Feeding value of supplemented diet with black tea by‐product silage: Effect of polyethylene glycol addition to the diet on digestibility of protein fractions in goats

Abstract

AbstractThe feeding value of black tea by‐product silage was studied both in vitro and in vivo. To estimate the effect of tannins in black tea by‐product on nutrient digestibility, polyethylene glycol (PEG) was also given to goats as a tannin‐binding agent. Regarding in vitro ruminal fermentation, supplementation of black tea by‐products silage to basal substrates at more than 1% on a dry matter (DM) basis decreased the ammonia‐N (NH3‐N) concentration in the medium, indicating that black tea by‐products suppressed protein degradation in rumen. In the in vivo digestibility trial, six castrated goats were fed diets of timothy hay + soybean meal (control), control + black tea by‐product silage (+BTS), and control + black tea by‐product silage + PEG (+PEG) in a replicated 3 × 3 Latin square design. Black tea by‐product silage was supplemented to the control diet at 5% on a DM basis. Black tea by‐product silage supply significantly decreased crude protein (CP) digestibility compared to the control diet (63.9 vs 67.5%), but the diminished digestibility tended to be improved by PEG addition (66.5%). Acid detergent insoluble N (ADIN) was slightly digested in control (1.5%), but the digestibility was negative (–19.1%) in +BTS. On the other hand, addition of PEG clearly improved the ADIN digestibility (2.9%). Urinary N output and ruminal NH3‐N concentration was altered in neither +BTS nor +PEG. Fecal N output was significantly higher in +BTS than in control, but it tended to be lower in +PEG. The amount of retained N was the highest in +PEG, followed by +BTS and control. This study confirmed indirectly that chemical compound in black tea by‐product silage that lowered protein digestibility was tannins. It was suggested that offering black tea by‐products silage at a 5% rate in a diet was such a high level that fecal N output was increased by tannin‐protein bindings fractionated as ADIN.