Effect of somatic cell counts on the production traits and estimation of genetic parameters for milk yield traits of Holstein cows

The reduction in milk yield on test day associated with somatic cell count up to 600 000 cells/ml using monthly cow’s-records collected for a 9-year period (2002-2010) on primiparous Iranian Holstein cows in 159 different herds was studied. The used dataset included 181226 records from 21469 cows. According to the results of this study, the increase in somatic cell score from 1 to 5 and above could resulted to reduction in average daily milk yield by 1.300 kg (0.04) and increasing of average daily fat and protein percentage by 0.14 and 0.08 %, respectively. Additionally in this study genetic parameters of production traits were estimated by a multi-trait random regression test-day model. Heritabilities of 305-d lactation for milk yield, fat and protein percentage 0.40, 0.39, and 0.69, respectively were estimated. Heritabilities of daily yields were lower than that for complete lactation (305-d) and varied from 0.08 to 0.297 for milk yield, 0.054 to 0.123 for fat percentage, and 0.044 to 0.286 for protein percentage. The highest value of genetic and permanent environmental correlations was observed between fat and protein percentage (0.60<).