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Groups of mature Merino ewes selected on the basis of either large or small bodysize were differentially fed during a pre-experimental period to produce subgroups of high and low body condition. These levels of body condition were maintained for 2 weeks before mating and for the first 5 weeks of gestation. Thereafter, the four subgroups grazed together until lambing.
Big ewes had more multiple ovulations than small ewes (14/41 υ 6/53; P < 0·01), and there was a significant linear regression of ovulation rate on body size. Body condition was positively related to the incidence of multiple ovulations, but the regression of ovulation rate on body condition at mating failed to reach significance. Body weight was significantly related to both ovulation rate and the incidence of multiple ovulations, and proved to be a more effective predictor of ovulation rate than either body size or condition.
These results make it clear that size and condition, the two components of body weight, each have a considerable independent influence on some aspects of reproduction, and that the precision of some experiments and the effectiveness of some production routines could be improved by considering them separately.