Secrets to share

Crop scientist outlines top six factors that impact soybean yield

The five-soybean pod. For many growers, it’s as mythical as Bigfoot and the Loch Ness Monster.

Not even seasoned crop scientist Dr. Fred Below of the University of Illinois has seen one of these elusive occurrences firsthand, even as he researches ways to help growers maximize production through better management.

“The typical number of seeds per pod is three; under stress you might see two or even one,” Below said. “But if you have the right growing conditions, the right genetics and the right management, you can get four or even five. I hate to admit it. I’ve never found a five-bean pod, even though I’ve looked like crazy.”

Three, four, five—why should growers care about how many soybeans are in each pod? It’s simple, Below said. Yield.

“Take the number of pods per plant and multiply it by the number of seeds per pod, and that gives you the seed number,” he explained. “And seed number is directly associated with yield. Add one more pod per plant, and you’ll get 2 more bushels per acre.”

Increasing that number is the focus of Below’s most recent research into factors that have the biggest impact on soybean yield. The result is the “Six Secrets to Soybean Success,” which he shared in August with growers at a presentation sponsored by MFA and Bayer Crop Science.

The soybean studies are a follow-up to Below’s popular “Seven Wonders of Corn Yield” treatise, which identified factors that could lead to a 300-bushels-per-acre crop. Soybeans tend to get less attention than corn when it comes to management strategies, Below said, even though U.S. acreage planted to soybeans grew more than 13% in the last two decades, from 73.7 million acres in 2003 to 83.5 million acres in 2023. Yields have also been on the rise, increasing from 38 bushels per acre to just under 50 bushels in 2022, largely based on seed genetics.

“Production practices for soybeans have not kept up with genetic improvements for yield, pest resistance and stress tolerance,” he said. “There is a huge opportunity to increase soybean yield by changing the way we manage the crop.”

Below and his fellow researchers at the university’s Crop Physiology Laboratory define “soybean success” as achieving yields of 80 bushels per acre. Here are the not-so-secret factors contributing to that goal, listed in order of impact on yield: 
1. Weather — 30+ bushels
2. Variety — 30 bushels
3. Row spacing — 7 bushels
4. Foliar protection — 6 bushels
5. Fertility — 5 bushels
6. Seed treatment — 2 bushels

Below emphasized that while each factor must be optimized to achieve top yields, all of them must work together in a holistic approach. They are also dependent on crucial prerequisites, such as adequate soil drainage, season-long weed control and proper soil pH.

1: Weather
The first “secret,” of course, isn’t really a secret at all. Weather dictates much of the success of any crop, and is, unfortunately, the factor growers have no control over, Below said. They can, however, determine the planting date, which is also somewhat governed by the weather.

“Planting date is important, and, yes, the weather plays a huge role in that decision,” he said. “But let’s say we have a choice. Plant soybeans early. The canopy closes faster, the plants put on more nodes, and that equates to more yields. In Illinois, about 30% of the soybeans went in the ground ahead of corn this year.

It’ll be more next year. This is one way to have some control over the weather.”

2: Variety
In the first iteration of Below’s studies in 2012, fertility was ranked as the second-most important factor impacting soybean yield. However, subsequent research now indicates that variety selection is No. 2, falling just below weather.

“When we initially developed this list, we didn’t have sufficient data to put an average value on each secret,” Below said. “As we’ve continued to evaluate this concept, we’ve seen that genetics ranks right behind weather as having the biggest influence on yield. Every year, I watch farmers agonize over what corn hybrid they’re going to grow, and then they turn around and buy any old soybean. Big mistake. Variety is by far the most important decision you make, and most growers are not giving it enough attention.”

Growers should select seed that is high-yielding, has the necessary agronomic and defensive packages and is adapted to the field and soil where they will plant it, Below said. However, he added, bear in mind that all varieties are not created equal. In fact, citing trials from 2021, Below pointed out that soybean varieties of the same maturity group ranged in yield by as much as 30 bushels per acre when grown at the same location.

“Usually, it’s the variety with the fullest relative maturity that gives the highest yield,” he said. “Remember, too, that soybean varieties differ in their response to fertility and foliar protection, so if using a high-management approach, choose an offensive variety that is known to respond.”

