Delaying post-emerge weed control in corn can be costly

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There are plenty of weedy corn fields across Ontario as a planting season with weather-induced stops and starts has some growers struggling to get crop in the ground and also stay ahead of the weeds.

Getting the jump on those weeds, especially in fields relying on post-emergent herbicide programs is critical, says University of Guelph weed scientist Dr. Peter Sikkema. This is especially true in high weed density environments where growers can take a significant yield hit when herbicide application is delayed by as little as 24 hours.

“If today is Tuesday and we delay spraying until Wednesday, our data says we lose 1.7 bushels per acre,” Sikkema told an Ag Business Breakfast Meeting with agronomists and ag business reps at Ridgetown, Ont., earlier this week. His findings are based on some robust post-emergent product research, including seven trials conducted over a three-year period.

Sikkema did some further calculations to highlight the toll high weed pressure can take on corn yield and profitability. Waiting to spray on Wednesday — a one-day delay — would cost growers $11.00 per acre in lost yield based on $6.52 corn (the five-year average price during the trial period). If growers waited until Friday to spray — a three-day delay — the financial hit would be $33 per acre.

Sikkema acknowledges that every field is different and weed spectrum does play a role, but under heavy pressure, losses are significant. He understands that getting to fields in a timely manner can be a challenge for growers, “but smaller weeds are always easier to kill than big ones.” He also notes that delaying application typically leads to bigger weeds, which can require higher product rates, and increaseed cost.

For growers who did apply a pre-emergent herbicide on corn and soybeans and are now planning a post application to optimize control, Sikkema recommends spraying the corn first. “The yield loss to weed interference in corn is greater than soybeans — corn is just more sensitive to weed interference,” he notes.

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