Abstract
In feeding trials to clarify the mechanism by which unsaturated oils depress milk fat percentage, oleic acid at 250 or 500ml per cow per day did not reduce milk fat percentage significantly. At 500ml these changes were significant (control, oleic): rumen acetate 61.6, 603%; rumen propionate 19.4, 21.0%; milk fat content of 18:1 trans fatty acid 3.0, 8.0%; and of 18:2 cis fatty acid 2.2, 1.4%.Feeding hydrogenated vegetable oil containing 13% trans acid at 454g per cow per day decreased slightly milk fat percentage and elevated plasma cholesterol 190 to 245 mg/100ml and 18:1 trans fatty acid in milk fat 4.2 to 6.2%. Hydrogenated vegetable oil containing 49% 18:1 trans acid at 454g daily decreased milk fat 3.9 to 3.1%. Milk fat triglycerides decreased in short chain fatty acids and increased in 18:1 trans 2.6 to 11.2%, 18:1 cis 22.9 to 29.0%, and 18:2 trans .2 to 1.8%. Milk phospholipids decreased 14.1 to 9.6% in 14:0 fatty acid and increased .3 to 3.1% in 18:1 trans and 20.5 to 31.4% in 18:1 cis. Blood cholesterol esters were increased 152 to 195 mg/100ml.The data lend support to the concept that trans acids or compounds produced in the rumen during their formation from polyunsaturated fatty acids are responsible for the milk fat depression from unsaturated oils.
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