Opportunities for genetic evaluation of reproductive performance in accelerated lambing systems

Data from a Polypay sheep flock managed in an accelerated lambing system were used to estimate means, heritabilities, and genetic correlations for ewe lamb fertility (ELF); age at first, second, and third lambing (AGE1, AGE2, and AGE3, respectively); first and second lambing interval (INT1 and INT2); and number of lambings by 38 month of age (LAMB3). Summer matings had lower fertility than autumn and winter matings. Heritabilities of ELF, AGE1, AGE2, AGE3, and LAMB3 were 0.14, 0.39, 0.28, 0.36, and 0.27, respectively (P<0.01) but heritabilities for INT1 and INT2 were only 0.00 and 0.09, respectively. The quantity AGEACC=[(age in months at ith lambing)-12i] was proposed to compare observed ages at lambing to those expected in an annual lambing system using a repeated-measures analysis and had heritability of 0.31 and repeatability of 0.90. Genetic correlations of ELF with AGE1, AGE2, and LAMB3 were −0.89, −0.91, and 0.89, respectively (P<0.01). Genetic correlations of LAMB3 with AGE1 and AGE2 were −0.49 and −1.00, respectively. Genetic correlations of ELF and LAMB3 with direct genetic effects on WW were approximately −0.70, but correlations with maternal genetic effects on WW were 0.88 (P<0.10) and 0.58, respectively. Litter size was independent of ELF and LAMB3. Initial selection on ELF, augmented by additional selection on ewe ages at subsequent lambings, may allow prediction of genetic merit for reproductive performance and increase the frequency of lambing in accelerated lambing systems.