SummaryThree experiments were conducted to determine the value of foliage from three tropical legume trees as low level protein supplements to napier grass diets for growing ‘Kacang’ goats. The average crude protein concentration in the napier grass was 12%.Napier grass and foliage of the legume trees Gliricidia maculata, Leucaena leucocephala and Sesbania grandiflora were subjected to in situ microbial fermentation and subsequent treatment with acid-pepsin solution. The levels of N solubilized after 2 h incubation were 46 and 43% for napier grass and sesbania respectively, which were higher (P < 0·05) than those for gliricidia and leucaena (27%). Rates of protein disappearance between 2 and 24 h incubation in the rumen averaged 2·6%/h for the legumes and 1·0%/h for napier grass. The proportion of water-insoluble, rumendegradable protein from the legumes was larger (P < 0·05) than that from napier grass.Napier grass intake by goats supplemented with gliricidia or leucaena at 15% of the dry-matter intake from napier grass was lower (P < 0·05) than that of controls receiving no legume supplement. Napier grass intake did not differ between controls and sesbania-supplemented goats. There was no difference among diets in total dry-matter intake, intake of cell wall constituents or digestibility. Average daily gain for control goats was – 1 g/day as compared with 21 g/day for supplemented goats.The feeding of formaldehyde-treated soya-bean meal (F-SBM) as a supplement to either napier grass or napier grass–legume diets increased (P < 0·05) intake of dry matter and weight gain of goats. Napier grass intake of animals supplemented with only F-SBM was higher (P < 0·05) than that of control animals. The efficiency of N utilization from F-SBM was higher than that in the legumes, but replacement of legumes by F-SBM above 4% F-SBM feeding had no effect on weight gain or efficiency of utilization.It was concluded that napier grass of 6–8 weeks' regrowth with 12% crude protein did not provide sufficient protein for growing goats owing to inefficient protein utilization. The increase in efficiency of protein utilization on supplemented diets is mainly associated with the larger proportion of water insoluble, rumen degradable protein and possibly acid-pepsin soluble protein in tropical tree legumes.

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