Effects of direct-fed microbial supplementation on ruminal acidosis, digestibility, and bacterial protein synthesis in continuous culture

A study was conducted to determine whether a bacterial direct-fed microbial (DFM) alone or with yeast could be used to minimize the risk of ruminal acidosis and to improve utilization of a feedlot cattle diet containing high concentrate. A dual effluent continuous culture (CC) system was used to investigate the effects of DFM on fermentation, digestibility, and microbial protein synthesis in a 4×4 Latin square design. Treatments were control (without DFM), Propionibacterium P15 (PB), Enterococcus faecium EF212 (EF), and E. faecium EF212 combined with a yeast, Saccharomyces cerevisiae (EFY) (Chr. Hansen Inc., Milwaukee, WI). Fermenters were fed twice daily a feedlot finishing cattle diet that consisted of about 871 g/kg barley grain, 79 g/kg barley silage and 50 g/kg supplement (dry matter (DM) basis). The DFM products (1×109 or 6×108 colony-forming units (CFU)/g for PB or EF and EFY, respectively) were delivered equally twice daily into the fermenters just before feed provision. Mean fermenter pH ranged from 5.86 to 5.91, and did not differ among the treatments. Total VFA concentration and its molar proportions were not affected by the DFM supplementation except for caproic acid which was higher (P<0.05) for control than for DFM addition. The ratio of acetate to propionate was similar among the treatments but was relatively lower (range of 0.83–0.93) than that usually observed in the rumen of feedlot cattle (range of 1.5–2.0) due to a considerably lower proportion of acetate (395 mmol/mol) but high proportion of propionate (460 mmol/mol). Ruminal lactate-utilizing bacterial numbers were (P<0.05) greater for control and EF diets than for PB or EFY diets. In vitro digestibilities of DM, organic matter, starch and crude protein were all in the range of in vivo findings but digestibilities of fiber including acid or neutral detergent fibers were significantly lower than those of in vivo findings. However, no differences in digestion of any nutrients studied were detected among the treatments. Microbial protein synthesis (range of 0.9–1.0 g nitrogen [N]/day) and microbial efficiency (range of 18–20 g N/kg of truly fermented organic matter) were also similar among the treatments. The present results indicate that addition of DFM such as PB, EF or EF combined with yeast had no effect on preventing ruminal acidosis and on fermentation or nutrient digestion in CC. The CC technique can be used as an alternative to in vivo techniques to assess effect of DFM supplementation on ruminal acidosis and digestion of nutrients.