Effects of the processing methods of corn grain and soybean meal on milk protein expression profiles in dairy cows

A proteomic approach was used to investigate the effects of the processing method of corn grain and soybean meal on the milk protein expression profile in lactating dairy cows. A total of 12 multiparous Holstein dairy cows were used in a 4×4 Latin square design with a 2×2 factorial arrangement. The primary factors examined were corn (finely ground (FGC) v. steam-flaked (SFC)) and soybean meal (solvent-extracted (SSBM) v. heat-treated (HSBM)), which were used to formulate four diets with the same basal ingredient: 27% FGC and 9% SSBM; 27% SFC and 9% SSBM; 27% FGC and 9% HSBM; and 27% SFC and 9% HSBM. Each period lasted for 21 days. Milk samples were collected on days 18, 19 and 20 of each period. Changes in the milk proteins were assessed by two-dimensional (2D) electrophoresis and ImageMaster 2D Platinum 6.0 software. A total of 13 spots displayed variations in protein spot abundance according to the statistical analysis. These spots were identified by a matrix-assisted laser desorption/ionization-time of flight/time of flight MS. According to the gels, the relative abundance of α(s2)-casein (CN) fragments was higher in the cows fed the SFC-HSBM than that for SFC-SSBM, whereas β-CN, α-lactalbumin and zinc-alpha-2-glycoprotein fragments were down-regulated in HSBM-fed cows. The relative decrease of β-CN expression was validated by western blot and agreed with the MS data. These results suggested that the method used to process soybean meal modified the synthesis and secretion of milk proteins in lactating dairy cows' mammary glands.