3: Row Spacing 
In the university’s studies, narrower rows typically out-yielded wider rows (20-inch vs. 30-inch). Narrow-row soybeans were also more responsive when adding intensive management factors. This approach can be a tradeoff, Below cautioned, because narrow rows can promote more disease pressure due to reduced air circulation.

“Using the same population of plants but narrowing the rows helps close the canopy faster,” Below explained. “Again, that promotes more nodes and more yield. If you have a problem with a disease like white mold, however, a narrow row isn’t the way to go.”
Optimum row spacing not only interacts strongly with the weather but also the population and the planting date.

“If you plant early, row spacing doesn’t matter. You’ve got plenty of time for the canopy to close,” Below said. “Plant late, and narrow rows are going to be better than 30s. It really is a systems approach, and row spacing is one of those tools. If you want to intensively manage your soybeans, narrow-row spacing will give you the most bang for your buck.”

4: Foliar Protection
Including both fungicides and insecticides as “foliar protection” is success secret No. 4. Below said the goal is to preserve the photosynthetic activity of leaves throughout pod fill, which is critical to overall yield.

“If you put fungicide on, you’re crazy if you don’t put insecticide with it,” Below said. “This is an absolute case of one plus one equals three. There is clearly a synergy between them. Not only do you see less damage from disease and insects, but you also keep the leaves greener longer, converting sunlight into more yield.”

When it comes to foliar protection, timing is key, he added.

“Most yield—60%—comes from the nodes in the middle of the plant,” Below said. “When you spray a fungicide-insecticide combination at R3, you’re protecting nodes 7 to 13, where more than half of your yield comes from. That’s why foliar protection is so important.”

5: Fertility
In another case of synergy, foliar protection and fertility work hand in hand to give growers the full value of both types of inputs, Below said, continuing to No. 5 on the list. But fertility is also one area that growers tend to neglect, mistakenly assuming that nutrient needs are less intense than they really are.

“Soybeans use a lot of nutrients,” Below said, displaying a chart showing the fertility needed to produce 80-bushel yields: 327 pounds of nitrogen, 227 pounds of potassium, 57 pounds of phosphorus, 23 pounds of sulfur, 6.4 ounces of zinc and 6.1 ounces of boron per acre.

Perhaps the biggest misconception is the need for nitrogen, he added. Despite being a nitrogen-fixing legume, soybeans draw more N out of the soil than they put back in. The root nodules, where bacteria make nitrogen in a symbiotic relationship with the soybean plant, only satisfy half the crop’s nitrogen requirement of 4 to 5 pounds for each bushel of beans produced.

“If I teach you anything here today, it’s that soybean removes more nitrogen than it fixes,” Below said. “Soybean has a very high nitrogen requirement, and it only gets about half of its nitrogen from the nodules. The rest must come from fertilizer.”

6: Seed Treatment
Recognizing that yield potential is greatest on the day seeds are planted, growers can promote enhanced early-season emergence and vigor, especially in stressful environments, with seed treatments. As the No. 6 soybean secret, this factor accounted for about 2 bushels of yield in Below’s studies.

“Naked seeds don’t emerge and grow as well as treated seeds,” Below said. “The right seed treatment can significantly contribute to every crop development stage, from emergence to harvest. If you plant early, which I’m telling you should be done, it’s even more important. Those treatments will protect the seed and seedlings from disease and pests, getting them out of the danger zone faster and setting the crop up for success.”

Currently, Below and his team are extending their studies of seed treatments to include the newer biological products, which he described as “the biggest area of research we have right now.”

In summarizing his discussion, Below reminded the audience that purposefully managing soybeans to optimize each of these six factors could potentially add up to yields of 80 bushels per acre—or more.

“Focus on the ones at the top that have the biggest impact first,” he said. “And remember these things interact with each other. If one of these is not optimized, you’ll grow less. Guaranteed. That’s what makes this so hard. To grow more than 80-bushel soybeans, you have to optimize the prerequisites, each of the secrets and their positive interactions.”

To read more details about Below’s previous and ongoing studies, visit the University of Illinois Crop Physiology Laboratory website at cropphysiology.cropsci.illinois.edu

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