Genetic correlation of average somatic cell score at different stages of lactation with milk yield and composition in Holstein cows

Genetic and non-genetic correlations of the average somatic cell score (SCS) at different stages of lactation (100، 150، 200، 305 and 365 days in milk) with lactation performance were estimated based on the first lactation records of Iranian Holstein calving during 2000 to 2009. (Co) variance components were estimated using derivative-free REML based on multiple traits animal models. The estimated genetic correlations between average of first 100 days in milk SCS (SCS100) and milk100، fat100، mil 305، fat305 and protein305 were 0. 14، 0. 19، 0. 11، 0. 15 and 0. 17، respectively. The corresponding environmental correlations were -0. 13، -0. 03، -0. 18، -0. 10 and -0. 13، respectively. The estimated genetic correlations of the average SCS during the first 150 days in milk (SCS150) with milk100، fat100، milk305، fat305 and protein305 were 0. 14، 0. 19، 0. 11، 0. 15 and 0. 17، respectively. The corresponding environmental correlations were -0. 13، -0. 03، -0. 18، -0. 10 and -0. 13، respectively. The estimated genetic correlations of the average SCS during 305 days in milk with milk، fat and protein yield، and fat and protein percentages were 0. 12، 0. 11، 0. 18، -0. 05 and -0. 08، respectively. The estimated genetic correlation between SCS100 or SCS150 with average SCS over longer stages of lactations (SCS305 and SCS365) was 0. 99 ± 0. 01. These results indicate that recording SCS data over shorter period of lactation is an alternative approach to reduce the costs of SCS data collection and therefore to expand the SCS recording in the country by increasing both the number of herds and cows involved.