Fermentation of a High Concentrate Diet as Affected by Ruminal pH and Digesta Flow

Continuous culture fermentations were conducted to determine the effect of pH and digesta flow rates on fermentation of a 65% grain, 35% forage ration by rumen microorganisms. The pH selected were 5.8, 6.2, 6.6, and 7.0. Imposed on each pH in a randomized complete block design were digesta flow rates of 30/4, 26/8, 22/12, and 18/16, where the first number represents solids retention time (hour) and the second the liquid dilution rate (percent/hour). Digestibility of organic matter, neutral detergent fiber, and nitrogen was significantly depressed at pH 5.8, increased markedly at pH 6.2, and increased only slightly at pH 7.0. Digestibility of total nonstructural carbohydrate averaged 91.4% and was not affected by digesta flow rate or pH. Production of total volatile fatty acid was highest at pH 6.2 and 6.6. Overall means for acetate production increased, but propionate and butyrate production decreased as pH was increased from 5.8 to 6.2 with little response to further pH increases. Microbial efficiency decreased with increasing pH but increased 36% as digesta flow rate increased. The percentage of the microbial population associated with effluent neutral detergent fiber was decreased when pH was reduced below 6.2, which reflects the pattern seen for fiber digestion.