Heritability of alternative somatic cell count traits in Italian Holstein cows

Mastitis is one of the most costly diseases in the dairy herd. The direct collection of data on clinical mastitis is not widely implemented worldwide and thus most countries refer to somatic cell score (SCS), i.e. the log transformation of milk somatic cell count (SCC, cells mL-1), as indirect measure to select against mastitis. Recently, alternative SCC traits have been proposed to improve udder health. The aim of this study was to estimate heritabilities of alternative SCC traits in Italian Holstein dairy cattle breed. The dataset of SCC was retrieved from the Italian Holstein Association (ANAFI) and included test-day records of first lactation cows (20 to 40 mo of age) calving between 1999 and 2014 in herds of Padova province. The dataset was edited to include records collected between 5 and 305 days in milk (DIM). Moreover, cows with less than 3 test-day records of SCC, with the first test-day beyond 60 DIM and with interval between two consecutive test-days greater than 70 DIM were discarded. Contemporary groups were defined as cows calving in the same herd-year-season (HYS) and HYS with less than 5 animals were removed. After editing procedure, 46,304 cows from 303 herds were available for subsequent analysis. Five SCC traits were defined: 1) SCSTOT, the lactation-mean SCS, 2) SCS150, the average SCS from 5 to 150 DIM, 3) SCS305, the average SCS from 151 to 305 DIM, 4) infection, a dichotomous trait (0/1), where 1 indicates that the cow had at least one SCC higher than 100,000 cells ml-1 during the lactation, and 5) severity (%), the ratio between the number of lactation test-days on which SCC was greater than 100,000 cells mL-1 and the total number of lactation test-days. Heritability of the aforementioned traits were estimated using an animal model that considered HYS, classes of age of the cow at calving, and number of lactation testdays as fixed effects, and additive genetic animal and residual as random factors. The pedigree included 144,255 animals. Means for SCSTOT, SCS150, SCS305, infection, and severity were 3.43, 3.10, 3.38, 0.82 and 39.0%, respectively, and all traits exhibited large variability. Estimates of heritability for SCSTOT, SCS150, SCS305, infection and severity were 10.6%, 6.4%, 10.3%, 2.9% and 13.2%, respectively. Despite low, these estimates suggest that selection for alternative SCC traits is feasible and they may potentially contribute to improve mastitis resistance in dairy cows.