The objective of the present study was to estimate genetic parameters and genetic trends for body weight traits in Markhoz goat using random regression (RR) models. In this study, 12,116 test-day body weight records of 3078 Markhoz goats, from birth to 365 days of age, were used to compare RR models in which fixed and random effects were modeled using Legendre polynomials. The best model for direct genetic and maternal temporary environmental effects (common litter) included cubic, for direct permanent environmental effect included quadratic and for maternal permanent environmental effect included linear Legendre polynomials with a heterogeneous error structure. Direct heritability increased from 0.17 at birth to 0.32 at 120 days of age then decreased suddenly to 0.27 at 240 days of age and increased thereafter. Some estimates for the highest ages with the least records tended to be overestimated, in particular heritability beyond 340 days. Variances due to direct permanent and maternal permanent environmental effects, respectively, were the highest and the lowest compared with the other sources of variation. This study has demonstrated the possibility of application of RR models for routine genetic evaluation of Markhoz goat in Iran. Although the results confirm that there is a scope for genetic improvement in growth pattern in Markhoz goat, but the genetic changes of body weight for selected ages (120-d and 200-d) have not shown a substantial tendency among the years.
