Genotype by environment interaction and local adaptation of commercial legume cover crop genotypes.

Inconsistent winter survival is one of the major challenges in fall-planted legume cover crops. Although several genotypes marketed as winter-hardy are available commercially, their performance is unknown across many US regions. Climate change has further escalated this problem with unpredictable weather conditions. To address this gap, we evaluated winter survival of 30 genotypes of hairy vetch, winter pea, crimson clover, red clover, and balansa clover in 31 US location-years over two growing seasons (2016-2018). We analyzed the effect of genotype, environment, genotype-by-environment interaction, and weather-trait associations. Hairy vetch cultivars 'CCS Groff', 'Purple Prosperity', and 'TNT' showed the highest overall winter survival combined with stable performance across environments. Variance partitioning indicated that environment (43.2%) and genotype-by-environment interaction (35.9%) explained more variation in winter survival than the genotype main effect (17.1%), indicating that cultivar performance was considerably context-dependent and that environment-specific recommendations are warranted. In most environments, winter survival was moderate (0.50-0.80) to high (greater than 0.80), except in two site-years where even the superior genotypes had less than 0.5 winter survival, and substantial year-to-year variation occurred even within the same locations. Environment-level correlation analysis indicated that temperature in November, February, and March (positive effect) and snowfall in these same months (negative effect) were most consistently associated with winter survival, though no single weather variable or combination dominated, reflecting the complexity of winter survival as a trait. The substantial genotype-by-environment interaction observed, together with very low winter survival in some environments, highlights the necessity of multi-environment testing and developing more winter-hardy and stable legume cover crop cultivars.