Testicular size has been linked to reproductive efficiency and changes in testicular size could provide an early indicator for the choice of ram lambs to be used in flocks selecting for out-of-season breeding ability. For 3 years, 20 ram lambs from each of the BV(DL)-ve (earlier lambing) and BV(DL)+ve (later lambing) flocks at Ruakura were monitored for testicular diameter (TD) and liveweight (LWT) from weaning until November the following year. In 1991 and 1992, BV(DL) class and season of birth were confounded while in 1993 both BV(DL) classes were represented in both seasons of birth (Autumn and Spring). At the same age Autumn born BV(DL)-ve rams had larger (P<0.05) TD than did Spring born BV(DL)+ve rams in all years. For the 1993 born lambs, the shape of the TD growth curve was different for each BV(DL) x season of birth sub-class (P<0.001). However, at any single age point the within sub- class variation in TD (rsd = 4.6 mm) was such that no meaningful selection for BV(DL) could be made within rams for that season of birth. Liveweight interacted with TD and this effect was also influenced by the season of birth. These data indicate that while TD is influenced by season of birth, the difference in TD between ram lambs of divergent BV(DL; selection for out- of-season breeding ability) was not sufficient for TD to be a useful parameter for early selection.
Proceedings of the New Zealand Society of Animal Production, Volume 56, , 310-313, 1996
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