Effect of the nature of the carbohydrate source in diets of fattening on ruminal fermentation under in vitro conditions

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 (from 6.50 to 5.50), adjusted according to the inclusion of bicarbonate ion in the incubation solution to simulate fermentation conditions under high concentrate feeding. This Experiment 1.0 highlighted the close relationship between medium pH and substrate fermentation, and allowed to establish the adequate buffering level for the next studies. In a second experiment (Experiment 1.1), all the experimental substrates (n equal to 13) were tested under optimal pH conditions (pH 6.5) using FI, in order to get a basic comparison of their fermentative characteristics. Throughout all the incubation period (8 h), the medium pH was maintained for all cases in the expected range of 6.5 plus minus 0.2. This experiment showed marked differences in fermentation characteristics among substrates, as expected, and gives a tool for their relative comparison in other fermentation conditions. In the third experiment (Experiment 1.2), the nine cereal sources were tested under a medium pH adjusted at 5.5 that is allowing for a maximum response of fermentation on incubation pH. According to the nature of inoculum (FI and CI) two different sets of incubation series were established. The inoculum source and the substrate type influenced significantly the medium pH, which was always lower with CI than FI. Among substrates, pH was lower with barley varieties, and higher with sorghum varieties, this effect being more apparent with CI. From this experiment it is apparent that barley varieties ferment at a higher extent, and promote a higher drop of medium pH with both inocula, although the CI inoculum also promotes a lower pH. Differences in gas production among cereal species were maximised with FI. There were only minor differences among cereal varieties, except among those of sorghum, probably because of their proportion of tannins. In another experiment (Experiment 1.3), four non-starch carbohydrate sources (BP, CP, WB and SU) were tested with inocula from different nature (CI and FI), under medium pH adjusted at 5.5, also including C2 as a control. As in the previous experiment, the drop in pH was more important with CI than FI. The interaction inoculum x substrate at 4 and 6, 8 and 10 h indicate that substrates behave differently for both inocula. With CI, at all times of incubation SU recorded the lowest pH whereas the other substrates maintained parallel trends, being lower with CP. With FI, SU abruptly dropped from 5.75 at 6 h to 4.87 after 10 h incubation, and CP became lowest at 6 h (5.52). The other substrates maintained medium pH between 6.0 and 5.7 throughout all the incubation period. At all times of incubation, the volume of gas produced with CI was higher. The interaction inoculum x substrate indicate differences between CP and BP with CI, at all incubation times, but with FI differences between CP and BP were only manifested from 6 h onwards. After 10 hours of incubation, the in vitro gas production of substrates ranked them as: CP, WB greater than SU, C2 and BP with CI, and CP greater than WB greater than BP, SU greater than C2 with FI. In terms of total VFA concentration, it was higher with CI, which showed more butyrate whereas that with FI had more acetate. At 4 and 8 h incubation, the VFA concentration was higher with CP. A higher acetate proportion was observed with BP, higher propionate with WB, SU, and CP, and the highest proportion of butyrate was recorded with WB and SU. In the Experiments 1.2 and 1.3 strong correlations were found between medium pH, gas production, total VFA concentration, VFA profile, and lactic acid (for Experiment 1.2)