Like most agronomic researchers, Fred Below has spent his career chasing down the mix of in-field factors that create 300-bushel corn yields. In his work running the University of Illinois Crop Physiology Laboratory Team, he’s continued to confirm the wide-held belief that the greatest success comes in the form of creating the best conditions for growth rather than a single practice that makes all the difference.

Back in 1985, Below’s first year as a professor, Herman Warsaw produced his record-setting 370 bushels per acre in Saybrook, Ill. Below, who had research plots on Warsaw’s farm, personally oversaw some 313-bushel yields. He’s been obsessed with getting to the bottom of what it takes to produce high yields consistently ever since. 

“I didn't end up seeing 300 bushels again for 30 years,” Below says. “That's like a whole career — a whole mortgage. But all of a sudden, we start growing 300 bushels again. And where did we see it? It showed up in our management hybrid evaluation trials. For the last 9 years, we have screened commercial hybrids for their response to agronomic management.” 

This was the genesis of Below’s Seven Wonders of The Corn Yield World approach; identifying and understanding the 7 primary factors that contribute to a strong corn yield. He developed his list in 2008 (in order of importance): weather, fertility, hybrid, plant population, crop rotation, tillage/no-tillage, and biologicals. Working with generation after generation of grad students with the Crop Physiology Lab Team on 3 separate sites in Illinois, he’s continued to test, retest and refine his assumptions. 

Since 2015, the team has hit over 300 bushels per acre on their research plots every single year except 2020 — seeing a high of 379 bushels per acre at their northernmost site in Yorkville, Ill. in 2017. With the other two sites in Champaign and Nashville, Ill., Below notes that some inherent fertility in the soil plays a role. 

Prerequisites

Before even contemplating the variables in Below’s list of wonders, he says there are several prerequisites that must exist in the field to make a high-yielding crop a possibility. Calling them table stakes, Below says it’s crucial to have control of weeds, pests and diseases — specifically with fungicides and insecticides. 

In separate studies examining a set of 36 hybrids and a set of 20 hybrids in 2020-2023, Below saw a yield advantage ranging from 4-26 bushels per acre across all 3 sites. 

“Last year, we didn't see a single disease symptom, but we still got 5-7 bushels from foliar protection,” Below says. “That's a prerequisite. I'm pretty sure you're not going to grow 300 bushels without it.”

Robust soil structure and adequate drainage are also prerequisites. While Below will be the first to tell you that he’s no evangelist for either no-till or tillage — opting instead for whatever practice pulls in the best yields given the field conditions — he concedes that no-tilled fields often check this box by default. 

“Good soil structure and drainage aren’t just important, they’re actually crucial,” Below says. “This is sort of a no-brainer, but can I improve my soil structure with long-term no-till and cover cropping? That's pretty much the whole heart of this operation. So, the answer is yes. It’s a great advantage of no-till.” 

Lastly, Below says that proper soil pH and adequate base levels of phosphorus (P) and potassium (K) present in the soil based on soil tests is a must. 

Weather & Fertility

Easily the 2 most consequential wonders in Below’s list are weather and fertility — each accounting for up to 90 bushels per acre on their own. 

Study after study has shown Below that weather, although it’s the condition a farmer has the least control over, can make or break a growing season. It can delay or expedite a farmer’s planting, drown a crop, cook a crop, or batter it with high winds, offering very little recourse. “Weather rules,” Below says. Interestingly, his studies provided a very clear yield loss estimate when it comes to August nights. 

“Every day in August when the nighttime temperature stays above 73 degrees, will result in a bushel per acre loss in yield — you can kiss a bushel goodbye,” he says. “High temperature is a real problem.” 

As for fertility, it’s roughly as important, Below says, but the farmer has a lot more control over it. Admittedly, he says the quantities of nitrogen (N), diphosphorus pentoxide (P₂O₅), potassium oxide (K2O), sulfur, zinc, and boron he recommends are “high,” but he’s taking into account production and removal coefficients. Higher yields result in more biomass and nutrients being harvested and removed from the fields. 

Product is one thing though, and placement is another. Below says his 3 preferred methods are 2-by-2 in-furrow liquid at planting, banding directly under the future crop, or in-season Y-drop applications adjacent to the crop row. 

“Fertilizer placement is so important specifically due to the horizontal spread of each plant’s root system,” says Below. “The horizontal spread of a corn plant root system is only 6-8 inches. 90% of that root is within that 6-8 inches. You shouldn’t put the fertility where there aren’t any roots.” 

Hybrid & Plant Population

Assigning approximately 50 bushels per acre to hybrid selection alone, Below says it’s often the variable that has the most “swing” to it. 

“Across our 3 sites, we tested 36 different hybrids in 2020 and 2021 and 20 more in 2022 and 2023,” Below says. “The difference between the highest and lowest was 49 bushels, which is a big difference based on the hybrid being used. As no-tillers know, there are certain hybrids more suited to no-till than others and it’s a big factor to get correct.” 

