Anabolic implant and frame size effects on growth regulation, nutrient repartitioning and energetic efficiency of feedlot steers

Rates of growth and partitioning of nutrients among tissues were measured in large (Simmental x Hereford x Brahman]; n = 34) and very large (Chianina x Angus and Maine Anjou x Angus; n = 37) steers implanted with different anabolic growth regulators. All cattle were fed individually a whole shelled corn (13% crude protein) diet. Implant strategies were: none (n = 13), Ralgro 36 mg (n = 15), Ralgro 72 mg (n = 14), Synovex-S (n = 15) and Ralgro 36-Synovex-S (n = 14) administered at d 0 and 90. Empty body composition of all cattle was measured initially and at 90 d by D2O dilution procedures and at slaughter (average, 182 +/- 4.1 d) by carcass specific gravity. Empty body weight for large and very large cattle averaged 274 and 324 kg (P less than .05) initially and 497 and 603 kg (P less than .05) at slaughter. Empty body protein differed (P less than .05) for large and very large steers and averaged 51 and 61, 67 and 79, and 87 and 103 kg initially, at midpoint and at slaughter, respectively. Percentage empty body fat was lower for very large steers (13.5 vs 15.6%) initially (P less than .05) but was similar for very large and large steers at the midpoint (18.7 vs 18.1%) and at slaughter (23.2 vs 21.9%). Daily rates of empty body gain (DEBG) were greater (P less than .05) for very large vs large steers for both growing and finishing periods and averaged 1.53 vs 1.26 kg/d overall. Daily rates of protein gain (DPG) were similar for very large and large steers for the growing phase (204 vs 202 g/d) but greater (P less than .05) in very large steers for the finishing phase and overall (253 vs 204, and 229 vs 202 g/d). All implant strategies, except R36, increased DEBG and DPG and tended to decrease the percentage of fat in daily gain. In both large and very large cattle, implant growth regulators increased growth rate and partitioned nutrient use away from fat toward protein accretion, with the magnitude of partitioning toward protein increasing with greater rates of growth. These data indicate that anabolic growth regulators are viable strategies to enhance lean beef production in steers, regardless of animal size.