MAMMARY UPTAKE OF ENERGY METABOLITES IN DAIRY-COWS FED FAT AND ITS RELATIONSHIP TO MILK PROTEIN DEPRESSION

Mechanisms of change in milk composition in response to dietary fat supplementation were examined by measurement of plasma metabolite arteriovenous differences across the udders of cows fed fat. Four primiparous cows fitted with ruminal cannulas were assigned to a factorial arrangement of treatments: two diets and two casein infusion sites in a 4 x 4 Latin square design. Diets were formulated with yellow grease at 0 or 4% of DM. An 8% sodium caseinate solution was infused continuously at 5.04 kg/d into the rumen or abomasum during the last 5 d of each 21-d period. Plasma acetate concentration and mammary uptake were not affected significantly by added dietary fat, but plasma triacylglycerol concentration and uptake were increased, which resulted in milk with a greater fat percentage and a higher proportion of long-chain fatty acids. Reduced lipogenesis from acetate and increased beta-hydroxybutyrate uptake spared glucose for lactose synthesis, allowing increased milk yield with reduced mammary blood flow rate. Increased energy supply to mammary tissue, resulting from changes in patterns of plasma metabolites with added dietary fat, was implicated in the reduction of mammary blood flow rates.