Research Highlights

Illinois Study Looks at Managing Weeds in Early-Planted Soybeans

By Carol Brown

Due to many industry advancements, treatments now exist to protect soybean seeds from soil-borne diseases and insects even before they begin to grow. These seed treatments have enabled farmers to plant soybeans earlier in the growing season than ever before.

But early planting also exposes the crop to weather-related issues such as cold snaps that can freeze emerged seedlings or heavy rains that can wash seeds away. When the weather does cooperate and soybeans begin to grow, other plants grow as well — including weeds. 

University of Illinois Extension weed specialist Aaron Hager was aware that some farmers are planting crops earlier in the spring, but planting soybeans during the first few weeks in April, and before corn, was not what he practiced growing up on the farm nor in his work experience.

“At the university, we always conducted our corn trials first over the 50 years of our herbicide evaluation program,” says Hager. “For farmers who had questions on managing weeds in their early-planted soybeans, we didn’t have anything we could base a recommendation around.”

When he began to think about this issue, the restrictions for dicamba products were still in place. Farmers had only a 45-day window in which to apply post-emergence dicamba after planting their crops. This meant that farmers had to be done spraying by the third or last week of May, which, Hager says, is when waterhemp starts to take off. Early planting may also change when and which weed species emerge as well. Hager says lambsquarters or giant ragweed may become an issue earlier in the growing season, which could necessitate a change in weed control practices.

Through a project supported by the Illinois Soybean Association, Hager is exploring herbicide management in early-planted soybeans. In 2021, he and his graduate student, Logan Miller, began replicated field trials with industry partners at four locations across central Illinois. The tests included at-planting applications of residual herbicide at a full rate, a half rate, and a control strip of no herbicide. When weeds reached 4 inches tall in the non-treated plots at each location, they then compared applications of:

• glyphosate plus glufosinate, 
• glyphosate plus glufosinate with a half rate of layered soil-residual herbicide, and
• glyphosate plus glufosinate with a full rate of layered soil-residual herbicide. 

Their data collection included recording when weeds emerged in the plots treated at-planting as well as the untreated plots. They also recorded the days until crop emergence and when weeds reached 4 inches tall. They evaluated chemical weed control and soybean injury each week for four weeks after post-emergence application.

“We wanted to see if an added residual with the post-application would help control later emerging weeds for the remaining growing season, and whether a full rate is needed or would a half rate be sufficient,” Hager explains. “The first year of this experiment we could not have asked for any better weather, but this year the weather had other plans. We ended up three weeks behind last year’s planting and applications.”

Even though the weather didn’t cooperate in 2022, Hager’s research is still sound.

“I recommend not to skip the residual herbicide application at planting,” Hager states. “Regarding the full rate versus half rate post applications, we’ll need to do some more testing. But using a full rate of pre-emergence herbicide plus a post program with layered residual will address the late-season weeds. Soybean canopy will help reduce weed pressure as well.”

He also offers some words of wisdom regarding weed management in general, which are applicable to farmers everywhere. 

“Farmers use herbicides and weed control tactics to reduce interference of other plants that are trying to use the same resources that the soybean needs to grow. When farmers do weed control, they’re not directly increasing yield,” he says. “However, they are preserving that genetic yield potential in the soybean seeds. It’s all about reducing the interference of the co-existing vegetation before it adversely impacts soybean yield potential.”

In addition to the post-application rate tests, Hager says the next step in this research is looking at soybean response when certain chemistries are applied in early April. He also plans to explore the relationship between soybeans and herbicide carryover from the previous year’s crop. 

Published: Jan 16, 2023