Net portal absorption of volatile fatty acids and L(+)-lactate by lactating Holstein cows.

Net absorption of L-lactate and VFA from the portal drained viscera of first lactation Holstein cows was measured at 4, 8, (four cows), 12, 16, and 20 wk (two cows) of lactation. Chronic indwelling catheters were installed 7 to 14 d postpartum in appropriate vessels to measure blood flow and net nutrient absorption. Cows were fed a completely mixed, 60:40 (dry basis) corn silage:supplement diet and milked every 12 h. Average metabolizable energy intake was 2.8X maintenance and mean milk production was 24.0 kg. Net absorption of lactate, any of the VFA, or their total was not affected by week postpartum. Net absorption of L-lactate, VFA, and alpha-amino N accounted in sum for 53.6% of metabolizable energy intake; contributions of each component to energy in absorbed nutrients were acetate and propionate, 29.5% each; alpha-amino N, 23.4%; L-lactate, 5.4%; n-butyrate, 5.3%; 2-methylbutyrate, 2.8%, and i-butyrate, i-valerate, and n-valerate, 1.2 to 1.6% each. Comparison of paired samples of blood and plasma showed that blood cells contribute to the transport of acetate, propionate, i-butyrate, and 2-methyl-butyrate but not of n-butyrate, i-valerate, or n-valerate.