How early weed pressure affects crop yield before plants even emerge

When it comes to weed control, the strategy of starting clean and staying clean is becoming even more important as weed scientists gain a greater understanding of just how early crops can sense the presence of weed competition. In this interview, RealAgriculture’s Peter Johnson speaks with Dr. Clarence Swanton, professor emeritus at the University of Guelph, about research that is reshaping how scientists understand crop competition and yield loss. Swanton’s latest work, which he shared at the Southwest Agricultural Conference, shows that crops can sense neighbouring plants extremely early, altering their growth and physiology long before traditional competition for water, nutrients, or sunlight begins. “As a seedling comes through the ground, as it gets close to the soil surface, it can detect what’s above it through light signals,” Swanton explains. “It can actually change how its growth pattern develops before it’s even out of the ground.” That early detection triggers a cascade of internal responses. Swanton’s research shows the plant can produce elevated levels of singlet oxygen — a reactive molecule associated with plant stress. The crop must then divert energy to manage this stress response rather than directing that energy toward growth and yield. “It’s like saying the ship is on fire,” Swanton says. “The plant has to expend energy to control that fire before it can continue on.” These responses can also affect gene expression. Swanton says neighbouring weeds can suppress photosynthetic genes and reduce chlorophyll production, while increasing the plant’s antioxidant activity to manage internal stress. Together, those changes can reduce the crop’s resilience to additional pressures such as drought, insects, or disease. “If weeds are present early and another stress comes along — like drought — that can become the perfect storm,” Swanton says. “That’s another argument for why early-season weed control is really critical.” The research also raises questions about practices such as planting green into living cover crops. While those systems can offer soil health benefits, Swanton says the presence of neighbouring plants during emergence may introduce additional risk if other stresses occur. Plant size and leaf area influence how strong the signal is, meaning larger weeds or cover crops can create stronger competitive signals for emerging crops. One management option may be creating a clean band around the crop row to reduce early signals from neighbouring plants. If growers can get those neighbouring plants out of the immediate bandwidth — perhaps 30 centimetres overall — that would be a good beginning, Swanton suggests. Website: https://www.realagriculture.com/ #weedcontrol #agronomy #farming #agriculture Find us on our other social media platforms: X/Twitter:   / realagriculture   Instagram:   / realagriculture   Facebook:   / realagmedia