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How well did your herbicide program work?

Tom J. Bechman tall waterhemp weed
CLASSIC REGROWTH: This weed, which appears to be tall waterhemp, was set back by a herbicide application, but it’s coming back strong and will be a factor this season.
Soybean Watch: Check for regrowth of weeds that were sprayed.

Weed control overall is good so far in the Soybean Watch ’21 field. The grower included a residual herbicide in the burndown applied for weeds since he no-tilled soybeans into cornstalks. Then he returned with a postemergence application, adding an extra herbicide on end rows and the outside round where weed pressure was heaviest.

You may think you can forget about weed control if you put as much effort into controlling weeds as this grower did. However, Steve Gauck, a regional agronomy manager for Beck’s, sponsor of Soybean Watch ’21, says the last and perhaps most important step is scouting the field at midseason, assessing weed control and looking for weed escapes or regrowth.

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In some past years, scattered patches of Canada thistle were burned back in the Soybean Watch field, but a few managed to grow back to some degree.

“Canada thistle is one of those tough perennials that you aren’t going to eliminate with a typical broadleaf postemergence herbicide,” Gauck says. “You can burn them back enough to get through the season. However, since they have rhizomes underground and an extensive root system, expect them to come back.

“The best advice is to target those areas for fall spraying after sufficient regrowth occurs with a mixture of glyphosate and 2,4-D. You may have to hit them again in the spring.”

Weed regrowth

The picture with this story tells its own tale. This weed was spotted in a neighboring field this summer, not the Soybean Watch ’21 field. It’s in the pigweed family and is likely tall waterhamp.

The field was custom-sprayed, and the farmer assumed his weed concerns were over. The applicator he uses always does an excellent job.

Nevertheless, this weed, set back by the herbicide but obviously not killed, was growing again within three weeks after application. It wasn’t a lone wolf, either. Many of its friends were also making a comeback, and could still factor into yields this season and in future seasons.

“We don’t know why he didn’t get complete kill this time on some of these weeds,” Gauck says. “Environmental conditions when spraying or a whole host of factors could have been at play here.

“The point is that some of these weeds survived. They regrew early enough in the season that they could still be a factor this year. Maybe they will impact yield, maybe not. They definitely could be a nuisance at harvest.”

The worst part, Gauck says, is that if the weeds are waterhemp, they likely will still be able to produce seed. Waterhemp is a prolific seed producer. Even a plant set back by herbicide that got a second chance could produce thousands, if not hundreds of thousands, of seeds and leave them behind for future seasons.

“If you don’t go back and scout fields to assess weed control, you may not know these are there until they stick up above the canopy and are obvious at harvest,” he adds. “By then, it’s too late to go back and assess what might have gone wrong and allowed the regrowth. The goal is to figure it out so you can make adjustments for next season.”

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