Genetic estimation of growth curve parameters by different nonlinear models in Kurdi sheep breed of the North Khorasan

The aims of this study were to estimate variance components, prediction of genetic parameters and correlations of growth curve parameters of nonlinear models such as Broody, Gompertz, Logistic and von in Kurdi sheep breed. Data comprised of 17659 body weights from birth to yearling of 5074 lambs belonging to 162 rams and 1968 ewes from 1996 to 2013. Growth parameters were estimated by NLIN procedure of SAS V.9.4., and parameters including A, B and K were calculated by individual fit according to the sex of the animal. Variance components were estimated by a mixed model of BLUPF90 and considering the influences of fixed effect including sex, birth type, year and month of birth. Heritability of mature weight was low in all models (0.05 to 0.09) and showed the low participation of genetic variance from the phenotypic variance. Therefore, to achieve the desired genetic gain and high accuracy, more records of lamb growth parameters are required for each ewe and ram, as well as, the improvement of environmental circumstances. Regarding high positive genetic and environmental correlations of A-B, it is expected that selection of animals with higher birth weight or improving the environmental circumstances, lead to a height mature weight, which needs more investigation over the increasing of lambing ease. The medium and negative genetic and phenotypic correlations between A-K imply the negative relationship between the mature rate and weight.