ABSTRACT
Summary A set of procedures for the selection of Rhizobium strains for enhanced dinitrogen fixation in forage legumes is presented. Although forage legumes are among the most efficient members of the Leguminosae regarding biological nitrogen fixation (BNF), they are generally neither productive nor persistent without effective nodulation by appropriate rhizobia. The following methods define a selection and evaluation process to obtain appropriate Rhizobium. strains. These include (1) regional survey of the production area, (2) strain collection and soil analyses, (3) strain characterization, (4) strain selection and manipulation, and (5) field evaluation of strains. The initial procedure is to survey the region where the host legume is produced. This is necessary to obtain soil and plant samples, to become familiar with the major soil and climatic zones within the region, and to select potential field sites. Specific sampling sites chosen within each zone, based on the history of the legume host, are used to collect soil and plant samples for analysis and nodule isolations. At least 10 Rhizobium isolates should be recovered from each site, and the plant-infection assay should be used to determine the density of the indigenous Rhizobium populations. The effectiveness of the Rhizobium isolates on those host plant cultivars that are grown in the region is the most important selection criterion. Other methods that may help in selecting strains for field trials might include growth in acidified media, salt tolerance, and survival under moisture/temperature extremes. In order to measure strain performance in the field it frequently becomes necessary to mark Rhizobium strains genetically so that they can be identified when recovered from nodules. Antibiotic resistance and/or serology are the techniques of choice, although there are problems with both. Eventually field plots will become cross-contaminated with different strains, and then identification of each strain is the only way that the evaluations can proceed. Field plots should be designed with at least 4 replicates and appropriate uninoculated controls. Data should consist of plant dry weights, nitrogen content, nodulation score, and root and leaf stage from samplings every 6 or 8 weeks. Nodule-occupancy tests should focus on those strains that over time are found to be most productive in BNF and host-plant responses. The best strains from the field tests should be considered for inclusion in inoculants or used in some distribution system for wider testing at the producer level.
