[en] Mastitis and milk quality in dairy cows are an ongoing concern of great relevance to animal welfare and productivity in modern dairy production. High somatic cell scores (SCS) are an indirect measure of presence of mastitis and low milk quality, they are relatively easy to record and values of heritability are higher than records of mastitis. However, published SCS values are highly variable across studies which makes it impossible to have a reliable reference value. The objectives of this study are to perform a meta-analysis (1) to estimate this reference value and the extent of its variability and (2) to identify whether and how factors of variation (year and country of publication, parity, and breed) influenced this value. Information on SCS was retrieved from 138 papers published between 1979 and 2020 in 40 countries and analyzed with the Metafor package in R software. Standard deviations were estimated from available data or imputed using a Bayesian hierarchical modelling approach. Results of the meta-analysis revealed a significant decrease of 0.04 units in mean SCS with the year of publication, an increase with the number of parities and a significant variability across countries. The reference SCS value was estimated at 3.68 (3.59 - 3.76) and total heterogeneity across studies at 1.24 (1.11-1.38). Further analyses are necessary to verify arguments provided to explain the results.
