This article was reposted from G.R.O.W. – Getting Rid of Weeds. 

Double the crop, half the weed control – that’s part of the allure of following wheat harvest with a second planting of soybeans.

“Weed control in double crop soybeans is generally easier than in full-season beans,” notes University of Kentucky weed scientist Dr. Travis Legleiter. “You’ve truncated the amount of time you have to deal with weeds.”

But there are a few caveats: The practice comes with a slightly different weed spectrum, some residual herbicide timing decisions and – if you’re using older Xtend soybeans – a spray cut-off date to consider.

Double crop soybeans emerge in wheat stubble. Photo credit: Rachel Vann, NCSU

The Weeds of Midsummer

First up, weeds do pay some attention to calendar dates, Legleiter explains. “With full-season beans, you’re planting right in the bulk of the emergence window for summer annual weeds,” he says. “Whereas with double-crop beans, a lot of summer annual weed emergence is done.”

You can expect to see some summer annuals still cranking out new seedlings, such as waterhemp and Palmer amaranth, though at lesser densities. But watch for the later-emerging crowd as well, including weeds such as prickly sida, hophornbeam copperleaf and morningglories, Legleiter says.

Timing Residual Herbicides

For the most part, time is on your side with double crop soybeans. Planted into warm, midsummer soils, the beans practically leap out of the ground, grow rapidly and canopy quickly. “Our window of weed control, from planting to canopy is much shorter – maybe only a month long,” Legleiter explains.

After wheat harvest, some growers follow so closely behind the combines with sprayers and planters that their burndown application essentially becomes an at-planting application and can see them through the season. “Some no-till growers here will put down a good residual with their burndown and be able to get away with essentially a one-pass program,” Legleiter notes.

But that requires timely and adequate rainfall for activation of those residuals, so don’t count on that equation every year, he adds. Some years and fields will require a postemergence application in double crop beans.

If you do add residuals to your burndown application, keep your next crop in mind, Legleiter warns. “Several residual herbicide products in soybeans have a 10-month rotation back to corn,” he explains. “You especially have to be mindful of that corn-soybean rotation if it gets really dry, because those residuals can really stick around for a while.”

Apply that same logic to any fall-planted cover crops you’re planning on and read labels carefully.

Watch Cut-off Dates for Dicamba

Many growers with dicamba-tolerant soybeans have adopted Xtendflex soybeans, which come with an option of glufosinate and glyphosate for postemergence applications, in addition to dicamba.

But for the smaller number of growers still growing Xtend-only soybeans, remember that the federal cutoff for applying existing dicamba stocks is still June 30th and may be sooner for some growers, depending on state regulations.

“So at best, for double crop beans, you can probably only use it in your burndown,” Legleiter cautions.

Double crop soybeans growing mid-season among wheat stubble. (Photo credit: Rachel Vann, NCSU)

For more help planning weed control in double crop soybeans, see this article from Ohio State.