Effect of divergent selection for weaning weight on liveweight and wool growth responses to feed intake in Merino ewes

Abstract

During a 6-week experiment, the feed requirements for maintenance of liveweight (LW) by adult Merino ewes were measured in 2 flocks selected over 32 years for divergent weaning weight. Also examined were the net efficiency of LW gain and wool growth in response to change in feed intake, and the ability to digest dietary organic matter. Ewes selected for high weaning weight (W+) were 31% heavier than those selected for low weaning weight (W-) at the start of the experiment. To maintain LW, W+ ewes ate 24% more digestible organic matter (DOM) per day than W- ewes. There were no differences (P>0.2) in the net efficiency of feed use for LW gain. The W+ ewes digested more (P<0.01) dietary organic matter, by 1.8 percentage units, than W- ewes and, consequently, required only 22% more dry feed to maintain their heavier LW. Ewes from the 2 flocks did not differ (P>0.4) in the amount of wool grown, and as W+ ewes were larger, they produced less (P<0.01) wool per kg LW for the same DOM intake.