Influence of subclinical mastitis on milk composition of crossbred Holstein x Zebu dairy cows.

Milk composition is influenced by nutritional and non-nutritional factors. Subclinical mastitis is one of the most important non-nutritional factors affecting milk composition. Noteworthy, the contents of lactose and non-fat solids (NFS) in milk from Brazilian dairy herds are usually lower than those found in the published literature, which could be related to the high prevalence of subclinical mastitis in Brazilian dairy her Thus, we aimed to evaluate the influence of somatic cell counts (SCC), as an indicator of subclinical mastitis, on milk composition of crossbred (Holstein-Friesian x Zebu) dairy cows in Brazil. Since there are two well-defined seasons (dry-mild; rainy-hot) in most part of the country, this source of variation was also included in our study. Data from 468,262 samples collected from bulk tank milk (BTM) of commercial dairy farms over a period of three years (2006-2008) were used in the analysis. SCC values were divided in seven classes, ranging from <50,000 to >1,600,000 cells/ml. The contents of fat, protein, lactose, total solids and SNF in milk during the dry and rainy seasons were, respectively: 3.77/3.75, 3.28/3.30, 4.42/4.42, 12.50/12.45 and 8.72/8.70. Considering 400,000 cells/ml as a threshold, the contents (%) of the same milk components were 3.72/3.80, 3.30/3.28, 4.47/4.37, 12.51/12.44 and 8.78/8.64 for SCC values below and above the threshold, respectively. Although SCC had influenced all milk components (p<0.01), the most pronounced changes occurred for lactose content which was decreased by 0.30 percent units at cach logioSCC increase. This reduction was more pronounced during the rainy scason when compared with the dry season (0.36 vs. 0.26 percent units, respectively). A similar inverse relationship was also observed between SCC and SNF (-0.20 percent units), with the magnitude of reduction being higher in the rainy season than in the dry season (0.26% vs. 0.16%, respectively). The season of year influenced milk fat and protein contents (P<0.05 and P<0.01, respectively). There was a significant interaction between year season and SCC for all milk components. In general, our data showed that the SCC was inversely associated with milk lactose and SNF contents in Brazilian commercial dairy farms.