Genetic Analysis of Milk Yield for Holstein Dairy Cows in different provinces of Iran

In order to genetic evaluate of milk yield in Iranian Holstein dairy cows a total of 276767 first lactation records from different provinces of Iran within 583 herds collected from 1991 to 2008 were used. The records were corrected for 305 days in milking, twice daily milking and mature equivalent age. The records less than 2500 and higher than 13000 kg were deleted before analyzing of data. To identify factors affecting milk yield a general linear model (GLM) procedure was applied and then herd-year-season and age at first calving were considered as fixed and covariate factors in the model respectively. A single trait animal model was applied for estimation of genetic parameters using restriction maximum likelihood (REML) method. Mean and heritability of milk yield were obtained 7944±1743 (kg) and 0.28±0.01 respectively. Genetic and phenotypic trends were 46.4±2.4 and 147.27±5.7 kg/y respectively (P