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The growth and carcass characteristics of 627 crossbred lamb progeny from 20 Poll Dorset sires was studied. The sires were selected on the basis of Australian sheep breeding values (ASBVs) for postweaning growth (PWWT), depth of loin muscle (PEMD) and depth of subcutaneous fat (PFAT). Lambs were weaned either at 20- or 30-kg liveweight and then within each of these groups were maintained at their weaning weight for 55 days or fully fed, giving four treatment groups. Restricted lambs were realimented subsequent to the period of feed restriction and all lambs within each treatment group were slaughtered when their mean liveweight reached 45 kg. After adjusting for the weaning group and weaning age, liveweight at weaning depended on birthweight, rearing type and sex. For the late-weaning group, it also depended on whether the lamb was born as a single or a multiple and on sire ASBVs for PWWT and PEMD. This was such that liveweight increased by 0.14 � 0.05 kg for each unit increase in sire ASBV for PWWT and decreased by 0.55 � 0.20 kg for a unit increase in sire ASBV for PEMD. During the period of feed restriction, sire ASBV for PWWT had an effect on the growth rate of lambs in all treatments with a coefficient of 1.67 � 0.52. Lamb growth rate during the realimentation phase increased with increasing ASBVs for PWWT at a rate estimated as 2.21 � 0.49 g/day for each unit increase in PWWT. Early-weaned (20 kg), restricted lambs grew on average 26.0 � 3.4 g/day faster during the refeeding period than the early-weaned, fully fed lambs, and for late-weaned lambs (30 kg) the difference was 35.5 � 5.4 g/day. Lambs subjected to a restricted intake of pasture produced carcasses 0.79 � 0.32 mm fatter on average than those fully fed as measured at the GR site (110 mm from the midline over the 12th rib). As the sire PFAT ASBV increased, the carcasses of the progeny were fatter at the GR site, fat C site (over the longissimus muscle at the 12th rib), the rump site (30-mm distal to the lumbar?sacral junction over the gluteus medius muscle) and in terms of carcass fat percentage. As the sire PEMD ASBV increased, progeny carcasses had larger loin muscle cross sectional areas, and as the sire PFAT ASBV increased, there was a decrease in loin cross sectional area. There was no impact of treatment group on lean percentage. As the sire PFAT ASBV increased, the carcasses contained less lean and as PEMD ASBV increased, the carcasses contained more lean equating to a potential 2% increase given the range of ASBVs in the experiment, which represents ~0.4 kg lean on average. |
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