Linear Mixed-effects model to quantify the association between somatic cell count and milk production in Italian dairy herds

Milk production loss due to mastitis in dairy herds is economically important. Before estimating the economic impacts of mastitis, it is crucial to quantify the association between mastitis and milk production. The objective of this study was to estimate the association between somatic cell count (SCC, as an indicator of intramammary infection due to mastitis) and milk production for dairy cows in Lombardy (Italy). The test-day (TD) records data of 3,816 dairy herds located in 3 different geographical areas of Lombardy from January 2016 to December 2018 were used. After data editing, the final dataset comprised 10,445,464 TD records from 2,970 farms and 826,831 cows. The analysis was carried out by using a mixed-effects model with seven fixed effects (geographical area, breed, days in milk, parity, season and year) and nested random effects for each cow and herd. The results indicated that the SCC had a negative association with milk production. When the SCC increased every two-fold, the milk production lost 0.830 (95% CI: -0.832, -0.828) kg/cow/day in the whole Lombardy. These results can be used for economic calculations on the costs of mastitis