Ruminal metabolism, fiber, and protein digestion by lactating cows fed calcium soap or animal-vegetable fat.

Effects of two levels of Ca soap or animal-vegetable fat on digestion of fiber and N and on microbial protein synthesis were studied in five ruminally and duodenally cannulated lactating cows in a 5 x 5 Latin square. Fat was added at 0, 2.5%, or 5% of dietary DM, which consisted of a 60:40 forage:concentrate mix. Treatment did not influence DMI, duodenal OM flow, or digestibility. Fat linearly increased liquid dilution rate (12.1 vs. 11.1 h-1) and reduced liquid (53.6 vs. 62.4 L) and ruminal (61.6 vs. 70.1 L) volumes. Source and amount of fat did not influence mean ruminal pH, ammonia N, or VFA concentrations; however, animal-vegetable fat reduced acetate:propionate ratios compared with Ca soap (3.47 vs. 3.64). Neither source nor amount of fat influenced fiber flow, digestion, or efficiency of microbial protein synthesis. Source and amount of fat did not change ruminal or total digestibilities of DM, OM, ADF, or NDF; however, dietary fat increased total tract apparent digestion of N (70.3 vs. 66.8). Estimates of ruminal DM digestibility were lower (24.2 vs. 39.0) and total tract DM digestibility was higher (63.3 vs. 60.5) with Cr marker compared with C31 hydrocarbon marker. Animal-vegetable fat or Ca soap can be supplemented up to 5% of the dietary DM in high forage diets without adverse effects on ruminal metabolism and digestion.