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

Thirty-two mature recently shorn wethers (mean liveweight 52.1 kg) were imp lanted with 1 of 4 different hormonal growth promotants (Ralgro, Compudose, Revalor-S and Synovex-H) to determine animal production responses on dry s eason pastures. The wethers were grazed for a period of 135 days on dry sea son native Mitchell grass (Astrebla spp.) pastures infested with Acacia nil otica. The wethers were weighed monthly. Four months after commencement of the experiment, the wethers were shorn to determine wool characteristics (m icron, 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 declini ng throughout the experiment. Relative to the controls, the Compudose impla nt had increased (P<0.05) average daily gain by day 29 of the experiment. A t days 93 and 118 post-implantation, compared with the control wethers, Com pudose and Synovex-H implantation of wethers had significantly (P<0.05) inc reased 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 1 2, 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 ho rmonal growth promotant treatment on hot carcass weight, fat depth or fleec e characteristics. We conclude that under declining nutritional conditions, hormonal growth pr omotant treatment of mature wethers with either Compudose or Synovex-H impr oves liveweight performance on dry season Astrebla spp. pastures. The small improvement in liveweight may not provide economic returns for sheep desti ned 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.