Energy and protein nutrition of ruminants present as major cost of feed for ruminants. Special interest has been given to the study of rumen nitrogen metabolism in dairy cows, given the cost of protein sources in diets. The evaluation technical of ruminal metabolism and especially estimation of microbial protein synthesis has been questioned, mainly by difficulties in estimating the post ruminal flow of amino acids. The literature is scarce information about the ruminal metabolism of nitrogen and especially the post absorptive metabolism of nucleic acids, when lipid sources are added to dairy cows diets. Some studies suggest reducing the degradability of protein diet and microbial synthesis with increased synthesis efficiency, as fat sources are used in the diet. Moreover, with the change in the metabolism as a result of increased energy density of the diet and serum cholesterol concentration, there may be change in muscle protein turnover and cellular ATP synthesis, leading to changes in the metabolism of nucleic acids, changing the composition of compounds excreted in urine and endogenous contribution of the purine derivatives excreted. This study aims to evaluate the ruminal and post-ruminal events associated with the addition of a lipid source in the diet, as well as increasing the concentration of degradable protein in the rumen in these diets.
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