Lab, greenhouse, and field experiments were conducted to determine whether plant and/or Rhizobia genetics could be selected to enhance symbiotic nitrogen fixation (SNF) in hairy vetch, especially when grown as a winter cover crop in colder climates. Thirty cold-hardy vetch lines were grown to determine how SNF and arbuscular mycorrhizal fungal (AMF) colonization vary with crop genotype. Other experiments were conducted to determine the effect of crop genotype, rhizobia strain, soil N levels, management history, climate, and genetic X environmental interactions on plant vigor, cold hardiness, and percent of N derived from SNF.

Vetch variety trials showed little correlation among %N derived from SNF, plant biomass, AMF colonization, and flowering date. Plant genotype and temperature drove SNF, which decreased dramatically under cold conditions. The ‘AU Merit’ vetch genotype, is considered cold susceptible, yet maintained the highest SNF rates under cool conditions (15°C/10°C).  Genotypes with greatest shoot N were not shown to select for more effective Rhizobia strains.  An evaluation of 16 Rhizobia strains from northern MN and 16 from NC identified five strains from MN and one from NC that consistently form nodules on vetch hosts; however, none outperformed a commercial inoculant in terms of vetch biomass.