Effects of pH and fibrolytic enzymes on digestibility, bacterial protein synthesis, and fermentation in continuous culture

Abstract

A dual effluent continuous culture system (CC) was used to investigate effects of pH and addition of fibrolytic enzymes to a dairy cow diet on fermentation, digestibility and bacterial protein synthesis. The experiment was a completely randomized design with two factorial treatments and four replications. Factors were fermenter pH level (5.5, 6.0 and 6.5) and fibrolytic enzyme supplementation (control and enzyme). Fermenters were fed twice daily an early lactation diet that consisted of about 50% forage and 50% barley-based concentrate (dry matter [DM] basis). The enzyme product used was a commercial blend which contained relatively high xylanase and cellulase activities (Promote ®, Agribrands Inc., St. Louis, MO). The enzyme (0.2 ml) was diluted into 20 ml of water and then sprayed onto 1 kg of total mixed ration (dried in an oven at 55 °C and ground through a 4.5-mm screen) prior to feeding. Total volatile fatty acids (VFA) concentration and its molar proportions increased linearly ( P<0.04) with increasing fermenter pH. Degradabilities of DM, organic matter (OM) and fibre, except crude protein (CP), were affected ( P<0.05) by fermenter pH. Considerable increase in digestion was observed when fermenter pH increased from low (5.6) to medium (6.0), but effect of a further increase from medium (6.0) to high (6.0) were negligible. Enzyme supplementation did not affect total VFA concentrations, but increased molar proportions of acetate ( P<0.01) and reduced that of propionate ( P<0.01), as a result of increased digestion of NDF ( P<0.01) and ADF ( P<0.04). However, degradation of CP and bacterial protein synthesis were not affected by adding fibrolytic enzymes into the diet. There was a numerical ( P>0.13) interaction between pH and enzymes for fibre digestion; the increase in digestibility of NDF due to enzyme supplementation was lower (8%) when mean fermenter pH was 5.6 compared to when pH was over 6.0 (18%), indicating that the effectiveness of enzymes can be reduced in low rumen pH conditions. The present results indicated that manipulation of fermenter pH in CC was highly effective in altering fermentation pattern and fibre degradation. Fibrolytic enzymes have the potential to improve in vitro fibre degradation, but are not expected to fully overcome the limits to fibre digestion caused by low pH.