Selection for weaning weight in Merino sheep. 3. Maintenance requirements and the efficiency of conversion of feed to wool in mature ewes

Abstract

Net and gross efficiencies of wool growth have been estimated for ewes from three Trangie selection flocks, one selected for high weaning weight (Weight Plus), one for low weaning weight (Weight Minus), and a random control flock. Weight Plus ewes had 15 and 11 per cent heavier body weights than Weight Minus ewes at maintenance and unlimited levels of feeding respectively. At each level, feed intake was proportional to body weight both between and within flocks. In addition, the feed costs of a unit gain and loss of body weight were the same for each flock at each level of feeding. The Weight Plus ewes grew 7 and 10 per cent more wool than Weight Minus ewes at maintenance and unlimited levels of feeding. Consequently, there was only a small difference between the flocks in efficiency of conversion of feed to wool at maintenance, and there was no difference on unlimited intake. These results indicate that there is no genetic correlation between weaning weight and efficiency of conversion of feed to wool. At the three levels of controlled feeding (543, 725, 902 grams of dry matter a day) there were no differences in gross efficiency of wool growth, either between flocks or between levels within flocks, despite differing changes in body weight. During unlimited feeding, gross efficiency of wool growth was lower than that during controlled feeding. This was due to a reduced response in wool growth, as feed costs per unit body weight change were constant. As both production and feed intake changed in proportion to body weight, it is concluded that attention to weaning weight has little place in a Merino breeding program aimed at increasing wool production. However, selection for high weaning weight may be a useful tool in selecting ewes for crossbreeding because of the associated increases in milk production and lamb growth rate.