Decoding weed pressure in corn Ralph Pearce Peter Sikkema shares insights from decades of research on weed competition, resistance, and the economics of control Long-term trials show corn yield loss from weeds can swing dramatically, even in the same field. Understanding weed biology, species pressure, and emergence timing is key to managing the risk. ONTARIO GRAIN FARMER AGRONOMY 26 For more than 30 years, Peter Sikkema dedicated his field work to the science behind weeds and their effects on crop production. It took more than monitoring research plots or memorizing herbicide labels and modes of action — it required a sense of vision, an understanding of the changing biology of weeds, and their evolving tolerances and resistances. Although Sikkema retired from his position as field crop weed management professor at the University of Guelph’s Ridgetown campus last year, he is still called upon to provide his expertise and understanding of weed science. He was invited to give a session at the 2026 Southwest Agricultural Conference in January titled “Weed Whisperer,” where he was emphatic about the need for a higher level of management in dealing with weeds, many of which have developed resistance to multiple modes of action. To start, Sikkema shared data from work he conducted with Susan Weaver at Agriculture and Agri-Food Canada’s (AAFC) Harrow research station. The primary message from their work was that corn yield loss from weed interference is unpredictable and varies from field to field and year to year. Over a 10-year period, 59 trials were completed on the same 40 acres at the University of Guelph’s Ridgetown campus. “What was really interesting to me was that on the exact same campus, in seven per cent of the experiments, corn yield loss due to weed interference was less than 10 per cent,” said Sikkema. In 46 per cent of the experiments, he said the corn yield loss was between 10 and 50 per cent, and in 44 per cent of the experiments, the yield loss was between 50 and 90 per cent. In three per cent, the yield loss was greater than 90 per cent. The same soil, the same 40 acres and the weed seed bank hadn’t changed appreciably in 10 years. Based on the available data, Sikkema concluded that corn yield loss from weed interference is variable, unpredictable, and influenced by: • weather • weed density • seeding date • tillage practices • relative time of weed and crop emergence • soil texture and nutrient status • weed species composition “A really important one is the relative time of weed and crop emergence,” said Sikkema. “If those weeds come up at the same time as the crop, they have a much greater impact on corn yield than if the weeds come up a week or two after the corn. Since yield loss is variable and unpredictable, farmers should invest in a good insurance policy or a robust two-pass weed management program.” SPECIES-SPECIFIC The next observation was corn yield loss by species, based on another project of Susan Weaver’s testing seven annual grasses — crabgrass, yellow foxtail, witch grass, barnyard grass, green foxtail, fall panicum, and giant foxtail. Weaver also tested seven broadleaf weeds, including black nightshade, lady’s thumb, velvetleaf, common ragweed, redroot pigweed, lamb’s quarters, and giant ragweed. “Broadleaf weeds cause a greater yield loss in corn than annual grasses,” noted Sikkema. “Lamb’s quarters and giant ragweed
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