Selection of Rhizobia for Inoculation of Forage Legumes in Savanna and Rainforest Soils of Tropical America

Summary

Legumes which are adapted for use as forage plants in association with grasses on the acid problem soils of tropical America are being selected by a network of national agricultural programmes coordinated by the Tropical Pastures Programme at CIAT. Inoculants containing rhizobia are used for crop legumes in many of the countries involved, and many legumes yield better when inoculated. Evaluation of rhizobial inoculants for tropical forage legumes is therefore considered to be justified. A large collection of rhizobial isolates has been built up, and methods for screening them and evaluating inoculation responses in the field avoiding the use of complex or expensive equipment, have been developed, with a view to their use in isolated places. The same control strains are tested at different sites, nitrogen mineralization is minimized by using reduced tillage establishment, and there is one treatment where high nitrogen availability is ensured by making frequent applications.

An international collaborative training and research programme has been initiated to assess responses to rhizobial inoculation of locally selected legumes. An agronomist and a microbiologist are responsible for each trial, and both are involved at all stages of planning and execution. This approach has been adopted in view of the recognition that such trials are simple to execute but require the experience of both disciplines, and can fail if this is not available. Failure may be misinterpreted to mean that the legume does not need to be inoculated, or that the technology is too complex to use in the region.

This paper gives examples of the methods which have been developed. Two experiments for screening rhizobia in the greenhouse and the field with the promising forage legume Arachis pintoi are described, including details of the methods used. Marked yield increases due to inoculation of this legume with some strains were observed. The most effective strains were isolates from A. pintoi and other legumes. Abundant, internally red nodules were formed in the uninoculated controls. This demonstrates that a promiscuous legume may respond to inoculation if suitably selected strains are used.