After you make your last postemergence herbicide pass, clean up the sprayer and park it in the toolshed, is that it? Will you say goodbye to weed control in soybeans for another year? After all, you’ve got more pressing things than weeds to worry about.

If that is your attitude, Steve Gauck believes you might want to rethink your approach. “You can learn a lot about what is working and not working in your weed control program by walking fields and observing how well your weed control program worked this year,” says Gauck, a regional manager for Beck’s, based near Greensburg, Ind. Beck’s sponsors Soybean Watch ’24. Gauck visits a field during the season, monitoring conditions and offering observations and suggestions that also could apply to your fields.

“[Late summer] is the time of year when you could see if you controlled weeds as well as you liked,” he says. “Yes, they will be there at harvest, but if you look now, you can pick up clues as to why some weeds survived your best weed control efforts.”

Problem annual weeds

On a late-season scouting trip in a past Soybean Watch field, Gauck spotted a few marestail that escaped control. “The grower sprayed a broadleaf herbicide postemergence, but marestail is hard to pick up with that pass,” Gauck says. “You really need to plan ahead to control marestail.”

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Some marestail plants he observed in that field were hit hard by herbicide, but not hard enough that they died. Once the plant began recovering, it sent out new growth, which was healthy by the time soybeans were setting and filling pods.

“Marestail can be a winter annual or summer annual weed,” Gauck explains. “It responds well to fall herbicide applications, because you can control winter annual marestail plants while they are in the rosette stage.”

Cover crops, especially cereal rye, also show promise toward limiting marestail problems in the spring.

These two methods are mutually exclusive — you can either sow cover crops or apply fall herbicides, not both. Decide which fits your system best, Gauck advises.

Battling perennial weeds

In the eastern Corn Belt, Canada thistle is a nemesis because it grows in thick patches and regrows from deep, virulent roots. In the western Corn Belt, other perennial weeds pose a threat.

“Canada thistle is another weed that you won’t do more than burn back with postemergence herbicides in soybeans,” Gauck says. “You really need a long-range plan to eliminate tough weeds.”

DANGER LURKS: So what if one Canada thistle plant escaped control? Look more closely. Where there is one, there is almost always more — see them lurking just below the canopy. An early post application set thistles back but helped little on actual control of this patch.

For Canada thistle, fall is one of the best times to spray, because regrowth is one of the most susceptible stages to herbicides.

“You are not going to wipe out a weed like that one with one application,” Gauck adds. “It takes multiple applications during the year with effective chemicals. But the first step is knowing you have a problem and deciding to address it. Have a plan and then execute the plan.”

From the field:
Beans reach reproductive phases

Flowering starts around the first day of summer due to how soybeans respond to day length. So, the reproductive phase is likely underway, whether plants are as big as you like or not. Here is what Beck’s field agronomists are observing across the Midwest:

In Illinois. “Most soybeans have received post herbicide applications within the past couple weeks. Majority have reached R1 or beginning flower, or even R2, which is full flower. No noticeable insect issues exist currently. Timely rainfall will allow for much-needed additional vegetative growth and node development.

“Fungicide timing at R3 is just around the corner. I always encourage growers to apply 1 quart per acre of a 10% boron product with fungicide and insecticide at R3 to improve flower retention and pollination, while protecting yield potential. If dry conditions persist, foliar potassium will likely provide a positive economic benefit, applied at 1 to 2 gallons per acre of potassium thiosulfate at R1 to R3. Make sure your chosen product is derived from potassium acetate for best efficiency of foliar-applied potassium.”  —Chad Kalaher, eastern and northeastern Illinois

In Missouri. “Moisture really spread out our planting window. First-crop beans got pushed back, and there are replant acres due to excessive moisture. So, there is a broad range of where we are with soybean stages across the state. There are some farmers finishing up post herbicide spray applications, while some have begun to put fungicide on R3 soybeans.” — Celena Kipping, southern Missouri

In Minnesota. “Recent large rain events across the state caused ponding and delayed growers from making post application of herbicide. With saturated soils, we are seeing some seedling diseases take hold, such as pythium and some rhizoctonia, especially in fields planted with untreated seed. Some of the very first planted soybeans within the state are nearing R1, with some flowers beginning to show.” — Dale Viktora, southern Minnesota