Summary In order to determine the capacity of rumen acidification and their level and rate of fermentation in situations that mimic those of high concentrate feeding, the in vitro pH and gas production pattern of different sources of carbohydrates, namely three varieties of each of barley (B1, B2, B3), corn (C1, C2, C3) and sorghum (S1, S2, S3) as cereals, plus wheat bran (WB), citrus pulp (CP), sugar beet pulp (BP) and sucrose (SU) as sources of non-starch carbohydrates, were studied according to the nature of the inoculum source (from a concentrate diet, CI, or from forage diet, FI). A first methodological experiment (Experiment 1.0) was arranged with five levels of pH (6.50; 6.25; 6.00; 5.75 and 5.50), adjusted according to the inclusion of bicarbonate ion in the incubation solution to simulate fermentation conditions under high concentrate feeding. The pH diminished linearly (P0.05) the medium pH at 2 h, but at 4 and 8 h the effect was significant. With these experimental conditions, CP always recorded the highest (P0.05). The gas production recorded with the CI as inoculum was always superior to that with FI (P SU, C2 and BP with CI, and CP > WB > BP, SU > C2 with FI. In terms of total VFA concentration, it was higher with CI (P<0.001), which showed more butyrate (P<0.001) whereas that with FI had more acetate (P<0.001). At 4 and 8 h incubation, the VFA concentration was higher (P<0.001) with CP. A higher acetate proportion was observed with BP (P<0.001), higher propionate with WB, SU, and CP (P<0.001), and the highest proportion of butyrate was recorded with WB and SU. In the Experiments 1.2 and 1.3 strong correlations were finding between medium pH, gas production, total VFA concentration, VFA profile, and lactic acid (for Experiment 1.2)
