Lines of Romney sheep selected for either weaning (WWW) or yearling body weight (TYW and HYW) for 25 years were compared relative to their respective controls in a common environment using two unselected crops of male and female progeny. The weaning weight (WWW) line and one of the yearling weight (TYW) lines were derived from a common base flock, of diverse origin. The WWW line maintained a similar body weight advantage of approximately 20% over its control at 5, 8, 10 and 14 months of age. All control lines showed very similar average body weight trends with age (28.0±0.5, 33.9±0.05, 35.3±0.06 and 42.6±0.7). In contrast the yearling weight lines showed increasing divergence with age from 22% in February to 28% (TYW) and 31% (HYW) at 13 months. All lines, and especially the yearling weight lines showed some weight increase over the winter (May to July). Eye muscle depth and width changed little over the winter compared with the previous autumn or the following spring, in contrast to fat depth trends which increased in all lines but for which however, there was little line divergence among the yearling weight lines during the winter. All selected lines had significantly lower eye muscle dimensions at the same liveweight than their controls (by 4% for width and 5% for depth). Lines also showed contrasting divergence in average weight-adjusted fat depth relative to their controls, especially those derived from the same base population. As judged by reductions in subcutaneous fat depth, the WWW line lost its initial leanness superiority (4% lower fat depth) during the winter, the TYW line increasing its superiority (16-17% lower fat depth) during the winter and spring. The other yearling liveweight line (HYW) showed only half the leanness superiority over its control at these times. Thus, lines selected for improved growth alone had different patterns of muscle and especially subcutaneous fat development in a desirable direction for commercial lean lamb production, but responses were variable.
Proceedings of the New Zealand Society of Animal Production, Volume 57, , 263-266, 1997
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