Data and pedigree information of 5173 Iran-Black sheep, maintained at the breeding station of Abbasabad, Iran, for a period of 25 years (1980 through 2004) were used to determine the level of inbreeding of the animals as well as to quantify the effect on the growth traits of the animals. The individual (and maternal) inbreeding depression of the animals was investigated by fitting the inbreeding coefficient of the lambs (and the ewes) as linear covariates under univariate animal models. The mean inbreeding for the ewes, whole population, females, and males, were 6.55%, 7.54%, 7.72%, and 7.36%, respectively, in the flock. The individual inbreeding depression per 1% increase in the inbreeding ranged from -3.4 (birth weight) to -44.3 g (9-month body weight), whereas the maternal inbreeding depression varied from -12.6 to -80.7g for the weights at birth and six months of age, respectively. The present results showed that inbreeding depression significantly reduced the body weights of lambs in this breed. Since the level of inbreeding of the lambs had a detrimental effect on growth traits, there is a need to control the inbreeding in the flock.
