Management of highly fecund ewe types and their lambs for 8-monthly lambing. 2. Effect of weaning age and sex on lamb growth and carcass traits

Abstract

The effect of weaning at ages ranging from 6 to 13 weeks on growth of lambs from Booroola Merino x Poll Dorset (BD), Trangie Fertility Merino x Poll Dorset (TD), and Border Leicester x Merino (BLM) ewes was examined over 2 years as a component of management in an 8-monthly lambing system. Skin weight and carcass traits were also analysed. In the first year, there was a significant (P<0.01) difference between growth rates to 3 months of age of lambs weaned at 12, 9, and 6 weeks (222.5 ¦ 4.4, 204.8 ¦ 4.4, and 180.0 ¦ 4.6 g/day, respectively). In the second year, lambs were weaned at weekly intervals from 6 to 13 weeks of age, and growth rates were higher because of better seasonal conditions. Using regression coefficients, lambs weaned at 12 and 9 weeks were estimated to have higher (P<0.01) growth rates by 17 and 8%, respectively, than lambs weaned at 6 weeks. There was no difference between lambs weaned at 6 and 9 weeks for liveweight at 5 months, but those weaned at 12 weeks were 5% heavier (P<0.01). Postweaning mortality was higher amongst early-weaned lambs. Progeny from TD and BLM ewes had 7% higher growth rates and liveweights than those from BD ewes (P<0.01). Male lambs (rams and wethers) grew faster (P<0.01) than ewes, and in the second year there was a 5% liveweight advantage of rams over wethers (P<0.01). Weaning age had no effect on any carcass trait. Carcass weight accounted for most variation in carcass traits, and its inclusion in the model removed the effect of type of rearing. There was a significant (P<0.01) crossbred type x carcass weight interaction for fat depth at the C site, due to progeny of BLM ewes being fatter than the other crossbred types at heavier carcass weights. The regression coefficients for fat depth at the C site on carcass weight were 0.39 ¦ 0.05, 0.31 ¦ 0.04, and 0.60 ¦ 0.05 mm/kg for BD, TD, and BLM progeny. Carcasses of rams were leaner than those of wethers, which were leaner than those of ewes (P<0.01). There was no advantage from early weaning in management for an monthly lamb production system because lamb growth and survival were depressed.