Genetic Trends for Milk Yield, Persistency of Milk Yield,Somatic Cell Count and Calving Interval in Holstein Dairy Cows of Iran

The objective of the present study was to estimate genetic trends for lactation milk yield, persistency of milk yield, somatic cell count and interval between first and second calving in Holstein dairy cows of Iran. The dataset consisted of 210,625 test day and 25,883 first parity cows with milk yield recorded from July 2002 to September 2007 comprising 97 herds in Iran. Breeding values of animals were predicted with Best Linear Unbiased Prediction methodology under multi-trait animal model. Model included region, herd–year–season of calving and age at first calving as fixed effects and Holstein percentage (covariate) and random animal additive effect. Genetic trends of studied traits were estimated by regressing mean of breeding values on calving year. Phenotypic trends were estimated using annual averages of first lactation traits on calving year weighted by the number of animals in each year. Genetic trend was positive and significant for lactation milk yield (21 kg/yr; p < 0.01).Although not significant, favorable genetic trends for most studied traits were reported. Considering that selection emphasis in Iran was on lactation milk yield, progress for this trait was low during the years of the study.