Rate, composition and efficiency of growth in feedlot steers reimplanted with growth stimulants

Eighty Charolais-cross steer calves (283 kg) were fed a moderately high-energy (2.89 Mcal ME/kg) diet for 189 d to examine the effects of reimplantation of 36 mg of zeranol (Ralgro) or 200 mg progesterone plus 20 mg estradiol benzoate (Synovex-S) on the rate, composition and efficiency of gain, skeletal size and carcass parameters in a comparative slaughter trial. The implant treatments included unimplanted controls (C), Ralgro initially (R1), Synovex-S initially (S1), Ralgro initially and a reimplant at 84 d (R2) and Synovex-S initially and a reimplant at 84 d (S2). Both implants increased (P less than .06) gains by 8.1% from 0 to 84 d. Ralgro and Synovex-S increased (P less than .01) daily gains by 11.5% and 25.2%, respectively, from 84 to 189 d. The duration of the response to a single implant appeared to be in excess of 140 d; thus, reimplantation did not further increase daily gains. Reimplantation did improve (P less than .05) feed utilization in Ralgro implanted steers, however. Ralgro and Synovex-S increased (P less than .01) the rate of empty body (EB) protein accretion by 14.1% and 24%, respectively, without affecting EB fat growth. The efficiency of protein gain per unit protein (P less than .05) or energy intake (P less than .04) was improved, but the efficiency of energy gain per unit energy intake was not affected by implantation. Carcass weights of implanted steers were 5% greater (P less than .04) when adjusted to an equal carcass fatness. Both growth stimulants increased hip height (P less than .02), wither height (P less than .08) and body length (P less than .08) over C steers at slaughter.