Throughout the African highlands forage legumes are relied on to add soil N, support increasing livestock populations, and reduce soil erosion. This research addresses a constraint designated by the International Livestock Center for Africa (ILCA) to identify African clover (Trifolium) germplasm and Rhizobium trifolii strain combinations with high productivity potential for the African highlands. A Vertisol and an Eutric Nitosol (Paleudalf), and seed from Trifolium decorum, T. quartinianum, T. rueppellianum, T. steudneri, and T. tembense were obtained from ILCA in Addis Ababa, Ethiopia. A commercial collection and R. trifolii strains isolated from the Ethiopian soils were evaluated for symbiotic effectiveness with these clovers. Effective combinations were evaluated for growth and biological nitrogen fixation (BNF) in a greenhouse on both soils with limited, adequate, and excessive soil moisture. Rhizobia were also evaluated for survival in desiccated soil (12.5 and 17.5 g H₂O g⁻¹ soil) and for competitive nodule forming ability. Effective strains were found among soil isolates but not in the commercial collection. Highest dry matter yields and total BNF accumulation were obtained from T. tembense on all soil and moisture treatments followed by T. decorum and T. quartinianum.

Trifolium rueppellianum and T. steudneri had low yields and BNF capacities. Plants receiving adequate and excessive moisture had higher yields than moisture stressed plants. The highest levels of cumulative BNF were obtained on the nitosol soil with either adequate or limited moisture. The moisture limited vertisol supported the lowest BNF levels. Rhizobial strains survived desiccation only in the vertisol at 17.5 g H₂O g⁻¹ soil. All strains could compete with background rhizobia populations to nodulate host plants. Nodule occupancy rates of 20 to 30% were required for high yields. Trifolium tembense, T. decorum, and T. quartinianum are adapted to soils with adequate or excessive moisture, T. rueppellianum and T. steudneri are suited to moisture limited conditions or short growing seasons. Effective rhizobia inoculants and selected clovers have the potential for increasing forage productivity in highland areas.