Effects of Somatic Cell Score on Milk Traits in Polish Holstein-Friesian Cows

The objective of this study was to investigate the effect of somatic cell score (SCS) on milk yield; fat, protein, and lactose percentage; and fat-to-protein ratio (FPR). The study dataset contained 41,593 records collected for 2709 Polish Holstein-Friesian cows from three herds. A total of eight classes were created based on SCS. For each study trait, a linear mixed model was used, taking into account fixed effects: parity, stage of lactation, season of calving, SCS class, and their interactions, and random effects of cow, herd test day, and residual. A higher SCS was found to be associated with a reduced milk yield and lactose percentage, while the lowest protein and fat percentage and FPR were observed in the lowest SCS class, and they tended to increase with increasing SCS. The effect of SCS on milk yield, protein, and lactose percentage was more pronounced in younger cows (up to third parity) than in older cows (fifth and sixth parities). A greater effect of SCS on milk yield was observed in the later stages of lactation (from about the fifth month).