INFLUENCE OF DIETARY-FAT SOURCE ON OXIDATIVE STABILITY AND FATTY-ACID COMPOSITION OF MILK FROM COWS RECEIVING A LOW OR HIGH LEVEL OF DIETARY VITAMIN-E

Fifteen primiparous Holsteins were used in a latin square design with three periods and three treatments to assess the effect of fat source on mirk flavour, yield and composition, and composition of milk fat. A basal diet was supplemented with one of three concentrates offered twice daily: soybean meal (no supplemental fat); cracked, micronized soybeans (containing 20 g kg(-1) oil); and soybean meal plus CSFA which supplies an amount of fat similar to that of the soybeans. Periods lasted 6 wk, and each cow received 6000 IU dL alpha-tocopheryl acetate d(-1) for the last 2 wk of each period. Measurements of milk yield, composition and flavour were made at the end of weeks 4 and 6 of each period. Feeding CSFA reduced DM intake (P = 0.064), but fat supplemented diets tended to supply more NE(1) than the unsupplemented diet (P = 0.085). Yields of milk (P < 0.001), fat (P < 0.001) and energy (P = 0.003) were increased by fat supplementation. Protein yield was reduced (P = 0.003) when CSFA were fed. Soybeans reduced fat concentration in milk, but only before supplemental vitamin E was given. Soybeans increased alpha-tocopherol output in milk (P < 0.05). Milk from cows fed soybeans was most susceptible to oxidation (P < 0.01). Intensity of oxidized flavour was similar before and during vitamin E supplementation, Fat supplementation reduced the proportion of short- and medium-chain fatty acids (P < 0.001) and increased the proportion of C-18 fatty acids in milk (P < 0.001). Soybeans increased the proportion of polyunsaturated fatty acids in milk but CSFA did not. Soybeans can escape biohydrogenation in the rumen and render milk susceptible to oxidation by increasing the proportion of polyunsaturated fatty acids. At the levels fed, vitamin E did not prevent oxidation due to its low transfer to milk.