When managing weeds in soybean fields, you need a game plan encompassing the current and coming season, says Brian Weihmeir, an agronomist with LG Seeds in south-central Illinois.

That plan should also go beyond soybeans. In the Midwest where a corn/soybean rotation is common, Weihmeir emphasizes growers need to keep their corn fields clean or risk an uphill battle the following season. 

The tall plant and quick canopy can hide a lot of mistakes, so Weihmeir says corn is typically one of the last crops they worry about for weed control. He advises farmers to look at their corn herbicide program and take advantage of the many tools available for keeping those corn fields clean, which will make it much easier to keep bean fields clean the following season. 

“Growers need to take a systems approach when looking at soybeans,” he emphasizes. 

Tip #1: Overlap residuals to prevent weed emergence

According to Weihmeir, there’s a need for a mentality shift regarding soybean weed management timing. He says the easiest way to keep weeds at bay is to prevent them from emerging via overlapping residuals. 

“Make sure you’re putting down a good pre-emergent herbicide with residual, and then come back for post-application residual herbicide to take us all the way to crop canopy,” he explains. 

Pre-emergence spraying should happen as close to planting as possible.

Tip #2: Quick canopy can keep weeds from germinating

The next major focus is keeping weeds from germinating, and Weihmeir says that hinges on getting a quick canopy. This should influence farmers’ row spacing and seed selections. 

“Talk to your seed provider about finding the right soybeans for your farm,” he adds. “If you have a light field, for example, make sure you’re talking to your seed provider about getting a soybean that gets a little bit taller and bushier so we can close the rows quicker.”  

Weihmeir says crop canopy and row spacing should also be important to farmers, an assertion backed by research from Iowa State University Extension. Research across Midwest states over the course of several years has consistently shown soybeans planted in rows less than 30-inches wide have a yield advantage compared to beans planted in wider rows. 

“The primary reason for this advantage is light utilization; canopy closure is approximately 15 days earlier in 15-inch rows compared to 30-inch rows,” states Dr. Mark Licht, an assistant professor and extension cropping systems specialist with Iowa State University Extension and Outreach. “Canopy closure earlier in the growing season results in greater light interception and higher growth rates.” 

Licht also says quicker canopy helps prevent weeds from germinating, which is increasingly important to preserving the value of herbicides, in addition to reducing moisture loss and erosion. 

Tip #3: Walk fields and identify weeds

Palmer amaranth and waterhemp cause Midwest soybean farmers the most trouble. 

“They can grow an inch per day, so you might walk out in the field and hardly see anything and then come back three days later and they’re ankle-tall and we’re too late,” warns Weihmeir. 

Palmer amaranth can cause up to 79% yield loss in soybeans and waterhemp can halve soybean yields, driving home the importance of scouting fields and taking a proactive approach to herbicide application. 

“Pick several random areas within each field to scout and identify weeds,” Weihmeir recommends. 

He adds farmers should make note of weed height and be ready to tweak herbicide plans if unexpected weeds like grasses or cocklebur appear and cautions against relying solely on a retailer for keeping fields clean. 

Tip #4: Switch up modes of action

It’s important to vary what herbicides you’re using on both corn and soybeans to stay ahead of herbicide resistance, according to Weihmeir.

“LG Seeds provides different trait offerings like Xtend®, XtendFlex®, Enlist E3® and conventional to give growers choices based on what works best for them,” he says. “We’re very fortunate to help the grower by giving them those choices.”  

Supply issues could force some producers to change herbicides more than they otherwise would, so Weihmeir says it’s important to know your options if your preferred herbicide isn’t available.

He offers this example: “If you’re planting Enlist E3® soybeans and can’t get Liberty® herbicide, know that you can also use 2,4-D or Roundup®.” 

Tip #5: Cover crops can suppress weeds

Producers using limited or no-till might find themselves more reliant on herbicides for soybean weed control. Weihmeir says that’s where cover crops come in. 

“Growers utilizing cover crops have been able to control some of our winter annuals like marestail and chickweed,” he explains. “Once cover crops are established, they can outcompete winter annuals. When cover crops are terminated in the spring, they also leave a nice mat that helps suppress weeds during the growing season.” 

Weihmer recommends farmers who are interested in lessening their dependence on chemicals and want to give cover crops a try should talk with others who have done it in the past. He says even the groundbreakers on the cover crop front are still learning and improving their systems.

“One positive about this situation is it’s pushing farmers to look at their system a little bit closer, ask more questions and start thinking outside the box,” says Weihmeir. 

He encourages farmers to reach out to their local LG Seeds agronomist both before and after planting to fine tune their game plan for this year and the next.