Control Weeds in Early-Planted Soybeans

01:49PM Feb 14, 2020
Waterhemp in soybeans
Weed management isn’t necessarily tougher than with normal planting — but it is different.
( Pam Smith )

Planting soybeans early, so they begin to flower before the summer solstice, can boost yield. But changing the plants’ growing pattern requires modifying weed control practices. Applying herbicides the way you always have could reduce your early-planting yield premium, says Ken Ferrie, Farm Journal Field Agronomist. 

Some Principles Apply to Both Normal- or Early-Planted Soybeans

  • Cost isn’t the only factor to consider. “It’s no longer a matter of cost per acre, as it was before weeds developed resistance,” Ferrie says. “Cost is a factor, but your goal is season-long control, without any residual running into the next crop. Follow harvest and cropping restrictions on the herbicide label.”
  • Plan a weed-control program for each field. “There are no one-size-fits-all weed control programs,” Ferrie says. “Now we must not only identify weeds, but understand which ones are resistant to which herbicides.”
  • Involve the entire pest team. “Field scouts must identify weeds and resistance issues,” Ferrie says. “Once you know the targets, select varieties based on their tolerance for various herbicides.”

Some Practices Might Need To Change

A few aspects of weed control might actually become a little easier. “Our studies show early-planted soybeans close their canopy sooner (by calendar date, although it actually takes more days) than normal-planted beans, regardless of row width,” Ferrie says. “Soybeans in 30" rows, planted early, might close the row sooner than soybeans in 15" rows planted at the normal time, due to more early vegetative growth.
“As a result we see better control of late-emerging weeds, with fewer escapes in August and early September. The earlier canopy helps considerably with waterhemp, which emerges later. So shift your focus a bit toward early-season control — starting clean and staying clean until canopy,” he adds.

Keep These Factors In Mind With Early-Planted Soybean Weed Control

  • Avoid conflicts that delay soybean planting. “With early planting, the time frame for applying burndown, preplant and pre-emergence soybean herbicides is likely to coincide with corn planting,” Ferrie says. “If your custom applicator is applying preplant and pre-emergence herbicides on corn, he might not want to stop, clean out a sprayer and apply soybean herbicides. I’ve seen soybean planting delayed three weeks while growers waited for a burndown herbicide application.”
  • Begin planning the previous fall, and discuss it with your retailer. “If he won’t stop spraying corn herbicides, you might need to find a herbicide compatible with corn or apply a preplant or pre-emergence treatment yourself,” Ferrie says. “Control tough winter annuals, such as marestail, early or control becomes difficult.”
  • Consider moving your burndown herbicide application to the fall. “There’s more time in the fall, and it will alleviate some pressure in the spring,” Ferrie says. “Then you can focus on annual broadleaf weeds and grasses.”
  • Post-emergence herbicide treatments require extra care. “The main focus is on weed size,” Ferrie says. “They must be emerged, but not so tall they require higher rates or can’t be controlled at all. You no longer can simply apply a post-emergence herbicide a certain number of days after planting. While you still spray based on the size of the weeds, you also must consider the growth stage of the soybeans. If they are at the reproductive stage, spraying can cause some flowers to abort, costing yield premium from early planting.”
  • You might need to consider new options, such as a preplant or pre-emergence soil-applied herbicide with residual control, Ferrie says. Follow that with a post-emergence application, applied a little earlier than you are used to. You might need to include a residual herbicide in the post-emergence treatment.
  • “You can use herbicides such as diphenyl ether that burn the soybeans (which can boost yield by stimulating branching),” Ferrie says. “But don’t apply them when soybeans are in full reproductive stage because that will cause them to abort flowers, pods and beans.”
  • Plot a rescue treatment strategy in case late-emerging weeds escape. “With normal planting, you might have changed additives or increased rates and burned the soybeans in the process of cleaning them up,” Ferrie says. “With early-planted soybeans, burning could reduce yield if the plants are in the reproductive stage. Consider less-destructive herbicide programs. If waterhemp is escaping when soybeans are at R2/R3, your options will be limited — maybe even to non-chemical practices such as cultivating and walking.”
  • Pest teams must understand label restrictions, such as no application after a certain date or so many days after planting. Some labels have restrictions based on the stage of crop maturity. “Early planting might move up harvest by seven to 10 days,” Ferrie says. “So check harvest restrictions.”

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