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Rate, Composition and Efficiency of Growth in Feedlot Steers Reimplanted with Growth Stimulants^1,2,

The Pennsylvania State University, University Park 16802

Abstract

Eighty Charolais-cross steer calves (283 kg) were fed a moderately high-energy (2.89 Meal 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 < .06) gains by 8.1% from 0 to 84 d. Ralgro and Synovex-S increased (P < .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 < .05) feed utilization in Ralgro implanted steers, however. Ralgro and Synovex-S increased (P < .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 < .05) or energy intake (P < .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 < .04) when adjusted to an equal carcass fatness. Both growth stimulants increased hip height (P < .02), wither height (P < .08) and body length (P < .08) over C steers at slaughter.

¹ Authorized for publication as Paper No. 7796 in the Journal Series of the Pennsylvania Agric. Exp. Sta.

² The authors gratefully acknowledge Syntex Corp., Palo Alto, CA, for partial support of this study.

³ Present address: Dept. of Anim. Sci., Iowa State Univ., Ames, 50011.

⁴ Dept. of Dairy and Anim. Sci.

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