Effectiveness of Sinorhizobium Inoculants on Annual Medics

Abstract

Annual medics (Medicago spp.) have potential to be important forage legumes in the southern Great Plains of the United States. Currently there are no commercial medic species‐specific rhizobia strains identified for their inoculation. However, commercially available alfalfa (Medicago sativa L.) inoculants may be effective at successfully nodulating annual medics. The objective of this study was to determine the relevant symbiotic rhizobial partner in association with seven annual medics. A greenhouse experiment arranged in three replications of a randomized complete block design quantified rhizobium effectiveness index and shoot and root dry matter (DM) of Medicago arabica (L.) Huds., Medicago lupulina L., Medicago minima (L.) Bartal., Medicago orbicularis (L.) Bartal., Medicago polymorpha L., Medicago rigidula (L.) All., and Medicago rigiduloides E. Small when inoculated with five Sinorhizobium spp. strains (M2, M49, Nitragin A, Nitragin N [a blend of three isolates], and WSM1115) and an untreated control (UTC). Strain M49 was the only strain that effectively nodulated M. rigiduloides, while Nitragin A commercial alfalfa strain was only effective on M. rigidula. Inoculants M2, Nitragin N, and WSM1115 were able to effectively nodulate M. polymorpha, M. arabica, M. minima, and M. orbicularis. To a lesser extent, M. lupulina, M. minima, and M. orbicularis were also effectively nodulated by Nitragin A alfalfa strain. These data indicate that there are differences in rhizobia specificity of annual medic species, and further research is warranted to improve nodulation of these legumes in the field.