Estimation of genetic parameters for reproductive traits in Lori-Bakhtiari sheep

Reproductive records of 5373 Lori-Bakhtiari ewes were collected from a research flock at the Lori-Bakhtiari sheep breeding station during 1989-2005. Genetic parameters were estimated for six basic and six composite traits. The basic traits were conception rate (CR), total number of lambs born (NLB), number of lambs born alive (NLBA), number of lambs alive at weaning (NLAW), litter mean weight per lamb born (LMWLB) and litter mean weight of lambs at weaned (LMWLW). The composite traits were number of lambs born per ewe exposed (NLB/EE = CR × NLB), number of lambs weaned per ewe exposed (NLW/EE = CR × NLWA), total litter weight at birth (TLWB = NLB × LMWLB), total litter weight at weaning (TLWW = NLAW × LMWLW), total litter weight at birth per ewe exposed (TLWB/EE = CR × TLWB) and total litter weight at weaning per ewe exposed (TLWW/EE = CR × TLWW). A derivative-free algorithm was used to obtain REML estimates of genetic parameters. Fitting a multi-trait animal model included fixed effects and also random effects due to animal direct genetic effects, permanent environment effects due to repeated record of ewe and residual effects. Estimates of heritabilities for animal genetic effects were mainly small (0.01-0.19) due to the typical high influence of environmental factors on reproductive traits and non-normal distributions of some traits. Estimates of repeatabilities for animal permanent environmental effects were small to medium (0.09-0.24). Genetic correlations ranged from −0.58 to 0.98 among traits. Phenotypic correlations ranged from −0.25 to 0.38 among traits. Some estimates of genetic correlations (such as CR with some traits) do not seem to have a biological explanation. These estimates of genetic parameters may provide a basis for deriving selection indexes for reproductive traits.