Acidogenic value of feeds. I. The relationship between the acidogenic value of feeds and in vitro ruminal pH changes

Rustomo, B., Cant, J. P., Fan, M. Z., Duffield, T. F., Odongo, N. E. and McBride, B. W. 2006. Acidogenic value of feeds. I. The relationship between the acidogenic value of feeds and in vitro ruminal pH changes. Can. J. Anim. Sci. 86: 109–117. The objective of this study was to use an in vitro technique (i) to assess the acidogenic value (AV) of feed ingredients, (ii) to evaluate the relationship between the AV of feed and ruminal pH changes, and (iii) to determine the relationship between the AV of feeds and chemical constituents of feeds. Assessments of AV were based on 24 and 48 h in vitro incubation in rumen liquor. A series of feeds, ranging from energy, fibre and protein sources were evaluated. Ruminal fluid pH changes in the incubation medium were measured at the end of 24 and 48 h incubation in buffered rumen liquor (60% buffer, 40% rumen liquor). The chemical constituents of the feed ingredients were determined using standard procedures. There were no differences (P > 0.05) between 24 and 48 h incubations on apparent AV and rumen fluid pH changes. The best predictors of AV for all classes of feed were non-fibre carbohydrate (NFC) fraction and acid detergent fibre (ADF; R2 = 0.81; P < 0.001). The best predictor of AV for energy sources were NFC and ADF (R2 = 0.70; P < 0.027); neutral detergent fibre (NDF) for fibre sources (R2 = 0.84; P < 0.027) and crude protein (CP) for protein sources (R2 = 0.73; P < 0.014). The rumen fluid pH changes had stronger relationships with apparent AV of all feeds after 24 h (R2 = 0.74, P < 0.0001) than starch (R2 = 0.35, P = 0.04) or NFC (R2 = 0.56; P < 0.0001). The results indicate that 24 h AV measurements and rumen fluid pH changes are acceptable measures for qualitatively describing or ranking feed ingredients.