Rumen fermentation parameters of grazing, lactating yaks offered supplementary 15 g/day RP-Lys and 5 g/day RP-Met (RPA), 1.2 kg/day concentrate (C) and 15 g/day RP-Lys, 5 g/day RP-Met and 1.2 kg/day concentrate (RPA+C), or no supplement (controls-CON). Values are means ± Standard Error of Mean (SEM); n = 6 for each group. p values for: Volatile fatty acids (VFAs) for RPA, C, and RPA × C were 0.734, <0.01, and 0.828, respectively; acetate values for RPA, C, and RPA × C were 0.773, <0.01, and 0.923, respectively; propionate values for RPA, C, and RPA × C were 0.743, 0.680, and 0.823, respectively; and butyrate values for RPA, C, and RPA × C were 0.469, 0.022, and 0.957, respectively.