VARYING DEGRADATION RATES OF TOTAL NONSTRUCTURAL CARBOHYDRATES - EFFECTS ON NUTRIENT-UPTAKE AND UTILIZATION BY THE MAMMARY-GLAND IN HIGH PRODUCING HOLSTEIN COWS

Six ruminally cannulated Holstein cows at 56 to 77 d of lactation were fed three total mixed rations that varied in ruminal degradation rate s of total nonstructural carbohydrates (6.04, 6.98, and 7.94%/h). The design was a 3 x 6 Latin square with 21-d experimental periods. Cows w ere catheterized in the jugular vein and in one of the caudal superfic ial epigastric veins. Increases in the ruminal degradation rate of tot al nonstructural carbohydrates 1) elevated mammary blood flow and bloo d concentrations of nonessential amino acids; 2) decreased arterioveno us differences and extraction rates of essential and most nonessential amino acids, but not of Gln and Glu (analyzed together) and glucose; 3) increased mammary uptake of Gln, Glu, and glucose; milk protein con centrations of Glu, Pro, and Asp; and total nonessential amino acids; and 4) did not alter uptake to output ratios of amino acids in the mam mary gland, however, of the nonessential amino acids, only the uptake to output ratios for Glu and Gln increased (19%). Glucose and amino ac id arteriovenous differences were not strongly correlated with. their arterial concentrations. Increases in the ruminal degradation rate of total nonstructural carbohydrates increased intestinal digestibilities of total nonstructural carbohydrates and protein and increased uptake of energy substrates by the mammary gland, resulting in a 13% improve ment in the utilization of nonessential amino acids for milk protein s ynthesis and in higher milk production and milk protein yields.