Effects of ensiling grape pomace and addition of polyethylene glycol on in vitro gas production and microbial biomass yield

Due to a shortage of ruminant feeds, co-products such as grape pomace (GP) which contain tannins, represent alternative ruminant feed resources. The aim of the current study was to assess effects of ensiling and addition of polyethylene glycol (PEG) on deactivation of tannins in the grape pomace by changes in the chemical composition and phenolics levels of fresh and ensiled GP for 5, 10 or 30 days. In vitro gas production (IVGP), estimated organic matter digestibility (OMD), ammonia concentration and microbial biomass yield (MB) of each treatment, with or without PEG addition were measured. Increasing the duration of ensilage decreased the content of WSC and pH (P<0.01). Ensiling decreased (P<0.01) total phenolics (TP), total tannin (TT), soluble condensed tannins (SCT), protein precipitable phenolics (PPP), and IVGP, OMD and MB. Adding PEG to ensiled GP, however, increased (P<0.01) IVGP, OMD, ammonia and MB. The increment in gas production at 24 h for fresh and ensiled GP due to addition of PEG illustrates the positive influence of PEG addition, but not ensiling, on inactivation of tannins.