Effects of saturation and esterification of fat sources on site and extentof digestion in steers: Digestion of fatty acids, triglycerides, and energy

Five steers (mean BW 526 kg) fitted with ruminal, duodenal, and ileal cannu las were used in a 5 x 6 Youden square design with 14-d periods. Diets cont ained chopped alfalfa hay, corn silage, and concentrate (25:35:40, DM basis ). Treatments were 1) control (no added fat); 2) tallow (T), iodine value ( IV) = 51.5; 3) partially hydrogenated tallow (PHT), IV = 30.7; 4) hydrogena ted tallow (HT), IV = 6.9; ii) blend (1:1) of HT and hydrogenated free fatt y acids (HTHFA), IV = 9.0; and 6) hydrogenated free fatty acids (HFA), IV = 11.2. Fats replaced cornstarch in the control diet to supply 5% added fatt y acids. Intake was restricted to 90% of ad libitum; DMI was similar among diets (mean 9 kg/d). Total fatty acid intake averaged 170, 500, 506, 525, 4 89, and 491 g/d for treatments 1 to 6, respectively. Flows of total C-16, t otal C-18, and total fatty acids to the duodenum were increased by suppleme ntal fat; flows of total C-18 and total fatty acids were greater than their intake for all treatments. Flow of total fatty acids associated with rumin al bacteria accounted for 50 and 17% of the total duodenal fatty acid flow for the control and fat-supplemented diets, respectively. Digestibility of total fatty acids entering the small intestine (74, 71, 62, 39, 53, and 63% for treatments 1 to 6, respectively) was greater for the control diet than for fat-supplemented diets and decreased as either saturation (T < PHT < H T) or esterification (HFA < HTHFA < HT) increased. Digestibilities of fatty acids in the total tract followed similar patterns. Ruminal lipolysis of d ietary triglycerides decreased linearly as the degree of saturation of fat sources increased. Small intestinal disappearance of triglycerides (89, 75, 51, 44, 64, and 73% of duodenal flow for treatments 1 to 6, respectively) decreased linearly as either saturation or esterification increased. Flows and digestion of gross energy followed patterns similar to those for fatty acids and triglycerides. Resistance to ruminal and small intestinal lipolys is is a major factor contributing to the poor digestibility of highly satur ated triglycerides.