Application of Wilmink's Exponential Function in Genetic Analysis of 305-d Milk Production and Lactation Persistency in Holstein Cows of Razavi Khorasan

To estimate heritability and genetic trend for 305-d milk production and lactation persistency, a total of 130,668 monthly test day milk yields belonging to 15,183 first lactation Holstein cows in 131 herds and calved from 2000 to 2009 were used. To calculate 305-d milk yield as well as lactation persistency, estimated parameters of Wilmink’s exponential function were applied. The parameters of the function were estimated by SAS software. Genetic and environmental variance components and heritability of the traits were estimated by single trait animal model using DMU software. Genetic trend was estimated based upon weighted simple linear regression of average breeding values on calving year. Heritability estimate of 305-d milk yield and lactation persistency were found to be 0.184 and 0.05, respectively. A positive significant phenotypic trend (166.11 kg per year) was observed for 305-d milk yield while a non-significant genetic trend (-2.0107) was revealed for the trait. For the lactation persistency, there were no significant phenotypic (0.054 % per year) and genetic (0.003 % per year) trends over the period of time.