EFFECT OF RUMINALLY DEGRADED PROTEIN ON PROTEIN AVAILABLE AT THE INTESTINE ASSESSED USING BLOOD AMINO-ACID-CONCENTRATIONS

The effect of increasing amounts of ruminally degraded protein on prot ein available at the intestine was studied using the concentration of blood plasma branched-chain amino acids as an indicator of protein flo w to the small intestine. Five ruminally cannulated cows in midlactati on were used in a 5 x 5 Latin square design experiment and were fed a diet containing 20% alfalfa silage, 40% corn silage, 30% shelled, coar sely ground corn, and 8% soybean meal (DM basis). Each experimental pe riod was 17 d. Each period contained subperiods: 1 (5 d), 2 (5 d), and 3 (7 d). In addition to the basal diet, cows were given casein infusi ons of 0, .5, 1.0, 1.5, and 2.0 kg/d. During subperiod 1, the full amo unt of casein was infused into the abomasum (A100); in subperiod 2, 10 % of each casein level was infused into the abomasum (A10); and, durin g subperiod 3, the full amount of casein was infused into the rumen (R 100). The concentration of branched-chain amino acids (Ile, Leu, and V al) in blood plasma increased as amounts of casein infused into the ab omasum increased to the full amount. There was no increase in branched -chain amino acids when the same amount of casein was infused into the rumen, suggesting that ruminally degraded protein was adequate in the basal diet and the increased supply of degraded protein from ruminall y infused casein did not increase the now of protein to the small inte stine.