Acidogenic value of feeds. II. Effects of rumen acid load from feeds on dry matter intake, ruminal pH, fibre degradability and milk production in the lactating dairy cow

Rustomo, B., AlZahal, O., Cant, J. P., Fan, M. Z., Duffield, T. F., Odongo, N. E. and McBride, B. W. 2006. Acidogenic value of feeds. II. Effects of rumen acid load from feeds on dry matter intake, ruminal pH, fibre degradability and milk production in the lactating dairy cow. Can. J. Anim. Sci. 86: 119–126. The previous study evaluated the acidogenic value (AV) of a series of feeds. The objective of the current study was to determine the effect of increasing feed AV on dry matter intake (DMI), ruminal pH, fibre degradability and milk production and composition in lactating dairy cows. Two isoenergetic and isonitrogenous concentrate diets with either a low AV (LAV) or high AV (HAV) were formulated and fed along with corn silage and alfalfa silage in a total mixed ration (TMR). The diets were fed for ad libitum intake with as-fed forage to concentrate ratio of 57:43. Four rumen fistulated dairy cows (230 ± 30 DIM) were randomly assigned to one of the two treatments in a crossover design. Increasing feed AV increased (P < 0.05) time below pH 5.6 and the area below pH 5.6 to 6.0. The time distribution curve of HAV cows was shifted to a lower pH range, and hence, HAV cows tended to have longer time spent at pH 5.0 to 5.6 and shorter time spent at pH 6.2 to 6.8 than LAV cows. Increasing dietary AV increased (P < 0.05) milk yield from 21.8 L d–1 to 23.4 L d–1 and reduced (P = 0.04) the 72 h in situ NDF digestion of alfalfa/grass hay from 54.7 to 45.6%. Increasing dietary AV increased (P < 0.05) protein yield, lactose yield and lactose percentage. Increasing dietary AV had no effect (P > 0.05) on mean ruminal pH, DMI, and 4% fat corrected milk. These results emphasize the significance of ruminal pH variations and frequency distributions over time when evaluating the effect of feed AV on ruminal pH. The results suggest that feed AV could be used to predict ruminal pH in vivo.