The present study was performed to study the effect of inbreeding on the pre-weaning growth traits including the birth weight (BW), weaning weight (WW), average daily gain (ADG), Kleiber ratio (KR) and lamb survival from birth to weaning (SUR) in Baluchi, Iran-Black and Zandi sheep. Significant positive inbreeding trends were found only for Baluchi and Iran-Black breeds as 0.038 and 0.278, respectively. Inbreeding depression was assessed by fitting individual inbreeding coefficient as a linear covariate for each trait applying the most suitable animal model detected via Akaike's Information Criterion (AIC) among the models containing different combinations of direct additive and maternal effects. Inbreeding had no significant effect on BW in these breeds (P>0.05) but WW decreased significantly through inbreeding as -29.35 g, -25.12 g and -42.20 g per 1% increasing in inbreeding for Baluchi, Iran-Black and Zandi breeds, respectively. The influence of inbreeding on ADG of Iran-Black and Zandi breeds was significant, the corresponding values of -0.30 g/day (P<0.05) and -4.00 g/day (P<0.01) were obtained, respectively. The ADG in Baluchi sheep was not influenced by inbreeding (P>0.05). Inbreeding significantly influenced KR in Baluchi (0.30) and Zandi (-0.13) breeds but not in Iran-Black breed (P>0.05). Inbreeding (P<0.01) decreased SUR in Baluchi and Iran-Black as -0.45% and -0.85%, respectively, but not in Zandi sheep (P>0.05). Inbreeding depression was observed for most of the pre-weaning studied growth traits (except for BW) and lamb survival from birth to weaning in these breeds. It was concluded that the mating policies designed to minimize the inbreeding effect in Baluchi and Iran-Black sheep have not been effective. Keywords: inbreeding depression, inbreeding coefficient, lamb survival, sheep
