Nutrient digestibility and ruminal fatty acid metabolism in lambs supplemented with soybean oil partially replaced by fish oil blend

•Mixing fish oil with soybean oil to manipulate rumen fermentation is proposed.•Fish oil supply increases the duodenal flow of C18:1 trans-11 vaccenic acid in lambs.•Small amounts of fish oil do not affect the intake and digestibility of nutrients by lambs. The objective of these experiments was to evaluate the effects of partially replacing soybean oil with small amounts of fish oil blend on the intake and digestibility of nutrients, ruminal SCFA and pH, and duodenal flow of fatty acids. Five ram lambs rumen and proximal duodenum-cannulated (51.7±3.1kg of initial BW; mean±SD) were assigned to a 5×5 Latin square design. The oils were added to a basal diet that contained 900g/kg DM of concentrate (corn, soybean meal, soybean hulls, urea, ammonium chloride, limestone, mineral mix, and Rumensin® 100) and 100g/kg DM of forage (fresh sugarcane bagasse). The treatments were as follows: (1) basal diet without added oil (CONT); (2) 40g/kg DM of soybean oil (0FO); (3) 2.5g/kg DM of fish oil blend+37.5g/kg DM of soybean oil (2.5FO); (4) 5g/kg DM of fish oil blend+35g/kg DM of soybean oil (5.0FO); and (5) 7.5g/kg DM of fish oil blend+32.5g/kg DM of soybean oil (7.5FO). All diets were isonitrogenous (160±3g/kg DM of CP). Intakes of DM, organic matter (OM), non-fiber carbohydrates (NFC), and neutral detergent fiber (NDF) (kg/d) were not affected by the treatments. The apparent digestibility of DM, OM, and NDF were similar for all diets. The soybean oil and fish oil blend supplements decreased CP digestibility. Total fatty acids digestibility and total digestible nutrients (TDN) were higher in the diets with oils in comparison to the control diet. Ruminal concentrations of acetate, butyrate, and total short chain fatty acids (SCFA) were higher for animals fed the control diet. The ruminal pH was lower for animals consuming the control diet in comparison to the diets with oils. The duodenal flow of C18:0 and C18:1 trans-11 was greater for ram lambs that received soybean oil supplements, and the duodenal flow of C18:1 trans-11 and CLA C18:2 cis-9, trans-11 was greater for diets with oils. In addition, C18:1 trans-11 flow to the duodenum increased linearly as fish oil blend was added to the diets. The ruminal biohydrogenarion of C18:2 cis-12; C18:3 cis-9, cis-12, cis-15, eicosapentaenoic (EPA) and docosahexaenoic (DHA) acids were similar for all diets. When associated with high amounts of soybean oil, only a small amount of fish oil blend (2.5g/kg DM) was necessary to opti