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Reduce your risk
Reduce your risk. “A total postemergence program is the most risky weed-control system, because the timing of a postemergence herbicide application is almost completely up to Mother Nature, and no one can control the weather,” points out Hager. “Instead, try using an integrated program with some soil-residual products. Also, farmers could consider a split application of an early preplant treatment followed either by a pre-emergence or a postemergence treatment to provide more consistent weed control than a single, early preplant application.”
A pre-emergence herbicide application will help keep late-emerging weeds small and uniform enough in height to boost odds for success when following up with a postemergence treatment, points out Gunsolus. “Using a pre-emergence herbicide buys you more time to apply your postemergence herbicide for optimal weed control,” he says.
In addition, a pre-emergence herbicide can be an especially good investment with irrigation, which ensures moisture is available at the right time to activate the chemistry, says Sandell. “With dryland corn in Nebraska, using a residual pre-emergence program will still be beneficial in reducing potential problems with glyphosate-resistant weeds, even if a lack of rainfall delays activation past the ideal time for starting corn out in clean fields,” he says.
Pay attention to timing. “Your main management focus should be on controlling those early weeds,” says Gunsolus. “Our research in Minnesota, and more comprehensive research in Wisconsin, shows that at about the V3-V4 stage, if weeds aren’t removed, fields will suffer an average 3 bu./acre/day yield loss up until the end of June. In fact, farmers should plan to have all their weed control completed by the fourth of July.”
Timing is important for both post- and pre-emergent applications, adds Hager. “For pre-emergent herbicide applications, try to time them closer to when you plant, especially if you have a weed spectrum in the field that can emerge later in the season, such as waterhemp.”
Weeds that re-infest after an initial herbicide application can also be very competitive, and Hager recommends being vigilant to control these later weed flushes, if necessary, while they are also still small.
Be careful with reduced rates. Many farmers run reduced herbicide rates of soil-residual herbicides to save costs, says Hager. “However, with reduced rates, you may be setting the product up to fail earlier, depending on weather conditions and weed pressure. Using a full, or a nearly full rate based on soil type often provides an extended period of weed control that you don’t always have with reduced rates.”
Gunsolus agrees. “Especially in the postemergence arena, good early season weed control has a lot to do with proper timing and not skimping on rates,” he says. “Also, when you do postemergence weed control, make sure you don’t go too fast and check to make sure you’re getting good spray coverage on weeds.”
Scout and reassess. After each weed-control practice, farmers should scout fields and evaluate how well their treatment worked and whether or not a remedial treatment might be needed, advises Gunsolus.