MILK COMPOSITION AND PROTEIN FRACTIONS FROM COMMERCIAL DAIRY HERDS

Abstract

The aim of this study was to evaluate the effect of breed, time of year, feeding system and mastitis on milk composition (fat, lactose, total solids) and milk protein fractions: crude protein, true protein and milk urea nitrogen. Forty-nine commercial dairy herds were randomly selected for monthly collection of bulk tank samples during the period of one year, divided by breed (17 Holstein, six Jersey and 26 Girolando herds) and the feeding system (six confined and 43 semi-confined). Milk samples were analyzed for fat, lactose, total solids, crude protein, true protein, milk urea nitrogen and somatic cell count. The results were submitted to descriptive statistics and analysis of variance to assess the effects of breed, seasonality, feeding system, somatic cell count and herd characteristics. There was no significant effect of breed on contents of fat, lactose and milk urea nitrogen. Highest concentrations of fat were observed in Jersey herds (3.97%) compared with those of Holstein (3.54%) and Gyr (3.45%), with no differences on fat contents. The nitrogen fractions were not affected by bulk tank somatic cell count. However, breed influenced milk true protein and non-protein nitrogen concentrations. The month of sampling had a significant effect on crude protein, true protein and non-protein nitrogen levels. Higher concentrations of crude protein and true protein were observed during October (3.30%), and the lowest levels in December (2.93%). No significant effect of feeding system was observed on true protein levels of Holstein cow’s milk.