Soybeans Under Siege

Yield loss. This is never something your growers want to hear, but it is even worse when soybean prices remain high. Studies have shown the value of early-season weed management is critical in maximizing soybean yield potential. Appropriately timed herbicide applications can help growers manage glyphosate-resistant weeds and improve their bottom line.

This was the subject of CropLife Media Group’s recent Webinar, “Soy­beans Under Siege: The Econo­mics Of Early-Season Weeds,” presented by Bryan Young, professor, Southern Illinois University, Kevin Bradley, associate professor, University of Missouri, and Bob Kacvinsky, technical support representative, Syngenta Crop Protection. The speakers provided key insight on how to protect soybean yield and increase profits.

Residual Herbicides

In systems where glyphosate is not providing the effectiveness of weed control as it has in the past, there’s a lot of value in the use of residual herbicides in the spring, said Young during the Webinar, which was sponsored by Syngenta.

“Growers need to look at soil residual herbicides that diversify their approach to weed management,” he said. “Soil residual herbicides can help manage your risk and improve yield. They also delay the onset of any glyphosate-resistant weed species.”

Delayed applications of gly­phosate in soybeans not only affects the return on herbicide investment, but impacts other costs as well, Young said.

“It’s not just about return on your herbicide dollar invested, it’s also about all of the variable costs invested on each acre, including seed, fertilizer, land use, and labor,” he said. “If you can spend more money on residual herbicides, it’s going to return more money on all of your investments.”

Choosing the right residual herbicide to manage weeds is also important, Young said.

“My advice is to select a residual herbicide that will address your primary weed species present in the field,” he said. “There are several different soil-residual herbicide options, so make sure you are buying one that is most effective for your target weed.”

Young also discussed field trial results that examined one-pass vs. two-pass glyphosate programs, including programs that used pre-emergent herbicide treatments.

“For long-term weed management, growers can make more consistent, timely applications using the pre-emergent program, followed by post-emergent glyphosate,” he said.

Managing Glyphosate Resistance

Both Young and Bradley offered best strategies to deter glyphosate-resistance weeds.

“I recommend making glyphosate work the first time with timely applications,” Young said. “Soil residual herbicides can reduce the number of weeds by the time you apply glyphosate, making sure the weeds aren’t too big. In doing so, we can also expose fewer weeds to glyphosate, which reduces the odds of resistance.”

Bradley added: “You want to reduce the selection pressure, which reduces the number of weeds that get sprayed with glyphosate in the first place,” he said. “That should really be your goal — preventing resistant weeds in the future, as well as controlling resistant weeds that are already there.”

Bradley said there are a couple of key reasons why he recommends growers in Missouri, which now have widespread populations of glyphosate-resistant waterhemp, use pre-emergent herbicide programs.

“First is the economics of it. You can make more money using a pre-emergence herbicide when you have glyphosate-resistant weeds in the field,” he said, “Secondly, it is a much more effective way to deal specifically with glyphosate-resistant waterhemp. It is more effective than trying to control the weed in-crop with glyphosate and some other tank-mix partner.”

Syngenta Learning Center

Kacvinsky shared results from ongoing research on soybean weed management conducted at the Syngenta Learning Center in York, NE.

“It’s important to remember that weeds typically outnumber crops,” he explained. “Weeds compete directly with soybeans for three main areas — nutrients, water, and space/sunlight.”

Studying soil moisture at the Learn­ing Center, Kacvinsky said they’ve concluded two rules of thumb when working with water sensors.

“First, we’ve found that three-inch tall weeds can remove one inch of moisture every three days,” he said. “Secondly, for every inch of water removed by soybeans, weeds will remove three inches of water. Even under irrigation, the loss of water from weed competition has a cost associated with it. At some point, you’re going to have to replace that water.”

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