TRANS-OCTADECENOIC ACIDS AND MILK-FAT DEPRESSION IN LACTATING DAIRY-COWS

We examined the role of trans-octadecenoic acids in milk fat depressio n when low fiber diets were fed. The study consisted of four experimen tal periods with a 2 x 2 factorial arrangement of treatments to test t he effects of dietary fat (saturated vs. unsaturated) and rumen fermen tation (high fiber diets vs. low fiber diets) on milk fat depression. Dietary fiber concentration and type of fat had significant effects On milk fat. Effects were most pronounced when unsaturated fat was added to the low fiber diet. When the low fiber diet plus unsaturated fat w as fed, milk fat percentage and yield were decreased by 30 and 35%, re spectively, compared with the percentage and yield when the high fiber diet plus saturated fat was fed. Alterations in rumen fermentation ca used by differences in dietary fiber concentrations had little effect on the amount of trans-octadecenoic acids in milk fat, and the total a mount did not correlate with changes in milk fat percentage. Further e xamination of the isomeric profile of trans-octadecenoic acid revealed substantial differences among the dietary treatments. Although the ad dition of unsaturated fat resulted in marked increases in the milk fat content of trans-octadecenoic acid, regardless of dietary fiber conce ntration, the low fiber diet plus unsaturated fat increased the conten t of trans-10-octadecenoic acid. This combination was also associated with a significant decrease in milk fat content and yield. When the lo w fiber diets were fed, circulating insulin concentrations were elevat ed, regardless of the type of fat supplement. However, marked milk fat depression occurred only when the low fiber diet was supplemented wit h unsaturated fat.