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The effect of supplementing a mixed rice straw and tropical grass diet with legume as a nitrogen (N) source on intake, digestibility, rumen ammonia and microbial protein production was evaluated in Bos indicus cattle. Four rumen-cannulated steers were used in a crossover design with two diets and two periods. The diets were T1 = 40% rice (Oryza sativa L.) straw + 60% grass (Brachiaria spp. cv. Mulato II hybrid) and T2 = 40% rice straw + 30% grass + 30% legume (Stylosanthes guianensis cv. CIAT 184) on DM basis. Supplementation with legume doubled (P < 0.01) rice straw and total N intake, and increased total DM intake by 32%. It did not affect the DM, organic matter, neutral detergent fibre and acid detergent fibre digestibility (P > 0.05) but did increase (P < 0.05) N digestibility. Faecal N and total N outputs from T2 cattle were higher (P < 0.05) than T1 cattle, but urinary N output did not differ between diets (P > 0.05). N retention in T2 cattle was improved by 83% (P < 0.05) compared with T1 cattle. Rumen ammonia concentration, microbial protein production and efficiency of microbial protein production were improved (P < 0.05) when the legume forage was included in the straw-grass diet. We conclude that when a mixed rice straw and fresh grass diet is supplemented with ~30% legume (DM basis), significant improvements in DM and N intake can be achieved. Cattle nutrition in tropical areas depends on rice straw and tropical grass diet, which is insufficient in fermentable energy and protein. By supplementing ~30% legume (DM basic) to a mixed rice straw and grass diet, intake and microbial protein production were improved to achieve weight gaining of Bos indicus cattle. Using legume (Stylosanthes guianensis), as part of an eco-farming system, is a key solution for smallholder farmers to improve their cattle production. [ABSTRACT FROM AUTHOR]