Competitiveness and Symbiotic Effectiveness of Five Strains of Rhizobium Trifolii on Red Clover

Little information is available on seasonal Rhizobium ‐red clover ( Trifolium pratense L.) interactions, competitiveness, and persistence, and dinitrogen fixation rates in livestock production areas of the southeastern United States. Five introduced strains of Rhizobium trifolii were examined on red clover at two locations (on a ultisol and a vertisol) over two consecutive growing seasons. Double antibiotic resistance acquisition and intrinsic antibiotic tolerance were used to measure the proportions of nodules produced by the inoculum strains. Strains 162BB1, RP113‐7, and LM1 were highly competitive and persistent, and accounted for > 94% of the assayed nodules at both soils over both seasons. Strain 162P17 occupied 90 to 93% of the nodules in the establishment year at both sites. However, in the following season nodule occupancy of 162P17 declined to 83 and 59% at the ultisol and vertisol sites, respectively. A major decline in nodule occupancy by strain TA1 was observed to the extent of its being virtually eliminated by the indigenous Rhizobium populations of both soils during the second season. Higher forage and crude protein production were associated with strains 162BB1, RP113‐7, and LM1 at both locations and seasons. Results showed that red clover, when effectively nodulated by competitive and persistent strains of R. trifolii , can become established and productive under conditions found in the Lower South.