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marestail plants under soybeans
UNWELCOME SURPRISE: Steve Gauck pulls back soybeans and finds marestail plants lurking just beneath the canopy. These weeds may have been dinged by herbicides but still regrew.

How well did your weed control program work in soybeans?

Soybean Watch: Assess 2019 weed control successes and failures before harvest.

Steve Gauck inspects the Soybean Watch ’19 field often to assess how the crop is developing. But when he’s there, he looks at more than just soybean growth. He looks for diseases, insect activity and a whole lot more.

“When we walk fields late in the season, we can gauge how well the producer’s weed control program worked,” says Gauck, a Beck’s sales agronomist based near Greensburg, Ind. Beck’s sponsors Soybean Watch ’19.

Even if the field looks clean to the eye, Gauck knows there may be weeds lurking under the canopy. “They may be weeds which germinated later after the residual herbicide ran out and after the postemergence products were applied,” he says. “Or they may be weeds which were already up when herbicides were applied, but which were only burned back. Then later, they regrew and hung around. Some of those can still produce seeds, depending upon the species, and add to the seed bank for next year.”

Problem weeds

Here is a closer look at two weeds Gauck sometimes finds this time of year that take a hit from herbicide but survive:

Marestail. Especially because marestail plants were larger than normal due to the wet, delayed spring planting season, they were difficult even for some burndown programs to take down in no-till situations. In some fields, marestail plants were taller than many postemergence herbicides can control effectively when the applications were made.

Some marestail is resistant to glyphosate, adding to control issues, Gauck says. “When marestail plants are relatively tall, it’s tough for anything to control them effectively,” he notes. “By now, we often see plants which were obviously dinged by a herbicide, but which survived it and grew anyway. The stem may be elbowed, or there may be evidence of yellowing on lower leaves, but all the herbicide did was set them back, not control them.”

If you’re not seeding cover crops and you weren’t satisfied with marestail control in soybeans in 2019, one way to get a jump on them is to apply herbicides this fall on fields going to soybeans in 2020, experts say. Marestail plants start out in a rosette stage. If you can control them before they bolt and begin growing vertically in the spring, you have much better odds of obtaining true control.

Canada thistle. These are also hard to bring down with postemergence sprays, Gauck says. Two years ago, there were patches of Canada thistle in the Soybean Watch field.

“The producer burnt them down effectively, but he didn’t kill the plants,” Gauck says. That’s because they’re perennials that come back from the roots, even if top growth is totally burned off by herbicides.

“If you couldn’t spray in a timely manner due to wet soils, they can ding yield through competition before you spray, even if you burn off top growth,” Gauck says.

He adds that fall is a great time to spray thistles and kill roots. Hit them before a freeze, when they’re actively growing.   

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