The aims of this study was to evaluate inbreeding effects for growth rate (average daily gain from hatch to 8 weeks of age (ADG1-8), average daily gain from hatch to 12 weeks of age (ADG1-12), average daily gain from 8 weeks of age to 12 weeks (ADG8-12), average daily gain from hatch to sex maturity (ADG1-SM), average daily gain from 8 weeks of age to sex maturity (ADG8-SM), average daily gain from 12 weeks to sex maturity (ADG12-SM), and Kleiber ratios (KR1-8, KR1-12, KR8-12) in Mazandaran native chickens. The data set of 82,446 pedigreed individuals over 21 generations was analyzed. All individuals were grouped into three classes based on the inbreeding coefficient: First class contained non-inbred birds (F = 0) and second and third classes contained inbred individuals (0 < F ≤ 5% and F > 5%, respectively). The effects of inbreeding coefficient on the studied traits were estimated by the restricted maximum likelihood method (REML) applying the WOMBAT software and fitting individual increase in inbreeding coefficient (ΔFi) of birds as linear covariates under 6 different animal models. ADG8-SM and ADG12-SM increased non-significantly by 0.003 and 0.005 g, respectively, due to 1% increase in ΔFi, while ADG1-8, ADG1-12, ADG8-12, ADG1-SM, KR1-8, and KR1-12 decreased (P < 0.001), respectively, by 0.012 g, 0.011 g, 0.010 g, 0.014 g, 0.003 g, and 0.002 g. Also, KR8-12 non-significantly decreased by 0.001 g. Estimates of direct heritability of traits based on the most appropriate models ranged from 0.05 (KR8-12) to 0.26 (ADG12-SM). Results showed that it is important to regularly analyze inbreeding level in the herd in an attempt to prevent the reduction of performance.
Keywords: Average daily gain; Inbreeding depression; Indigenous chicken; Kleiber ratio.