Plastic response to early shade avoidance cues has season‐long effect on Beta vulgaris growth and development

Early‐emerging weeds are known to negatively affect crop growth but the mechanisms by which weeds reduce crop yield are not fully understood. In a 4‐year study, we evaluated the effect of duration of weed‐reflected light on sugar beet (Beta vulgaris L.) growth and development. The study included an early‐season weed removal series and a late‐season weed addition series of treatments arranged in a randomized complete block, and the study design minimized direct resource competition. If weeds were present from emergence until the two true‐leaf sugar beet stage, sugar beet leaf area was reduced 22%, leaf biomass reduced 25%, and root biomass reduced 32% compared to sugar beet grown season‐long without surrounding weeds. Leaf area, leaf biomass, and root biomass was similar whether weeds were removed at the two true‐leaf stage (approximately 330 GDD after planting) or allowed to remain until sugar beet harvest (approximately 1,240 GDD after planting). Adding weeds at the two true‐leaf stage and leaving them until harvest (~1,240 GDD) reduced sugar beet leaf and root biomass by 18% and 23%, respectively. This work suggests sugar beet responds early and near‐irreversibly to weed presence and has implications for crop management genetic improvement.