Effects of dietary n-3 polyunsaturated fatty acids and ... - FINS

1 st WorkshopXIII International Feed Technology SymposiumGriinari and Bauman (31) have proposed that dietary factors which affect milk CLAcontent could be grouped into of two categories. The first would be factors that providelipid substrates for formation of CLA or trans C18:1 oleinic acid in the rumen. Thesecond would be factors that change the microbial activity associated with ruminalbiohydrogenation (4). Literature data are presented in Table 2. according to the ranges ofobserved milk CLA content.Table 2. Effects of dietary factors on milk fat CLA content in dairy cows (literaturereview) alow values(0.2–0.8%)- corn silage- grass silage/hay/pasture- animal or vegetable saturatedfats- raw soybeans- micronized soybeans- soybeans treated by heatprocessing- extruded soybeans orcottonseedsmedium values(0.8–1.6%)- fresh pasture/younggrass- low fiber diets- restricted feeding- extruded soybeans- peanut oil- rapeseed oil b- soybean oil b- linseed oil b- calcium salts ofrapeseed oilhigh values(> 1.6%)- rapeseed oil c- soybean oil c- sunflower oil c- linseed oil c- calcium salts ofsoybean and linseedoils- fish oilsa (1, 3, 5, 12, 17, 18, 21, 22, 33, 34, 35, 41, 43, 45).b At low dosesc At medium dosesHowever, no definitive conclusions can be drawn from these data, because a largeproportion of the data arose from indirect comparisons of experiments in differentlaboratories or experimental conditions, and also because a lot of the potentialinteractions in practical farm conditions have not yet been studied. It neverthelessappears that plant oils high in linoleic acid (e.g. sunflower, soya and rapeseed) are veryefficient at increasing milk CLA content. Besides directly increasing the yield of CLAand trans C18:1, it is likely that linoleic acid inhibits the final reduction of trans C18:1,thus increasing its accumulation in the rumen (31). Calcium salts of rapeseed alsoincrease milk CLA content (17), in agreement with the concept that calcium salts are notresistant to ruminal biohydrogenation. Limited research shows that the frequency offeeding cows fats and the physical form of oils (free or in raw seeds of oleaceous plants),as well as thermal processing of seeds of oleaceous plants, exert a relatively lowinfluence on altering milk fat composition (7, 27). Furthermore, vegetable oils are moreefficient than extruded seeds (which are themselves more efficient than raw seeds) atincreasing milk CLA content (32) (Tab. 2). This potency could be inversely related tothe protection of PUFA against biohydrogenation. On the other hand, supplementationwith animal fats is not very efficient at increasing CLA content because of their lowPUFA content. When soya oil was offered 24-times daily, instead of twice, the milk fatcontent increased, and the percentage of trans C18:1 decreased whereas that of C18:0246