Effects of saturation and esterification of fat sources on site and extent of digestion in steers: ruminal fermentation and digestion of organic matter, fiber, and nitrogen.

Five steers (average 526 kg) fitted with ruminal, duodenal, and ileal cannulas were used in a 5 x 6 Youden square design with 14-d periods. Diets contained chopped alfalfa hay, corn silage, and concentrate (25:35:40, DM basis). Treatments were 1) control (no added fat), 2) tallow (T), 3) partially hydrogenated tallow (PHT), 4) hydrogenated tallow (HT), 5) blend (1:1) of HT and hydrogenated free fatty acids (HTHFA), and 6) hydrogenated free fatty acids (HFA). Fats replaced cornstarch in the control diet to supply 5% added fatty acids. Intake was restricted to 90% of ad libitum; DMI was similar among diets (average 9 kg/d). Ruminal pH and molar proportion of propionate (P) were greater (P < .05) but total VFA concentration, proportion of acetate (A), A:P, and percentages of OM digested in the rumen and total tract were less (P < .05) when fat-supplemented diets were fed than when the control diet was fed. Total VFA concentration increased linearly (P < .05) as esterification of fat sources increased (HFA < HTHFA < HT). Acetate and A:P increased linearly (P < .10) but propionate and apparent total tract digestibility of OM decreased linearly (P < .05) as either saturation (T < PHT < HT) or esterification of fat sources increased. Ruminal NH3 N concentration increased linearly (P < .001) as saturation increased. Apparent ruminal digestibilities of ADF (P < .05) and NDF (P < .10) increased linearly as esterification increased. Flow of nonammonia nonmicrobial N to the duodenum was less (P < .10) but flow of microbial N was greater (P < .05) for the control diet than for fat-supplemented diets. Flows and small intestinal digestibilities of N and efficiencies of microbial protein synthesis were not altered by degree of saturation or esterification. Results confirm previous in vitro observations that T or HFA can alter ruminal digestion; however, because these effects usually are not observed in dairy cows, feed intake likely is very important in responses to supplemental fats.