Digestion of ryegrass pasture in response to change in pH in continuous culture.

The ruminal pH of dairy cows fed high quality pasture is often below values recommended to optimize digestion. Four continuous culture fermenters were used to determine the pH required for the optimal digestion of pasture. High quality pasture was fermented at four controlled levels of pH (5.4, 5.8, 6.2, and 6.6) according to a 4 x 4 Latin square design. Automatic infusion of 5 M NaOH and 5 M HCl controlled pH to +/- 0.1. Digesta samples were collected during the last 3 d of each of the four 9-d experimental periods. Digestion and synthesis of microbial protein were largely insensitive to pH across a broad range of pH (5.8 to 6.6), but a large reduction in both occurred when pH was 5.4. The digestibility of pasture dry matter and synthesis of microbial protein were optimized at pH 6.35 and 6.13, respectively. The proportions of individual volatile fatty acids were not changed as pH increased. Digestion of high quality pasture in continuous culture was comparable to that predicted by the Cornell Net Carbohydrate and Protein System between pH 6.2 and 6.6. However, the model underpredicted organic matter and fiber digestibility between pH 5.4 and 5.8, compared with values obtained in continuous culture. This suggests that when ruminal pH is less than 6.2, the model may over-predict the production response to supplementation of high quality pasture with an effective fiber source.