Utilization of sulfur and other mineral elements by growing dairy heifers treated with bovine somatotropin

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Abstract
Apparent absorption and retention of Ca, P, Mg, K, Cu, Mn, Zn, and S were determined in Holstein heifers treated with recombinant bovine somatotropin (bST). Six heifers (initial BW averaged 104 kg) were assigned to various doses of bST in a 6 × 6 Latin square design with six consecutive 21-d periods. Treatments were daily i.m. injections of 0, 6.7, 33.3, 66.7, 100, or 200 μg of bST/kg of BW on d 8 to 21 of each period. All heifers were fed a hay-concentrate diet, and nutrient digestibilities and balances were determined on d 14 to 20 of each period. Mineral concentrations in feed, water, excreta, and blood plasma were determined with an inductively coupled argon plasma-emission spectrometer. Exogenous bST did not affect apparent absorption or retention of Ca, P, Mg, K, Cu, Mn, or Zn. Plasma CU concentration was greater (P < .05) in heifers receiving bST (82 μg of Cu/dl) than in those receiving excipient (73 μg of Cu/dl); plasma concentrations of other elements were not affected by bST. Injection of bST did not affect S absorption, but urinary S excretion decreased (P < .05) and S retention increased (P < .05) as dose of bST increased. The amount of S retained (1.5 g/d) when the heifers received the largest dose of bST was approximately 50% greater than the amount retained when heifers received excipient. These results demonstrate that bST affects the postabsorptive metabolism of S. Generally, bST did not affect nutrient digestibilities. Effects of bST on dietary mineral requirements were not determined because of the relatively short (14 d) treatment periods.

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