Rumen protected fat reverses the conjugated linoleic acid induced low milk fat content in dairy cows

Conjugated linoleic acids (CLAs) were protected by encapsulation in a matrix of formaldehyde-treated protein; these rumen protected (RP-CLAs) were fed in two separate trials using Holstein cows grazing pasture to examine their effects on milk fat content, yield and fatty acid composition. In trial 1, three groups of nine cows were fed pasture alone, RP-CLA (61.5 g d-1 equivalent to 10.3 g of trans-10, cis-12 isomer) and rumen protected canola/soybean (RP-C/SB) (65.6 g d-1 to have the equivalent level of fat containing no CLA). RP-CLA reduced milk fat yield by 27, 29 and by 38%, respectively, after 4, 7 and 10 d of supplementation. Milk fat content was also reduced and after 10 d of CLA supplementation declined from 3.7 to 2.3%. RP-CLA supplementation resulted in a gradual decline in the yield of C 10-C14, C16 and C18 fatty acids in milk during the 10-d feeding period. In trial 2, feeding RP-CLA for 4 d reduced milk fat content (pasture 3.4%, RP-CLA 2.4%) and yield (pasture 826 g d -1, RP-CLA 594 g d-1) by the same magnitude of 29%. Milk fat content and yield was restored by feeding RP-CLA together with RP-C/SB supplement (1-2 kg d-1), which provided an additional 328 g or 656 g of fat per day; this suggests that CLA preferentially inhibits mammary gland lipogenesis.