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Abstract
It has been suggested that selection based on insulin‐like growth factor‐1 (IGF‐1) concentrations could be used as a mechanism to improve efficiency of growth in young animals. However, there is the strong possibility that selection on IGF‐1 levels could cause correlated changes in other traits. The present study was designed to determine if selection based on phenotypic IGF‐1 concentrations has any impact on reproductive performance of ewes. In study one, the breeding performance and ovulation rate of oestrus‐synchronised ewes selected for either “Low” or “High” phenotypic IGF‐1 concentrations, and that of a control line, were compared. In study two, ewes were also superovulated using eCG and the above parameters measured.
In study one, both Low and High line ewes were less (P < 0.05) likely to be bred in the 4‐day period post‐synchronisation only, and more (P < 0.05) likely to return to service than Control ewes. These relationships were not observed in study two. In study two, but not in study one, High line ewes were more likely (P < 0.05) to be mated in the post‐synchronised period only, than Control and Low line ewes. In both studies the proportion not mated did not differ between the ewe lines. Conception rates were lower (P < 0.05) in both the Low and High lines than in the Control lines in study one, although the Low and High lines did not differ. In study two, conception rates did not differ between ewe lines. In study one, average ovulation rates were lower (P < 0.05) in the Low than in the Control and High line ewes, which did not differ (0.96, 1.15 versus 1.21 corpora lutea per ewe respectively). Average ovulation rates in study two differed (P < 0.05) between all groups such that the Control line ewes had higher rates than the Low line, which in turn had higher rates than the High line (1.74, 1.46 versus 1.22 corpora lutea per ewe respectively). In study two, eCG increased (P < 0.05) the proportion of ewes with multiple ovulations (19.9 versus 64.4%) and the average number of corpora lutea (1.82 versus 1.13). The inconsistency of results across the two studies makes clear conclusions difficult. This variation may have been caused by between year differences in nutrition and/or reproductive interventions. However, given that differences between IGF‐1 selection lines were apparent for some reproductive parameters, within individual studies, the results suggest care should be exercised when selection is based on phenotypic IGF‐1 concentration.
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