The use of different sources of supplemental fat in diets of dairy cows has been common practice in feed, mainly to allow improvement of energy status of these animals. However, when evaluating the supplementation of diets for ruminant with fat sources, all physiological processes should be taken into consideration, because the rumen metabolism, intestinal absorption, transport and systemic metabolism, secretion and deposition of fat in the organism are aspects directly related to lipid metabolism and may influence the physiological responses of animals receiving fat in the diet. The biohydrogenation is important step in the rumen metabolism of fatty acids and can be considered self-defense process of rumen microorganisms that convert unsaturated fatty acids in saturated fatty acids, less toxic to the rumen microbial population.Moreover, this process reduces the intestinal flow of polyunsaturated fatty acids for the duodenum and contributes to the accumulation of CLA isomers, cis and trans, in products derived from ruminants depending of the fat source used. Such isomers of CLA have been evaluated and found in products derived from ruminants raised on pasture or receiving diets rich in linoleic and linolenic acids, usually from oilseeds such as soybeans, and soybean oils and sunflower. While most of the fatty acid is modified by the rumen metabolism, the biohydrogenation is usually not complete, resulting in a wide variety of fatty acids. When there is incomplete biohydrogenation of polyunsaturated fatty acids, occur an increase of duodenal flow of trans fatty acids - C18: 1 and conjugated linoleic acid cis-9, trans-11-CLA and trans-10, cis -12 - CLA showing the first effects nutraceuticals and the latest proven inhibitory effect on the synthesis of milk fat, by reducing the expression of lipogenic genes.Thus, besides being considered a source of energy, the fatty acids are also indicated and used as moderators of the metabolism and physiology. However, the scientific hypothesis to be evaluated in this experiment suggests that, depending of the source of fat used, there may be changes in milk yield and its components. This experiment will be conducted to evaluate the effects of different sources of supplemental dietary fat for lactating cows on milk production and composition, and fatty acid profile of milk.
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