Alteration of bovine milk fatty acid profile caused by the dietary crude protein content supplemented or not with rumen protected conjugated linoleic acids

Twenty cows housed in 4 pens, homogeneous for parity, days in milk (DIM), and milk yield (MY) received 4 diets with 150 (CP15) or 123 (CP12) g of CP/kg DM, supplemented or not with rumen protected conjugated linoleic acids (rpCLA) (6.34 and 6.14 g/d of C18:2c9,t11 and C18:2t10,c12, respectively), according to a 4x4 Latin Square design. Dry matter intake and milk yield were recorded daily and milk samples were collected during the last of 3 wks of each of the 4 experimental periods. Milk fatty acids (FA) were extracted with hexane-isopropanol and methylated with sodium methoxide. Commercial standards for FA molecules were used to identify milk FA and their detection was performed using a bi-dimensional gas chromatographer. Data were analyzed with a mixed model which considered period, dietary treatment as sources of variation and group as residual error term. The dietary CP had small influence on the FA profile, as only few FA, mainly some odd and branched FA, were altered. In particular, cows receiving the low protein diet produced milk with a significantly greater proportion of branched and odd FA (P<0.001) compared to control. Differently, rpCLA had a strong influence on the FA profile, as it reduced the proportion of short and medium chain FA and that of saturated FA (P<0.001) and increased the proportion of CLA (P<0.001).