Random regression models (RRMs) were employed to estimate variance components for body weight(BW) records of Moghani sheep from 60 to 365 days of age. A total of 9165 BW records of 2811 lambs,collected at a breeding station of Moghani sheep were used in this study. Various combinations of Leg-endre (Leg) polynomials with 2 and 3 orders of fit and linear and quadratic B-Splines (BS) with 2 or 3coefficients were fitted for fixed and random regression parts of the RRMs. The models were comparedbased on Akaike’s and Bayesian Information Criteria. Different combinations of Leg and BS functions hadalmost similar estimates of variance components as ratios to total phenotypic variance, but the modelswith Leg polynomials especially in random regression parts had easier convergence and better fittingcriteria on average. However, a model fitting quadratic B-Splines with 3 coefficients in fixed regressionpart and 3, 3, 2 and 2 coefficients for random direct additive genetic, permanent environmental, mater-nal genetic and maternal environmental effects, respectively was the best model. According to the bestmodel, direct heritability (h2) was estimated in the range of 0.14–0.32 with a decrease from 60 to 120 daysof age and a subsequent increase to the end of trajectory. Coefficient of permanent environmental effects(p2) was in the range of 0.23–0.61 with an increasing trend throughout the trajectory. Maternal heri-tability (m2) and coefficient of maternal environment (c2) had low and negligible estimates, respectively.The results of the present study indicated potential of B-Splines for genetic evaluations of body weightin sheep, using RRMs.
