Net nutrient absorption in beef steers fed silage or high concentrate diets containing four levels of limestone

Net absorption of ammonia N, urea N, glucose, L-lactate, and volatile fatty acids was measured in four Hereford x Angus steers fed ad libitum orchard grass-clover silage. Net absorption was calculated as the product of blood plasma flow from portal-drained viscera and the difference in concentration between plasma from the portal vein and femoral artery. Measurements were made at 15-minute intervals for 4 hours. The steers were then adapted to four 92% concentrate diets containing 0.64, 1.42, 2.21, or 3.00% limestone. Net absorption measurements were repeated in a 4 x 4 Latin square design. Portal blood flow was slower (P less than 0.05), net ammonia N absorption was greater (P less than 0.05), and net absorption of L-lactate and propionate was lesser (P less than 0.10) when steers were fed silage than when they were fed 92% concentrate. There was net loss of urea N and glucose from plasma to the gut regardless of diet. Dietary limestone level did not affect net absorption rates of any nutrient measured except L-lactate, which decreased (P less than 0.10) as limestone level in the diet increased.