Soybean Success: No Longer a Secret

By Mace Thornton

Agronomists at the University of Illinois haven’t yet cracked all the codes to the secret formula for soybean production in the Prairie State. But they are on the leading edge of the curve, thanks to their shared commitment to continuous improvement alongside the state's soybean farmers.

Although yield trends are an upwardly moving target because of the breakneck speed of agronomic innovation, University of Illinois Postdoctoral Researcher Dr. Connor Sible spilled the beans during the recent Soybean Summit about what might be the optimal soybean growing strategy in Illinois for 2024.

According to Sible, who works alongside noted Professor of Crop Physiology Dr. Fred Below in the Department of Crop Sciences at the U of I, the recipe to soybean yield success in Illinois requires a comprehensive and season-long approach.

"The big picture is holistic,” he said. “Soybeans have a lot of potential the day they are planted, but soybeans can also be lazy. The big key is that once the plant makes it through the soil, it’s important to maintain a consistent push throughout the growing season.”

Farmers pushing for optimal yields need to deploy a full-package strategy, Dr. Sible explains. That starts with fall fertility before planting, then with selecting the right planting date and caring for the plants all the way through foliage maturity.

"It all ties together from a systems approach,” he said. “If you do one thing here or there, you can see little bumps, but it's when you piece it all together that you are going to see the greatest advantage.”

Old Fertility

For starters, Sible emphasizes the importance of a long-known agronomic saying, “Soybeans like old fertility.” candidate Marcos Loman, who has suggested that fall-applied fertility stimulates the breakdown of corn stalks and residues.

“Soybeans really have a preference for residual nutrients from the previous year's corn crop,” he said. “In a four-year study comparing fall- and spring-applied phosphorus ahead of soybeans, we observed consistent positive phosphorus responses in the first three years, where fall treatments outperformed spring applications.” Sible said the 2023 phosphorus study showed no fertilizer-age preference, which he theorizes might have been weather-related.

Sible said the “old fertility” preference is likely related to an innovative theory by Ph.D. candidate Marcos Loman, who has suggested that fall-applied fertility stimulates the breakdown of corn stalks and residues.

“This stimulation potentially aids in the mineralization of residue, benefiting soybeans in the subsequent spring,” Sible explained. “This theory proposes advantages such as improved residue breakdown, enhanced nutrient cycling and smoother planting conditions, supporting the notion that fall fertility may provide a substantial advantage for soybean cultivation in Illinois.”

Planting Date

In a challenge to traditional farming practices, Sible said, research is showing that earlier planting dates for soybeans can play a crucial role in increased production.

“There is an evolving understanding,” he said. "We used to plant corn first, then worry about soybeans. We're learning you can put soybeans in early and not only get higher yields, but they are able to tolerate suboptimal conditions quite a bit better. This shift in strategy is supported by research findings, demonstrating that early soybean and later corn plantings create higher dual-crop yield advantages compared to early corn and later soybeans.”

This year, the research group planted its first soybean plots between April 10 and April 15. Sible said the team got “a little nervous” when it got cold between the 15th and 25th, but the soybeans emerged and prospered through the growing season with greater tolerance. Yields were "pretty decent.” If corn had been planted in that early plot, Sible said, it likely would have required replanting. He encourages farmers to run their own small-scale trials to see if early soybean planting works on their farms.

“It’s a potential win-win situation,” he said. “Give it a try. Don't put the whole farm in, but if you have a smaller field that could go and you usually would wait, test it, put the planter out there and then, by the end of the year, you'll know whether the decision was worth it or not.”

Foliar Protection

Once the soybeans are in beginning pod (R3) stage, achieving yield success becomes a matter of foliar protection, according to Sible. The recommended application timing at R3, according to Sible, is during the second half of July for early-planted soybeans and the first two weeks of August for late-planted soybeans.

“Foliar protection is essential. Even in dry years with limited disease pressure, we see around a 4 bu.-per-acre response, with expectations of greater impact in years with higher disease pressure,” Sible said. This protection involves the application of fungicide with insecticide. Sible noted that most growers who apply foliar treatment do it only once, with the primary focus on safeguarding the leaf foliage.

"Everything about boosting soybean yield at that point is about protecting that leaf foliage,” he said. “The unique nature of soybeans is that the majority of photosynthates come from the nearest trifoliate [a compound leaf made of three leaflets]. Protecting that foliage is one key to higher yields.”

Seed Treatments and Biologicals

Another soybean production practice creating “a lot of market buzz” these days is the use of seed treatments and biologicals, according to Sible. But there are hits and misses.

Sible said that has been the case with bradyrhizobium, a traditional microbe used on soybean nodules. “Sometimes it works, sometimes it doesn’t. It’s basically a plus or minus 2 bu. per acre.”

“The reason is that if you have been in a long-term corn-soybean rotation, you already have bradyrhizobium in your soil,” he said. “Inoculating your seed is just kind of like a drop in an already full bucket. Some still swear by it, but we already have it in our soil, so we prefer adding a symbiotic play.”

Sible is exploring alternative approaches, such as the introduction of the micronutrient molybdenum to the seed to feed existing nitrogen-fixing microbes.

“We have been putting molybdenum on the seed instead of a microbe with the idea that you are now giving the existing microbes the tools they need,” he said. “We are seeing good, consistent results now on soybeans, around 2.5 bu. per acre. That may not sound like much, but the cost of these seed treatments would pay at that response.”

Sible also discussed the role of bacillus in the biologicals space. He noted their overall prevalence in soils and their contribution to phosphorus cycling and organic matter breakdown. He explained how advanced gene sequencing tools are helping to identify and concentrate specific microbes for use as more effective inoculants to enhance organic matter cycling and release nutrients such as sulfur and nitrogen.

“It’s like giving the whole organic-matter cycling process an espresso boost, accelerating it a bit compared to what it would do on its own,” he said.

Cover Crops and Double Crops

The use of cover crops and double cropping are also hot topics on the Illinois soybean circuit.

"There are many benefits to cover crops that have been well documented,” Sible said. “You think about the soil moisture penetration, aeration, soil structure and reduced runoff.”

During periods when cover crops might not work as well, such as dry weather, Sible said, there are options to give them a boost. He credits research conducted by graduate student Darby Danzl for providing a solution through early cover crop termination, and the addition of ammonium thiosulfate (ATS), microbial additives and a microbe-stimulating mixture, such as simple sugar and humic acid, to the termination tank to enhance cover crop decomposition.

“When implementing ATS and microbial additives, yields rebounded, though it did not surpass the control without cover crops,” Sible said. “But it shows we are developing the right processes and heading in the right direction.”

When questioned about the impression that cover crops don't work in Illinois, Sible underscores the need for adaptation and management changes and a bit of “trial and error.”

The U of I researcher also addressed the potential of double cropping soybeans following winter wheat, noting higher soybean yields with similar microbial strategies to break down and release nutrients from winter wheat straw. The strategy is so effective that Sible is an advocate for extending cover crop benefits and incentives to double crops, such as wheat followed by soybeans.

"I would 100% love to see double cropping winter wheat and soybeans qualify for the same benefits that cover crops do,” he said. “In some cases, it’s almost better season-long coverage than terminating a cover crop early.”