DIETARY AND RUMINALLY DERIVED TRANS-18 1 FATTY-ACIDS ALTER BOVINE-MILK LIPIDS/

The objectives of this study were to determine whether dietary fish oi l enhances production of trans-18:1 isomers in the bovine rumen and wh ether flow of ruminally derived and/or dietary trans-18:1 to the small intestine is correlated with milk fat production. Four lactating Hols tein cows with cannulas in the rumen and proximal duodenum were fed di ets with 0% supplemental fat (control diet), 3% hydrogenated tallow fa tty acids (HTFA diet), 1.5% menhaden oil plus 1.5% stearic acid (MO+SA diet), or 1.5% soybean oil plus 1.5% partially hydrogenated soybean o il (SBO+HSBO diet) in a 4 x 4 Latin square design. Intakes of trans-18 : 1 were 0, 12, 0 and 69 g/d when cows were fed the control, HTFA, MOSA and SBO+HSBO diets, respectively. However, the extent of 18:1 + 18: 2(n-6) + 18: 3(n-3) biohydrogenation in the rumen was reduced by consu mption of the MO+SA and SBO+HSBO diets. As a result, flow of trans-18: 1 to the duodenum was 163 g/d for MO+SA-fed cows and 152 g/d for SBO+H SBO-fed cows compared with 38 g/d for cows fed the HTFA and control di ets. Incomplete biohydrogenation accounted for all of the trans-18:1 f low when the MO+SA diet was fed. Compared with results when the HTFA a nd control diets were fed, milk fat percentage was lower and concentra tion of trans-18:1 in milk fat was higher when the MO+SA and SBO+HSBO diets were fed. Across all treatments, milk fat percentage decreased l inearly with the amount of trans-18:1 flowing to the duodenum and the concentration of trans-18:1 in milk fat. Results indicate that bovine milk fat percentage is depressed by trans-18:1, whether derived from t he diet or from incomplete biohydrogenation of unsaturated fatty acids .