Two phase selection of Zandi ewes breed

A study was conducted to evaluate of genetic parameters and composite reproduction traits (Total Weaning Weight) of a Zandi sheep flock using 5100 lambs, 165 rams and 1422 ewes for growth and reproduction traits including birth, weaning, 6, 9 and 12 month weights and total weaning weight for lambing periods of 1 to 4 per every ewe joined during 1991 to 2006. Data were analyzed by REML method using DFREML, 1998 version. According to the results based on the best model (number 3) for birth and weaning weight containing direct and maternal additive genetic effect, so direct and maternal heritability for birth weight were 0.098±0.037 and 0.152±0.019 and for weaning weight were 0.118±0.031 and 0.040±0.015 respectively. The best models for 6, 9 and 12 month weights was model number 1 containing only direct additive genetic effect, and estimates were 0.189±0.036, 0.185±0.042 and 0.269± 0.044 respectively. Genetic and phenotypic correlation between body weights in different ages were positive and moderate trend to high, ranging from 0.646 (between birth and weaning) to 0.983 (between weaning and 6 month weight). Phenotypic correlation between these traits were positive and moderate ranging from 0.185 (between birth and 12 month weight) to 0.701 (between 6 and 9 month weights), but were lower than genetic correlation estimates. Genetic correlation estimates for total weaning weights at different mating periods were low and negative (between period 2 and 3) to high and positive (between period 1 and 2) and were -0.361 and 0.905 respectively. Phenotypic correlation for these traits were lower than genetic correlations, ranging from -0.004 (between period 2 and 3) to 0.113 (between period 3 and 4). Estimates for direct additive genetic correlation between growth and composite reproduction traits were positive with ranging from 0.631 (between weaning weight and TWW4) to 1.00 (between birth weight and TWW1). As a final result, selection based on weaning weight can improve both growth and reproduction traits in the later growth stages of lambs and ewes meaning the kg of lamb weaning weight per ewe joined in every lambing period.