Environmental factors which might limit the growth of leucaena in the Ryukyu Islands are temperature, soil pH and soil salinity. Growth response of leucaena to these environmental factors indicated that leucaena is one of the most productive forage resources for Rhodualfs land of coastal areas in the southern part of the Ryukyu Islands.
　As far as agronomic treatments are concerned, the inter-row spacing of 15-30 cm for leucaena to which napiergrass was mix-planted and harvesting when the leucaena reached 150-200 cm in height, produced the highest annual yield of usable forage. In this case, 28 t/ha of edible dry matter, 15 t/ha of digestible dry matter and 2.44 t/ha of CP, were produced. However, more yield can be expected by the application of phosphorus and some micro-nutrients such as Zn and Fe.
　The yield of leucaena mentioned above is a moderately high yield of protein rich, palatable forage. More nitrogen can be obtained by the sole cropping of leucaena but additional supply of other components than nitrogen is needed to make full use of the high protein content of leucaena as the feed. Animal disorder caused by mimosine contained in leucaena is a disadvantage, but the existence of mimosine degrading microorganism in rumens was proved by an indirect method.
　It is quite likely that the cost of producing forage by growing leucaena in the southern part of the Ryukyu Islands is much less than that of the present fodder cropping systems which require heavy application of N, rotational cultivation and much labor. Leucaena grown in rows interplanted with a grass seems to be the most useful forage production system in the southern part of the Ryukyu Islands, provided that the mimosine problem is solved.