The actions and interactions of a number of factors on the response of the growing lamb to oestrogen implants

Abstract

A multifactorial experiment using 576 lambs is described, designed to determine the potential value of oestrogen implants in the Australian fat lamb industry. Actions and interactions of age at implantation (5 or 9 weeks), type of oestrogen (hexoestrol or stilboestrol), dose (15 or 30mg), crossbreed (Border Leicester x Merino or Dorset Horn x Corriedale), sex (ewe or wether), and type of birth (single or twin) were examined. Owing to drought conditions nutrition was only moderate. Sixty-three lambs were lost. Mortality tended to be concentrated on 30mg-oestrogen, Dorset Horn x Corriedale, male lambs (P < 0.05). Oestrogen-treated lambs (15 and 30 mg) reached 75 lb liveweight 2.5 and 1.9 weeks earlier than untreated lambs (P < 0.001). Wether lambs grew faster than females (P < 0.01). Treatment reduced dressing percentage and carcass weight by 1.8 per cent. and 1.1 lb respectively (P < 0-001). The dose of oestrogen was unimportant but the deleterious effect of stilboestrol was greater than that of hexoestrol. Treated carcasses were downgraded (P < 0.001) and there was a highly significant interaction between sex and treatment (P < 0.001). Carcasses of ewe lambs were more seriously affected than those of wethers. Treated carcasses were longer in the leg (P < 0.05) and thinner in the loin and flank than untreated carcasses (P < 0-001). The general effect was seriously to delay maturity and market quality, with a reduction in weight of carcasses despite increased liveweight gains. It is concluded that oestrogen implantation is unlikely to have wide commercial application in fat lamb production in Australia.