Ringing in at 25 bushels, Below’s fourth variable is plant population — a factor that can cut both ways. 

“If you have way too many plants, you're going to lose yield, if you don't have enough plants, or have them in the wrong row spacing, you're going to lose yield,” Below says. “This is actually one of the factors that has changed the most over time.” 

Looking at average yield in the U.S. against average planting population over the past 60 years, Below notes that planting density has been on a steady path upward. 

“Overall, it’s been increasing about 2 bushels per acre per year,” Below says. “There are better genetics and better management, and associated with that increase is a linear increase in the number of plants planted. Back in 1965 you’re seeing around 70 bushels per acre with 20,000 plants per acre probably on 36-inch rows. Now, 50 years later, we’re near 180 bushels per acre average, with 32,000 plants per acre on 30-inch rows. Guess what? It’s going to go up, despite the high seed costs.”

Plant population is such an important variable because it’s a component of yield, adds Below. 

“If you’re going to make corn yield a math equation, it’s multiplying 3 yield components together; plants per acre, kernels per plant and weight per kernel,” Below says. “The one we have the most direct control over is plants per acre. And that’s why the plant population in the U.S. goes up a little under 400 plants per acre per year.” 

Given the trend, Below does expect both the average yield and plant population to continue to rise. But he suspects there’s a limit to the population on 30-inch rows.

“38,000 is the top end for 30-inch rows,” he says. “You can plant more, but it's a risk.” 

Beyond that, Below says he thinks rows will continue to narrow for two main reasons: more dense corn crops mean better sunlight interception and the root size appears to depreciate less in 20-inch rows as population increases than in 30-inch rows. 

“With 44,000 plants per acre on 30-inch rows, they're less than 5 inches apart — it’s too close,” he says. “You want some elbow room at the dinner table, right? Put that in a 20-inch row and now they're 7 inches. So, you could manage a higher density of plants.”

Extensive root mass research with the Crop Physiology Lab Team has shown Below that while the obvious result of a smaller root system in higher density planting holds true, the effect is diminished slightly in more narrow rows.

“We've looked at this numerous times with numerous hybrids, in different places and what we see is for every thousand plant increase in population, you have 2.5% less roots,” Below says. “It’s very consistent. But what happens when we narrow the row at the same density? If we go to 20-inch rows, the root system gets bigger.” 

Crop Rotation, Tillage & Biologicals 

The last 3 variables in Below’s 7 wonders list are crop rotation, tillage/no-tillage and biologicals — to which he applies a yield advantage of 20, 15, and 10 bushels respectively. 

Once a grad student on the research plots himself, Connor Sible is now a Postdoctoral Research Associate with the Crop Physiology Lab Team and has been working with Below to better define the factors at play in 300-bushel corn. Sible has been examining yield disadvantages seen specifically on long-term continuous corn, and how even moderate crop rotation can alleviate that impact. 

“Established in 2003, we have 2 different sites with 17th or 19th year continuous corn that we worked with,” Sible says. “To make matters worse in 2019, I took it to no-till. So, for continuous corn, it’s often recommended that you're going to do some tillage at some point. But by taking it to no-till we were setting up a worst-case scenario for the residue side of it.”

Acknowledging that there are management techniques that could have been employed to reduce the impact of a no-till continuous corn system, Sible noted that they saw a 25% yield penalty when compared to a no-tilled plot that used a corn and soybean rotation instead. 

“The corn-and-soybean-rotated field yielded 201 bushels per acre where the long-term continuous corn yielded 153 bushels per acre, so it’s a 48-bushel loss,” Sible says. “Again, it’s an entirely unmanaged worst-case scenario but it’s a reference if you do nothing — not a recommended practice.”

If one were to take a more active role in management, Sible notes that the residue could be sized mechanically and the biologicals in the soil can be actively nourished.

“We started where we had either standard stalk rollers or a sizing knife roller — we used the BT chopper,” Sible says. “We added ammonium sulfate — about 200 pounds dry to the acre, 48 pounds of sulfur and 42 pounds of N. With broadcasting N in the fall, the entire intent is to immobilize this as we break down the residue, and then it will remineralize in the next year. That's giving the microbes a little fuel for the fire to go after that residue a little easier. And talking about biologicals, if you have that fall burn down, can you add a little residue stubble buster to the tank?”

Sible notes that these techniques diminished yield penalty significantly. 

“Sizing the residues gave us 13 bushels more,” he says. “So, the penalty (for continuous corn) was less at 166 bushels per acre. Then on top of that, when you give those microbes that nutrition — the sulfur and N — we gained another bushel. It’s an additive approach. What got really exciting is when we added that third component with that burn down and added microbial blend — there were another 11 bushels. When you sum it up together, that’s a 25-bushel reduction in the yield penalty. That's pretty exciting.”