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The effect of ammonia treatment and rice bran supplementation on the in situ rumen degradation of rice straw was determined using three Japanese Corriedale wethers fitted with permanent rumen cannula. About 4 g samples of diets containing 100% untreated rice straw (URS); 100% ammonia treated rice straw (ARS); 65% URS+30% rice bran (RB)+5% soybean meal (SBM) (T1); and 85% ARS+15% RB (T2) were incubated at 0, 4, 8, 16, 24, 48, 72, and 96 hours in the rumen of sheep to measure dry matter (DM), crude protein (CP) and neutral detergent fiber (NDF) degradability. The DM disappearance of ARS based diets were about 20% higher than that of URS based diets. Rice bran supplementation improved DM disappearance of URS but not on ammoniated straw. Degradation parameters showed that ammoniation increased rate (c) of straw degradation resulting to higher DM and fiber degradability but RB supplementation did not. ARS gave similar DM and CP solubility and effective rumen degradability (ED) with that of the supplemented groups indicating that ammoniation alone can give the same effect on rumen degradability of sheep receiving low quality roughage. All degradation parameters for NDF were consistently higher in ARS based-diets indicating improved fiber solubility. Rice bran supplementation did not affect degradation characteristics of the diets except on soluble DM and CP fraction (A) of URS but not on ARS.