential amino acids (EAA), being the essential amino acid component of the TAA treatment and (4) Basal plus 101 g d(-1) of essen tial amino acids plus 50 g d(-1) of glucose (EAA + G), being the glucose eq uivalent of the non-essential amino acid component of treatment TAA. All in fusions increased (P < 0.05) the concentration of milk protein compared wit h Basal but only for TAA was the increase in the yield of milk protein stat istically significant (P < 0.05), amounting to 68 g d(-1). Both TAA and EAA reduced (P < 0.05) the concentration of milk fat. There was no difference between EAA and EAA + G treatments. In Experiment 2, five dairy cows were given the following treatments in a 5 x 5 Latin square design with periods of 7 d: (1) basal diet (Basal), (2) B asal plus 182 g d(-1) of amino acids simulating casein (TAA), (3) Basal plu s 182 g d(-1) of non-essential amino acids as in casein (NEAA), (4) Basal p lus 100 g d(-1) of glucose (G100) and (5) basal plus 230 g d(-1) of glucose (G230). G100 supplied the glucose equivalent of NEAA whereas G230 supplied the caloric equivalent of NEAA. Again, only for TAA was the increase in yi eld of milk protein statistically significant (P < 0.05), amounting to 83 g d(-1). Neither glucose treatment caused any statistically significant (P > 0.05) effect on the yield of milk protein nor the yield of milk lactose. It is concluded that, in both experiments, the primary nutritional limitati on on milk protein output was the supply of amino acids as precursors of mi lk protein, there being no evidence to support a primary limitation due to glucose supply.