Conditions stimulating neutral detergent fiber degradation by dosing branched-chain volatile fatty acids. Ⅱ: Relation with solid passage rate and pH on neutral detergent fiber degradation and microbial function in continuous culture

摘要

To support improving genetic potential for increasedmilk production, intake of digestible carbohydrate mustalso increase to provide digestible energy and microbialprotein synthesis. We hypothesized that the provisionof exogenous branched-chain volatile fatty acids(BCVFA) would improve both neutral detergent fiber(NDF) degradability and efficiency of microbial proteinsynthesis. However, BCVFA should be more beneficialwith increasing efficiency of bacterial protein synthesisassociated with increasing passage rate (k_p). We alsohypothesized that decreasing pH would increase theneed for isobutyrate over 2-methylbutyrate. To studythese effects independent from other sources of variationin vivo, we evaluated continuous cultures without(control) versus with BCVFA (0 vs. 2 mmol/d eachof isobutyrate, isovalerate, and 2-methylbutyrate), lowversus high k_p of the particulate phase (2.5 vs. 5.0%/h),and high versus low pH (ranging from 6.3 to 6.8 diurnallyvs. 5.7 to 6.2) in a 2 × 2 × 2 factorial arrangementof treatments. Diets were 50% forage pellets and50% grain pellets administered twice daily. Without aninteraction, NDF degradability tended to increase from29.7 to 35.0% for main effects of control compared withBCVFA treatments. Provision of BCVFA increasedmethanogenesis, presumably resulting from improvedNDF degradability. Decreasing pH decreased methaneproduction. Total volatile fatty acid (VFA) and acetateproduction were decreased with increasing k_p, eventhough true organic matter degradability and bacterialnitrogen flow were not affected by treatments. DecreasingpH decreased acetate but increased propionate andvalerate production, probably resulting from a shiftin bacterial taxa and associated VFA stoichiometry.Decreasing pH decreased isobutyrate and isovalerateproduction while increasing 2-methylbutyrate productionon a net basis (subtracting doses). SupplementingBCVFA improved NDF degradability in continuouscultures administered moderate (15.4%) crude proteindiets (excluding urea in buffer) without major interactionswith culture pH and k_p.