Phenology and developmental stage mediate symbiotic N fixation in a hairy vetch breeding program

Hairy vetch ( Vicia villosa Roth) is an overwintering annual legume valued for its ability to provide N through symbiotic N fixation (SNF). Although SNF is a high‐priority trait for farmers, evaluating SNF in a breeding program is a challenging endeavor. Our objective was to evaluate how SNF varies within and among hairy vetch genotypes and how SNF covaries with traits directly targeted by breeders (i.e., vigor and flowering time). We evaluated SNF in two hairy vetch breeding nurseries (2017 and 2019) based on the proportion of N derived from atmospheric fixation (Ndfa) measured using the δ 15 N natural abundance technique. We also considered plant size (vigor) and flowering stage. In 2017, we sampled plants on the same day, at varying developmental stages. In 2019, we sampled plants at a common developmental stage (early flowering) on different days. The 2017 data could not be confidently interpreted because of confounding effects between developmental stage and genotype. The 2019 data collection controlled for developmental stage effects and showed a positive relationship between vigor and Ndfa. This relationship was partly driven by flowering time, since genotypes with delayed anthesis produced greater biomass and fixed N. All three traits showed discernable variation among genotypes and likely reflect cold late winter/early spring conditions that limited growth and N‐fixation of early‐flowering plants. Taken together, our study shows that developmental stage and community composition are important considerations for evaluating SNF in annual legume breeding programs.