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

The 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 tanninbinding 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 (NH 3 -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 NH 3 -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 byproduct 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.