Effects of differing levels of glycerol on rumen fermentation and bacteria

The effects of substituting corn with glycerol as a feed alternative were investigated using continuous fermenters. Four fermenters were used in a 4×4 Latin square design with four 10 days consecutive periods. Treatments diets contained 0 (T1), 36 (T2), 72 (T3) and 108 (T4)g glycerol/kg dry matter (DM). Diets consisted of 600g/kg alfalfa hay and 400g/kg concentrate (DM basis) and glycerol replaced the corn in the concentrate. Effluents were collected from each fermenter during the last 3 days of each period and analyzed for nutrients composition. On day 10 of each period, additional samples were collected from each fermenter at 3h after the morning feeding and analyzed for volatile fatty acids (VFA), ammonia-nitrogen (NH₃-N), and microbial DNA concentration. Results showed that neutral detergent fiber (aNDFom) digestibility decreased (P<0.05) with the T3 and T4 diets compared with the T1 diet. Glycerol substitution had no effects on fermenters pH, NH₃-N concentration, and digestibility coefficients of DM and acid detergent fiber (ADFom). The molar proportion for acetate decreased (P<0.05) while the molar proportions for butyrate, valerate and isovalerate increased (P<0.05) with the glycerol diets compared with the T1 diet. The DNA concentrations for Butyrivibrio fibrisolvens and Selenomonas ruminantium decreased (P<0.05) with the T3 and T4 diets compared with the T1 diet. The DNA concentration for Clostridium proteoclasticum also decreased (P<0.05) with glycerol substitution. No differences in the DNA concentrations for Ruminococcus albus and Succinivibrio dextrinosolvens among diets were observed. Results from this study suggest that substituting corn with glycerol at low level had no adverse affects on fermentation, digestion or ruminal bacteria. Higher substitution levels, however, may adversely affect rumen fermentation through reducing fiber digestion, acetate production and bacterial populations.