Ewes with higher embryo survival rear lambs that grow faster

A key economic driver of a meat producing sheep flock is the total kilograms of lamb liveweight at weaning per ewe exposed to the ram. Optimization of key reproductive steps is required to achieve peak performance of a sheep flock. The goal of this paper was to use more than 56,000 records from a commercial flock to investigate the key reproductive steps affecting flock reproductive performance. We also applied a maximum-likelihood based technique to predict the embryo survival and ovulation rate for daughters of individual sires based on measurements of the number of fetuses at midpregnancy (detected by ultrasound-scanning). The model was used to determine how changes in premating liveweight, age, predicted ovulation rate, embryo survival, number of fetuses at midpregnancy, lamb survival, and lamb growth rate affect the total lamb liveweight at weaning per ewe exposed to the ram in the flock. The data from the commercial flock was also used to investigate the role of ewe age and premating liveweight on each reproductive step. Sensitivity analyses were performed to identify the key reproductive steps affecting flock reproductive performance. The elasticity for embryo survival was 80% of that for lamb survival. There was also significant between sire variance in the estimates of ovulation rate and embryo survival. The reproductive performance of daughters of sires with high (top 50%) and low (bottom 50%) embryo survival was investigated. Embryo survival was 0.88 in the high group and 0.82 in the low group (a 6% reduction in embryo survival). The expected total weight of lambs weaned per ewe exposed to the ram was 42 kg in the high embryo survival group and 37 kg in the low embryo survival group (a 12% reduction in the total weight of lambs weaned per ewe exposed to the ram). The proportion of twin litters was 70% in the high group and 60% in the low group, highlighting the potential importance of embryo survival for the rate of twinning in flocks with ovulation rates greater than two ova. Although lamb survival was similar between the high and low embryo survival groups, lamb growth was reduced by 10% in the low embryo survival group for the same litter size (P < 0.001). This novel positive phenotypic association between embryo survival and lamb growth rate can potentially be exploited to improve flock performance.