EFFECT OF DIETARY-FAT AND POSTRUMINAL CASEIN ADMINISTRATION ON MILK-COMPOSITION OF LACTATING DAIRY-COWS

Perturbation of the amino acid status of lactating dairy cows was undertaken to characterize the mechanism of dietary fatinduced milk protein depression. Four first lactation heifers fitted with rumen cannulas were assigned to a factorial arrangement of treatments: two diets and two casein infusion sites within 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 (SE = .05) into the rumen or abomasum during the last 5 d of each 21-d period. Dry matter, N, energy, and NDF digestibilities were reduced by dietary fat treatment, but intakes of digestible DM, N, and energy were not affected. Yellow grease supplementation improved milk yield 14% and milk fat content 7%, whereas lactose, CP, and casein N percentages in milk were reduced. Abomasal sodium caseinate infusions had no effect on milk yield or composition other than to increase protein and casein N concentrations. Dietary fat-induced milk protein depression remained evident during abomasal casein treatments, indicating a mechanism independent of amino acid status of the cow. Improved intestinal availability of amino acids partially reversed detrimental effects of dietary fat on milk protein fractions.