Refat, B., Anele, U., He, Z. X., Bassiony, S. M., Abdel-Rahman, G. A. and Yang, W. Z. 2015. Effect of sainfoin hay and pomegranate peel extracts on in vitro fermentation and protein degradation using the RUSITEC technique. Can. J. Anim. Sci. 95: 417-423. The objectives of this study were to determine the effect of supplementation of sainfoin hay extract (SHE) and pomegranate peel extract (PPE) on in vitro rumen fermentation characteristics, protein metabolism and nutrient digestibility using rumen simulation technique (RUSITEC). The experiment was a completely randomized design with three treatments and four replications. Dietary treatments were: (1) control (10% barley silage and 90% concentrate), and control supplemented (2) with SHE (4.1 g catechin equivalent kg^-1 dry matter; DM) or (3) with PPE (2.8 g tannic acid equivalents kg^-1 DM). The experiment consisted of 10 d of adaptation and 7 d of data collection. Concentration of total volatile fatty acid (VFA) was lower (P<0.02), but molar proportion of acetate was higher (P<0.01) with SHE or PPE than control. Branched-chain VFA decreased (P<0.01) with PPE supplementation compared with control. Concentrations of large and small peptides were not affected but ammonia nitrogen (NH₃-N) concentration decreased (P<0.01) with PPE supplementation. Digestibility of DM, starch and crude protein decreased (P<0.02) with SHE and PPE supplementation compared with control. Bacterial nitrogen production (mg d^-1) and bacterial efficiency (g bacterial nitrogen kg^-1 digested organic matter) were not affected by SHE or PPE supplementation. Overall, there were no differences in VFA concentrations, nitrogen fractions and nutrient digestibility between SHE and PPE except for the concentrations of branched-chain VFA and NH₃-N, which were higher (P<0.01) with SHE versus PPE supplementation. In conclusion, supplementation of high-grain diet with SHE or PPE can decrease rumen by-pass protein; however, suppression of feed fermentation may decrease the feeding value of high-grain diet.