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

Amanzougarene, Zahia
Fondevila Camps, Manuel (dir.)

Universidad de Zaragoza, VET, 2015
Producción Animal y Ciencia de los Alimentos department,

Máster Universitario en Nutrición Animal

Abstract: 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 (P<0.001) with the buffering of the media, and remained constant throughout the 12 h incubation period except for treatments 5.75 and 5.50, whose pH continued decreasing to 5.51 and 5.31 at 12 h, respectively. Gas production decreased linearly (P<0.001) with the medium pH at 2, 4, 6, 8 and 10 h, and quadratically (P= 0.012) with the medium pH at 12 h. The total volume of gas produced after 12 h was correlated to pH at 12 h (R2= 0.629, P<0.01) and 6 h (R2= 0.836, P<0.01). 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= 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 ± 0.2. The type of substrate did not affect (P>0.05) 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 (P<0.001) volume of gas and the three varieties of sorghum recorded the lowest (P<0.001). The concentration of total VFA was influenced by the substrate type (P<0.001): at 4 h this concentration was highest with CP followed by WB, and the lowest concentration was recorded with the two corn varieties C1 and C2 and the three varieties of sorghum, whereas at 8 h WB was higher than CP, and S2 recorded the lowest concentration. For the VFA profile, similar pattern was observed at both 4 and 8 h: the proportion of acetate was highest with S2 and lowest with WB, whereas WB recorded the high proportion of propionate, together with the barley X varieties and SU, and S2 recorded the lowest proportion; and the lowest butyrate proportion was recorded with CP and BP. 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 (P<0.001). Among substrates, pH was lower with barley varieties, and higher with sorghum varieties (P<0.001), this effect being more apparent with CI. The interaction inoculum x substrate (P<0.001) indicates that cereal varieties behaved differently depending on the inoculum source, but no differences in incubation pH were detected among varieties of the same cereal (P>0.05). The gas production recorded with the CI as inoculum was always superior to that with FI (P<0.001). Despite the inoculum, after 10 h incubation the barley varieties recorded the highest volume of gas (P<0.001) followed by corn and sorghum varieties, which recorded the lowest volume mostly with S2 when incubated with FI. A significant interaction between the inoculum type and the substrate was recorded at all incubation times (P= 0.001), indicating different behaviour among varieties of each cereal species for each inoculum. The DMd was higher with FI than with CI (0.360 vs. 0.280; P<0.001), being highest for barley (on average, 0.511), followed by corn (0.314) and then sorghum varieties (0.136; P<0.001). Total VFA concentration was higher with CI than FI (P<0.001), with higher (P<0.001) proportions of acetate and branched-chain VFA and lower (P<0.001) of butyrate and valerate with the latter. Among substrates, total VFA concentration was higher (P<0.001) with barley than corn and sorghum varieties. At 4 h there was not difference between corn and sorghum in the total VFA concentration, whereas at 8 h it was higher with corn than sorghum. At 8 h with FI, sorghum varieties were more acetate than barley, whereas no substrate differences were detected with CI. For butyrate, its proportion with CI was lowest with barley but did not differ among substrates with FI inoculum. The lactic acid concentration was affected by the inoculum (P<0.001), being higher with CI. Barley was the type of substrate which recorded the highest concentration, and sorghum the lowest. Differences among varieties within species were only recorded with FI at 8 h, and with CI at 4 h. From this experiment it is apparent that barley varieties ferment at a higher extent, and promote a higher drop of XI 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 (P<0.001). The interaction inoculum x substrate at 4 (P<0.05) and 6, 8 and 10 h (P<0.001) indicate that substrates behave differently for both inocula. With CI, at all times of incubation SU recorded the lowest pH (P<0.001), whereas the other substrates maintained parallel trends, being lower with CP (P<0.001). With FI, SU abruptly dropped from 5.75 at 6 h to 4.87 after 10 h incubation, and CP became lowest (P<0.001) 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 (P<0.001). The interaction inoculum x substrate (P<0.05) 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 > 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).

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