A pedigree file consisting of 5860 individuals, 167 sires and 1582 dams collected at Makuie sheep breeding station (MSBS) during a period of 24 years (1990 to 2013) was used to calculate the inbreeding coefficients to reveal any probable effects of inbreeding (F) on the studied traits. The studied traits were: greasy fleece weight at 6 months of age, greasy fleece weight at 18 months of age, conception rate, gestation length, number of lambs born, number of lambs alive at weaning, litter mean weight per lamb born and litter mean weight per lamb weaned. The inbreeding coefficient among the individuals ranged from 0 to 25 % with an average of 0.33 %, and ranged from 0 to 25 % among the ewes with an average of 0.21 %. Fluctuations in individual and ewe inbreeding were observed in the period under study. The average generation interval was calculated as 3.6 years. The effective population size of the flock was 51.8 animals. The rate of inbreeding was 0.08 % per year and 0.53 % per generation. Six different models were applied and likelihood ratio test (LRT) was used to select the appropriate model. Based on the LRT, model II was selected as an appropriate model for greasy fleece weight at 6 months of age. Quadratic regression coefficients of greasy fleece weight at 6 months of age and greasy fleece weight at 18 months of age were determined as significant (P<0.001) - 0.007 and - 0.40, respectively per 0.01 change in the individual F. The reproductive traits were studied basing on the ewe inbreeding coefficient. Number of lambs born was affected negatively (P<0.01) by the linear effect of ewe F. The lambs produced by the Inbred ewes had significantly (P<0.05) more survival ability from birth to weaning than those produced by the non-inbred ones. The significant quadratic regression coefficient of conception rate was determined (- 0.22; P<0.05). Significant quadratic regression coefficient (P<0.01) of litter mean weight per lamb born was determined (- 0.63 per 0.01 change in the ewe inbreeding coefficient). Gestation length and litter mean weight per lamb weaned were not affected significantly by linear or quadratic effects of ewe F. Therefore, inbreeding should be avoided, except for purposes of genetic breeding, whose main objective is the fixation of certain alleles in the population.
