Environmental and physiological determinants of somatic cell count and clinical mastitis in primiparous Holstein cows

This study aimed to evaluate resistance to Somatic Cell Count (SCC) and Clinical Mastitis (CM) in Holstein dairy cattle. In this research, data representing 45,316 primiparous cows from 1026 herds, calved between 2002 and 2018, were analyzed. The goals of this study were to evaluate factors affecting somatic cell scores and to compare parameters of the lactation curve fitted by the incomplete gamma function in healthy cows and those with clinical mastitis. The results showed that the highest least square means (LSM) of Somatic Cell Score (SCS) were observed in spring and winter (April through June and January through March) (2.95 vs. 2.94) (p < 0.001). The lowest and highest LSM of SCS were related to the maximum and minimum classes of milk production (>50 kg/day, 2.60 and <10 kg/day, 3.63). The SCS in consecutive calving years decreased annually by 13,000 cells/year, and the age at first calving around 23 and 24 months was related to the lowest LSM for the SCS in Holstein first parity cows (p < 0.001). The highest milk reduction was observed in the third part of lactation, resulting in a 305-day milk yield reduction exceeding 500 kg in cows affected by CM compared to healthy cows.