Titration of the recombinant bovine somatotropin dosage that maximizes the anabolic response in feedlot steers.

The objective of this study was to determine the minimum dosage of recombinant bovine somatotropin (bST) required to elicit maximum depression in plasma urea nitrogen (PUN), an indicator of anabolic activity. Twenty-four steers (389 kg) were blocked by weight into six pens. Six steers were placed on each of the following bST doses: 0, 8, 16 and 32 mg bST/d. Treatments were administered once daily via subcutaneous injections for 21 d. Steers were weighed and jugular blood samples were taken on d 0, 1, 4, 7, 10, 13, 16 and 21 at 1400, approximately 4 h after feeding. Delta PUN (DPUN) was calculated as PUN - d 0 PUN. There was no dose x time interaction (P = .94) in DPUN. Maximum reduction in DPUN with bST occurred by d 7 (P less than .05). Linear (P less than .01) and quadratic (P less than .05) orthogonal contrasts indicated that DPUN depression increased with bST administration, with maximal reduction calculated to occur with 23 mg (59 micrograms/kg) bST/d. There was no further decrease in DPUN with 32 than with 16 mg bST, indicating that the minimum daily dose is at least 16 mg but no more than 23 mg. A similar dose response was observed in daily gain. Results from this study indicate that bST reduced PUN in a dose-dependent manner and that 41 to 64 micrograms/kg body weight maximized the anabolic effect of bST in growing steers.