Effect of Conservation Method on in Vitro Ruminal Fermentation of Purple Prairie Clover (Dalea purpurea Vent.) in Batch Culture

Little information about the effects of forage conservation method on the biological activity of condensed tannins (CT) in the rumen is available. The objective of this study was to determine the effects of forage conservation method and CT in conserved forage on rumen fermentation. Purple prairie clover (PPC; Dalea purpurea Vent. ) containing 84.5 CT/kg DM was harvested at full flower and preserved as freeze-dried forage (FD), hay (H) or silage (S). Batch culture incubations were conducted using conserved forages as a substrate with and without inclusion of polyethylene glycol (PEG) which inactivates CT. The treatments were arranged as 3×2 factorial design, and the incubation was repeated twice with quadruplicate fermentation vials for each treatment in each incubation. ¹⁵N labelled ammonium sulfate was used as microbial N marker and headspace gas was sampled to determine methane production. Concentrations of NDF and ADF were lower (P<0.01) in FD than in H and S. Ensiling decreased (P<0.001) total phenolics and extractable CT content but had no effect on total CT. Hay did not affect either extractable or total CT content. Hay and silage had lower (P<0.01) true DM disappearance (TDMD) and NDF disappearance (NDFD) than S and FD, whereas inclusion of PEG did not affect TDMD or NDFD at 8, 24 or 72 h of incubation. Hay had lower microbial protein (MP) production (P<0.001) than FD and S, and inclusion of PEG decreased (P<0.001) MP at 24 or 72 h of incubation. Productions of total gas, methane and total VFA and the rate of gas production were similar, but ammonia accumulation was higher (P<0.05) for PEG than non-PEG treatments during 72-h incubation. Incubation with S produced VFA with a lower (P<0.001) proportion of acetate, but higher (P< 0.05) proportion of propionate, resulting in a lower (P<0.001) acetate: propionate ratio as compared to that of FD or H after 8 h of incubation. No difference was observed between H and S in VFA profile. Condensed tannins in PPC decreased protein degradation in the rumen but had minimal effects on overall rumen fermentation, and conservation of PPC as hay or silage had little effect on the efficacy of CT in modulating rumen fermentation.