INFLUENCE OF DIETARY-FAT SOURCE ON OXIDATIVE STABILITY AND FATTY-ACIDCOMPOSITION 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 tw ice daily: soybean meal (no supplemental fat); cracked, micronized soy beans (containing 20 g kg(-1) oil); and soybean meal plus CSFA which s upplies an amount of fat similar to that of the soybeans. Periods last ed 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, compo sition and flavour were made at the end of weeks 4 and 6 of each perio d. Feeding CSFA reduced DM intake (P = 0.064), but fat supplemented di ets tended to supply more NE(1) than the unsupplemented diet (P = 0.08 5). 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 soybea ns 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 fatt y acids (P < 0.001) and increased the proportion of C-18 fatty acids i n milk (P < 0.001). Soybeans increased the proportion of polyunsaturat ed fatty acids in milk but CSFA did not. Soybeans can escape biohydrog enation in the rumen and render milk susceptible to oxidation by incre asing the proportion of polyunsaturated fatty acids. At the levels fed , vitamin E did not prevent oxidation due to its low transfer to milk.