As herbicide-resistant weeds continue to proliferate and diversify, farmers are going to have to spend more money on controlling and diversifying crop rotations to avoid exacerbating yield losses. IMPACTS OF LATE POSTEMERGENCE APPLICATION Dr. Peter Sikkema, (now retired) professor and field crop weed management expert at the University of Guelph Ridgetown Campus, recently concluded two studies analyzing the effectiveness of different herbicides and herbicide mixtures, and timing of application, on controlling problem weeds – including those with multiple resistances to different activity groups. For the first study – on emerging weed management issues in corn, soybean, and cereals – Sikkema says the take-home message is that post-emergence weed control is not enough to prevent yield losses, particularly in corn, and when application occurs late. “I think many farmers across Ontario do not appreciate how much money they’re losing by going with a total post-emergence weed control program for corn. If weeds are an inch or two inches in height, they think that’s okay. They can have a perfectly clean field at harvest time and not realize how much they lost,” says Sikkema, noting soybeans are comparatively less susceptible to weed interference, though still somewhat affected. A delayed post-emergence herbicide application in corn when weeds are five centimetres in height, for example, results in a yield loss of 5.5 bushels per acre, or $26.62 per acre. When herbicide application is delayed until weeds are 10 centimetres in height, yield losses total 16.3 bushels per acre or $79.24 per acre. For weeds of five and 10 centimetres in height in soybeans, a delayed post-emergence herbicide application results in a yield loss of one bushel per acre, or $13.37 per acre, and 1.5 bushels and $18.67 per acre, respectively. Asked why such yield losses have gone unnoticed, Sikkema believes it stems in part from the introduction of better postemergence herbicides in the 1990s – the doctrine at the time being to let weeds come up so applicators knew what needed to be targeted and where. The introduction of Roundup-Ready corn amplified this in the early 2000s. However, things have dramatically changed in the last 10 to 15 years. “The research since then has reiterated starting clean, whether you’re growing corn, soybeans, dry beans, or anything. That means putting down your best soil-applied herbicide and managing the escapes,” says Sikkema. Knowledge of weed pressure in previous years and the herbicide sensitivity of future crops is also critical. “In some years, you will not have to put on any post-emergence herbicide…In other years, you’re going to have weed escapes and need to put herbicide on that matches those escapes.” ONTARIO GRAIN FARMER RESEARCH 26 Matt McIntosh Controlling herbicide-resistant weeds Highly effective weed control is still possible with the right chemistry and the right approach to application. Early application and crop diversity critical
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