Conjugated linoleic acid, a bioactive fatty acid (FA) found in milk and dairy products, has potential human health benefits due to its anticarcinogenic and antiatherogenic properties.

Conjugated linoleic acid concentrations in milk fat can be markedly increased by dietary manipulation; however, high levels of CLA are difficult to sustain as rumen biohydrogenation shifts and milk fat depression (MFD) is often induced. Our objective was to feed a typical Northeastern corn-based diet and investigate whether vitamin E and soybean oil supplementation would sustain an enhanced milk fat CLA content while avoiding MFD. Holstein cows (n=48) were assigned to a completely randomized block design with repeated measures for 28 days and received one of four dietary treatments:

  • Control (CON)
  • 10,000IU of vitamin E/d (VE)
  • 2.5 percent soybean oil (SO)
  • 2.5 percent soybean oil plus 10,000IU of vitamin E/d (SO-VE).

A 2-week pretreatment control diet served as the covariate. Milk fat percentage was reduced by both high-oil diets (3.53, 3.56, 2.94, and 2.92 percent for CON, VE, SO, and SO-VE), whereas milk yield increased significantly for the SO-VE diet only, thus partially mitigating MFD by oil feeding. Milk protein percentage was higher for cows fed the SO diet (3.04, 3.05, 3.28, and 3.03 percent for CON, VE, SO, and SO-VE), implying that nutrient partitioning or ruminal supply of microbial protein was altered in response to the reduction in milk fat. Milk fat concentration of CLA more than doubled in cows fed the diets supplemented with soybean oil, with concurrent increases in trans-10 18:1 and trans-11 18:1 FA. Moreover, milk fat from cows fed the two soybean oil diets had 39.1 percent less de novo synthesized FA and 33.8 percent more long-chain preformed FA, and vitamin E had no effect on milk fat composition.

Overall, dietary supplements of soybean oil caused a reduction in milk fat percentage and a shift in FA composition characteristic of MFD. Supplementing diets with vitamin E did not overcome the oil-induced reduction in milk fat percentage or changes in FA profile, but partially mitigated the reduction in fat yield by increasing milk yield.

Source: Journal of Dairy Science/A.M. O’Donnell-Megaro, J.L. Capper, W.P. Weiss, D.E. Bauman