Effects of Lipid Intake and Degree of Saturation on Nutrient Digestion and Nitrogen Balance in Steers Consuming Corn-Based Diets

Increases in dietary lipid can augment energy density; however, lipids can mitigate ruminal fermentation of feed. We evaluated effects of amount and source of dietary lipid on nutrient digestibility, ruminal fermentation, and N balance in cattle consuming corn-based diets. Five steers (BW = 392 ± 15 kg) fitted with ruminal, duodenal, and ileal cannulas were used in a 5 × 5 Latin square with 12-d periods. Diets contained no added fat, or 4 or 8% added lipid from either a predominantly saturated (i.e., tallow) or predominantly unsaturated (i.e., linseed oil) source. Unsaturated fat decreased (P = 0.01) DMI compared to saturated fat and increasing amount of lipid supplementation tended (Linear = 0.14) to decrease DMI. Neither added amounts or source of lipid affected (P ≥ 0.33) ruminal NDF digestion. However, increased diet lipid decreased ruminal DM digestibility (Linear = 0.04) and tended (Linear = 0.15) to decrease ruminal starch digestibility. Apparent (Linear = 0.03) and truly (Linear = 0.05) fermented OM in the rumen decreased in response to increased dietary lipid. Ruminal pH (P ≥ 0.50), ammonia (P ≥ 0.18) and total organic acid content (P ≥ 0.31) were not affected by diet lipid. Additionally, amount and source of lipid had no effect on ruminal acetate (P ≥ 0.21) or propionate (P ≥ 0.21) concentrations; however, lipid amount tended (Linear = 0.07) to decrease acetate:propionate. Similarly, total tract digestion of DM (Linear < 0.01), OM (Linear < 0.01), and NDF (Linear = 0.02) were decreased by added dietary lipid. Greater lipid intake also tended (P = 0.12) to decrease total tract starch digestion. Unsaturated lipid tended (P = 0.06) to decrease total tract NDF digestibility, but saturated lipid tended to decrease total tract DM (P = 0.12) and OM (P = 0.10) digestion. Microbial N flow to the duodenum was not affected (P ≥ 0.18) by lipid, but microbial efficiency tended (Linear = 0.06) to increase with increased lipid intake. Fecal N excretion was decreased (P < 0.01) by unsaturated lipid. Nitrogen balance (P ≥ 0.28) and N efficiency (P ≥ 0.58) were not affected by lipid source or amount.