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Genetic variability exists for enhanced SNF (Kamfwa et al. 2015; Oladzad et al. 2020). However, under conventional systems, modest amounts of nitrogen (50kg N/ha) are applied to most breeding nurseries. SNF capacity of beans is diminished in the presence of N fertilizers preventing easy selection of improved lines (Reinprecht et al. 2020). By selecting under low N-inputs or soils, breeders could enhance SNF. Any gain in SNF would be advantageous for conventional growers, who could reduce the nitrogen being applied to the crops. This would be particularly favorable for growers in the Great Lakes watershed or numerous other regions that have significant drainage basins where issues of both surface and ground water contamination are serious. This would also help organic production, as organic system producers cannot apply N‐fertilizer and have to rely on crop rotations, animal manures, or compost to meet the nutritional needs of the crop, and it has been found that nitrogen is a limitation in organic bean production (Heilig et al. 2017b). Since beans are also an important crop of small, low‐income producers in developing countries, improved N‐fixation would be a most beneficial trait for these resource poor farmers (Graham et al. 2003).

Fig. 2.6. Direct combining navy beans in Michigan.

The use of marker‐assisted breeding for SNF related traits is one way to circumvent the need to select under low N conditions. Multiple studies have been undertaken to identify QTL that enhance SNF. One study was undertaken in a black bean population that was a cross between a commercial variety Zorro and a Mexican landrace, long reported to have high SNP capacity (Chaverra and Graham 1992; Heilig et al. 2017a). GWAS studies on Andean and Middle American diversity panels both identified multiple SNF related QTL (Kamfwa et al. 2015; Oladzad et al. 2020). These studies offer potential candidates for marker‐assisted breeding.