No abstract

Growth-finishing programs for Holstein calves in feedlots have become popular recently in Northwest Mexico. Holstein steers respond with greater gains to high-grain diets than beef steers and are more tolerant to heat stress (1); however, Holsteins require 10% to 12% more energy for maintenance than beef breeds, and they have less muscling than beef breeds (2). In order to increase energy efficiency for growth and muscle development, the use of growth promoters (as steroid implants) is a feasible alternative. Many studies have shown that Holstein cattle have better performance responses when they are implanted at heavier weights (approximately 325 kg live weight) (3). In the same way, the administration of exogenous recombinant somatotropin (recombinant bovine growth hormone, bST) is one biotechnology strategy that increases production (meat or milk) per unit of feed consumed (4,5), but bST promotes protein accretion through different mechanisms than the steroid implants (6,7). A positive response of the effect of combining exogenous bST and implants in feedlot steers has been reported previously (8). Those researchers concluded that the anabolic effects of implants and bST are additive and possibly independent in feedlot steers; however, there is no information available on the comparative effectiveness of both additives in intact cattle. Since, at present, the finishing of intact cattle (bulls) is a widespread system in North Mexico, the objective of this experiment was to determine the effect of combining recombinant bST and anabolic implants on the growth performance and dietary energetics of Holstein bull calves fed a finishing diets. With this aim, 21 Holstein bull calves (initial weight at the start of the experiment: 368 ± 5.2 kg) were individually assigned (7 repetitions/treatment) to pens of 16 m 2 with automatic waterers and 1.2 m fenceline feed bunks. The cattle were processed and adapted to the basal diet and facilities 3 weeks before the start of the experiment. The processing consisted of vaccination against bovine rhinotracheitis and parainfluenza 3 (TSV-27, SmithKline Beecham, West Chester, PA, USA), clostridials (Fortress 7, Pfizer Animal Health, New York, NY, USA), and Pasteurella haemolytica (One Shot, Pfizer Animal Health). The cattle were treated against parasites (Ultramectin, RXV Products, Kansas City, MO, USA) and were injected with 1 × 10 6 IU vitamin A (Vita-Jec A&D "500", RXV Products). Cattle were weighed (electronic Abstract: Twenty-one Holstein bull calves (368 ± 5.2 kg) were used in order to evaluate the effects of combining recombinant bovine growth hormone (bST) and anabolic implants on growth performance and dietary energetics. Steers were assigned (7 repetitions/ treatment) to individual pens (16 m2) equipped with automatic waterers and 1.2 m fence-line feed bunks. The experiment lasted 56 days. Treatments were: 1) no bST, no implant; 2) implant; and 3) bST + implant. Compared to implanted cattle, the combination of exogenous bovine somatotropin and anabolic implants did not e...