Linear and Threshold Model Comparisons for Genotypic and Phenotypic Parameters Estimation on Reproductive Traits in Zandi Sheep

Reproductive records of 5327 Zandi ewes were collected from a research flock at the Zandi sheep breeding from Varamin station during 1991-2011. Genetic parameters were estimated for main traits such as conception rate (CR), number of lambs born (NLB), number of lambs at weaning (NLW) and the composite traits including number of lambs born per ewe exposed, number of lambs weaned per ewe exposed, total litter weight at birth, total litter weight at weaning, total litter weight at birth per ewe exposed and total litter weight at weaning per ewe exposed. Year, age of ewe, breed of ewe on breeding were used as fixed effects. Direct and maternal genetic effects, permanent environmental effects of ewe and mate of ewe were considered as random effects in the model. The effect of environmental factors such as mating year and age of ewe at mating on all of reproductive traits were significant (P >0.01) exception CR, NLB¡NLB/EE, NLW NLW/EE, TLWB. Parametric and variance components were estimated by linear and threshold models. Estimates of heritability for animal genetic effects were mainly small and ranged from 0.0011 to 0.1 in linear model while the estimates resulting from thershold were ranged from 0.009 to 0.063. Estimates of heritability's for animal genetic and permanent environmental effects were low mainly due to the typical strong influence of environmental factors on reproductive traits. Estimates of repeatability's for animal permanent environmental effects were low to medium by linear analysis, and medium by threshold analysis.