Effect of dietary protein level and conjugated linoleic acid supply on milk secretion and fecal excretion of fatty acids

Little is known about the influence of dietary crude protein (CP) restriction and rumen-protected conjugated linoleic acid (rpCLA) supply on digestibility and metabolism of nutrients in dairy cows. This study investigated the effect of low dietary CP level combined with rpCLA supply on FA secretion into milk and excretion via feces in dairy cows. Twenty Friesian cows were involved in a 4 × 4 Latin square design over 3-week periods. Cows were fed 4 different rations based on 2 dietary CP levels (150 and 123 g/kg dry matter of CP), each one with or without the addition of a 80 g/d supplement of rpCLA. Feed, feces and milk samples were analyzed for their fatty acid content (FA) and data were used to calculate apparent FA transfer as the ratios between the daily FA secretion in milk, or excretion in feces, and intake. Dietary CP restriction had little influence on apparent FA transfer from the feed to the feces and the milk, but slightly increased the apparent transfer to the milk of 14:0iso (P = 0.031) and 15:0iso (P = 0.006), and tended to increase transfer of other branched FAs of microbial origin, i.e. 15:0anteiso (P = 0.06), 16:0iso (P = 0.06) and 17:0iso (P = 0.07). The rpCLA supply lowered the secretion/intake ratio of total FAs in the milk (P = 0.02), particularly those with the shortest chains and with even numbers of carbons, but had no effect on apparent transfer of branched and odd FAs of microbial origin. The rpCLA supply greatly reduced (P < 0.001) the secretion/intake ratio of 18:2c9,t11 and 18:2t10,c12 in the milk but also increased (P < 0.001) the milk secretion of 18:2c11,t13 (not supplied with the rpCLA) leaving no trace of it in the feces. In conclusion, the CP × rpCLA interaction had negligible effects of FA profile of both milk and feces. Our results suggested that rpCLA supply alters milk FA profile, mainly reducing the de novo synthesis of short-chain FAs and the Δ9 desaturase activity in the bovine mammary gland. Little influence of the dietary CP on animal digestive and metabolic processing of nutrients at the levels used in this experiment were suggested by small variations in the transfer of FA from feed to milk and feed to feces.