Effect of growth promotant implants on liveweight change, wool and carcass characteristics of mature wethers grazing dry season pastures

Abstract

Summary. Thirty-two mature recently shorn wethers (mean liveweight 52.1 kg) were implanted with 1 of 4 different hormonal growth promotants (Ralgro, Compudose, Revalor-S and Synovex-H) to determine animal production responses on dry season pastures. The wethers were grazed for a period of 135 days on dry season native Mitchell grass (Astrebla spp.) pastures infested with Acacia nilotica. The wethers were weighed monthly. Four months after commencement of the experiment, the wethers were shorn to determine wool characteristics (micron, yield and staple length). At 2 weeks post-shearing the wethers were slaughtered to determine carcass attributes. Serial rumen ammonia concentrations indicated that diet quality was declining throughout the experiment. Relative to the controls, the Compudose implant had increased (P<0.05) average daily gain by day 29 of the experiment. At days 93 and 118 post-implantation, compared with the control wethers, Compudose and Synovex-H implantation of wethers had significantly (P<0.05) increased liveweight and average daily gain. Average daily gain of the control, Compudose and Synovex-H wethers to day 93 and 118 was 3, 32 and 33, and 12, 43 and 43 g/day respectively. There was no effect of Ralgro or Revalor-S on liveweight or average daily gain. There was no significant effect of hormonal growth promotant treatment on hot carcass weight, fat depth or fleece characteristics. We conclude that under declining nutritional conditions, hormonal growth promotant treatment of mature wethers with either Compudose or Synovex-H improves liveweight performance on dry season Astrebla spp. pastures. The small improvement in liveweight may not provide economic returns for sheep destined for Asian export or manufacturing meat markets. An implant dose of 8 mg of oestradiol (Compudose) or a combined dose of 75 mg of testosterone and 7.5 mg of oestradiol (Synovex-H), was adequate to stimulate liveweight gain under these conditions. The effects of implantation on carcass and fleece attributes require further investigation.