Effect of dose of bovine somatotropin on nutrient utilization in growing dairy heifers

Growing Holstein heifers [n = 6; 104 kg initial body weight (BW)] were used to investigate the effects of a daily dose (0, 6.7, 33, 67, 100 and 200 μg/kg BW) of bovine somatotropin (bST) on nutrient utilization. A Latin square design was used, and treatments (daily doses of bST) were administered by intramuscular injection for 14-d periods. Intakes of a total mixed ration were adjusted according to BW at the start of each period. Energy was supplied to allow an estimated daily gain of 0.75 kg, and crude protein intake was increased 15% above recommendations to allow for anticipated increases in protein deposition with bST treatment. Nitrogen balance and nutrient digestibilities were determined on d 7 to 13, and blood samples were obtained on d 14 of treatment. Digestibilities of dry matter, organic matter and N were similar (70 ± 1%) and not affected by treatments (p > 0.1). Nitrogen retention increased, and plasma urea N and urinary N excretion decreased in a dose-dependent manner. Retention of N in heifers receiving the highest bST dose was 23% greater (p < 0.001) than the zero dose treatment. Serum concentration of insulin-like growth factor I (IGF-I) was also increased with bST treatment in a curvilinear dose-dependent manner. Plasma glucose increased (5 to 8%, p < 0.001) with the three higest doses of bST, but serum concentration of insulin was not altered. The ability of bST to enhance N utilization in a dose-dependent manner was due to postabsorptive changes in nutrient utilization, and the changes in IGF-I are consistent with a possible role for this somatomedin in mediating a portion of the effects of ST.