Milk phenotypic characterization and heterogeneity of variance across herds in Italian Jersey breed

Jersey dairy cattle breed is characterized by relatively low milk yield (MY), but high milk solids content. The low productivity and its udder conformation make Jersey a suitable breed for once-a day-milking, resulting in less stress load for animals and higher milk quality. For these reasons Jersey is appreciated by the dairy industry. The present study aimed to assess the phenotypic variation of MY and milk quality traits in Italian Jersey (IJ), and to evaluate the heterogeneity of variance across IJ herds. Sources of variation were investigated through a mixed linear model, including the fixed effects of days in milk, parity, calving season, milking frequency, recording type, the interaction between days in milk and parity, and the random effects of herd-test-day (HTD) and cow. Heterogeneity of variance was assessed across herds classified as low (L), medium (M) or high (H) according to their average MY and milk quality traits. Results highlighted the high contents of milk fat (5.18%), protein (4.08%) and casein (3.16%) of IJ cows. Somatic cell score (SCS) averaged 3.35 units, which is quite high and suggest that SCS should be monitored carefully in IJ herds, especially through an improvement of management practices. Milk yield showed moderate negative correlations with milk composition and SCS. Milk with greater fat content had also greater protein and casein content, whereas SCS resulted moderately and negatively correlated with lactose content. Phenotypic variances of MY and milk quality traits were mainly related to animal effect; on the other hand phenotypic variance of milk urea nitrogen content was mainly affected by HTD effect. Finally, heterogeneity of variance was observed across L, M and H herds, in a way that highly ranked herds showed greater standard deviation. Future research should investigate the impact of accounting or not for heterogeneity of variance in genetic evaluation of IJ breed.