LAUSR

Abstract Most non-genetically modified (GM) soybean ( Glycine max Merr.) cultivars are bred and performance-tested under conventional conditions and have rarely been tested in organic production. Twelve non-GM soybean cultivars were evaluated in weed-free and weedy conditions on 5 organic farms and 1 transitional farm in southern Manitoba in 2014 and 2015. The mean cultivar yield ranged from 1384 to 1807 kg ha −1 . Weed biomass at soybean maturity ranged from 1289 to 2553 kg ha −1 and was significantly affected by cultivar. Significant site–cultivar interactions were observed for soybean biomass, height, and yield. Site accounted for 72.4% of yield variability; cultivar accounted for only 1%. Our hypothesis that cultivars with greater early season height are more competitive with weeds was not supported. Yield loss due to weeds ranged between 20 and 44%; lower yield loss was associated with timely weed management. Partial least squares regression was used to assess the main factors controlling grain yield. Higher soil nitrate (N) status negatively impacted final grain yield in this study, suggesting that soil nutrient status impacted the soybean cultivars’ competitive ability against weeds. Results suggest that weed management and soil N status are of equal importance to cultivar choice for successful organic soybean production in Manitoba.