THE EFFECT OF ABOMASAL INFUSION OF CASEIN AND RECOMBINANT SOMATOTROPIN HORMONE INJECTION ON NITROGEN-BALANCE AND AMINO-ACID FLUXES IN PORTAL-DRAINED VISCERA AND NET HEPATIC AND TOTAL SPLANCHNIC BLOOD IN HOLSTEIN STEERS

Six Holstein steers, 6 mo of age, with a BW range of 180 to 200 kg wer e used to determine effects of abomasal casein infusion and recombinan t bovine somatotropin on amino acid absorption and metabolism in the g ut and liver. Catheters were positioned in the hepatic vein, the hepat ic portal vein, two mesenteric veins, and a mesenteric artery. Using t he same basal diet, treatments consisted of 1) basal diet only (Contro l), 2) abomasal casein infusion of 300 g/d (Casein), and 3) abomasal c asein infusion plus daily injection of 20 mg of bovine somatotropin (S T). All steers were fed the basal diet at a rate of 24 g/kg of BW on a DM basis, in 12 equal meals at 2-h intervals. Initially, Casein and S T treatments were in a balanced cross-over design, and then all steers received the Control treatment. Casein infusion tended to increase BW gain but did not affect BW gain per unit of N intake. The ST treatmen t increased weight gain (P < .01) and efficiency of dietary DM (P < .0 1) and N (P < .01) utilization by more than 40% and increased the perc entage of absorbed N retained. Nonessential, essential, and total amin o acid fluxes in portal-drained viscera (PDV) and their net removal by the liver were not affected by casein infusion. However, ST tended to reduce PDV flux of essential, nonessential, and total amino acids by approximately 30%, and their removal by the liver (P < .08). As a resu lt, ST increased, compared with casein, release of essential(P < .06) and total (P < .10) amino acids from the total splanchnic pool (TSP) i nto peripheral blood. The ST treatment increased the availability of a bsorbed amino acids to peripheral tissues, but it did not affect the a mino acid profile delivered from splanchnic tissues, especially that o f essential amino acids.