Ken DeSmith is a believer in higher plant populations, but that wasn’t always the case. The Atkinson, Ill., grower was skeptical when corn plant populations started steadily creeping upward a decade or more ago. “The talk was that seed companies were just pushing rate recommendations higher to sell more seed,” he says. “I may have had that thought in the beginning, but I don’t anymore.”

DeSmith and his brother Jerry average about 35,000 plants per acre across their fields, ranging from 34,000 on light ground to up to 36,000 on productive fields. “We were stuck at 32,000 for several years, but we were able to jump up with the better hybrids,” he says. “Our yields consistently push 200 bushels per acre or more.”

As recently as 20 to 30 years ago, if a grower planted at today’s higher populations, there was a risk of barrenness and lodging. But corn genetic advances have made higher populations a tool to  increase yields.

There’s no shortage of research to find the “sweet spot” of plant population. The burning question is, increased by how much?

University of Illinois crop sciences Professor Emerson Nafziger’s research shows that as plant population increases, both kernel weight and the number of kernels per ear decrease, with the decrease in kernel number greater than the decrease in kernel weight. Yield is maximized when kernel weight per ear drops at the same rate that the number of ears per acre increases. Under high-yielding conditions, this normally occurs around 35,000 plants per acre, give or take a few thousand.

Across 20 recent University of Illinois trials, yield leveled off at only about 32,000 plants per acre. Under lower-yielding conditions, responses to population tended to flatten out at populations in the high 20,000s or low 30,000s. Under higher-yielding conditions there were some responses up to the low 40,000s.

Although Nafziger believes that 32,000 plants per acre is not enough to maximize yields on the most productive ground, he also says growers may not need more than 36,000 to 38,000 plants to get to the 300-bushel mark. “Some use populations above where they need to be, under the belief that 35,000 plants can’t maximize yields,” he says. “Expecially in a stressed year like a drought, higher populations can be harmful to yield and profits.”

Pushing populations into the 40,000s is unlikely to increase yields and even less likely to pay for the seed, Nafziger says.

Higher plant populations are one of the most important ways to achieve the highest possible corn yields. says University of Illinois Plant Physiologist Fred Below, known for his research on 300-bushel corn yields.

Seed companies continually test their hybrids to discover which plant populations will work for different hybrids in a broad range of yield environments.

“There’s a different answer to what the ideal plant population is for every hybrid,” says Dale Sorensen, field research lead for Monsanto’s Integrated Farming System. You also have to consider field and growing conditions, and make assumptions about the weather, he adds. A population of 30,000 plants per acre may yield 260 bushels per acre in a good environment, he suggests. But that same hybrid at 40,000 plants per acre and 1 more inch of rainfall may yield another 50 bushels per acre. “It’s about fine-tuning our recommendations and matching the population to what the yield potential is based on the unique characteristics of individual fields and using variable-rate seeding.”

Syngenta studies indicate that a starting point for 200+ bushels yield is 38,000 to 39,000 plants per acre. But as yield environment drops, so must the plant density, says Chris Cook, head of agronomy for Syngenta. “If you have a field that historically yields lower, 150 to 175 bushels per acre for example, then 35,000 plants per acre is where you’re going to maximize that yield,” he says. Growers have to be honest with themselves about the yield environment that they are really in, he says. Then they can consider other factors like anticipated grain prices, fertility, weed spectrum and more.

DuPont Pioneer’s Steve Butzen, agronomy information manager, says one important key to its research on plant populations is testing in a range of growing environments from best to worst, and in different soils and diverse locations.

“For about 80% of corn acres, the seeding rates generating the most income for growers range from 32,000 to 38,000 seeds per acre,” Butzen says. “The low end of this range is appropriate for yields of 150-170 bushels per acre, and 38,000 seeds per acre is often best for yields exceeding 250 bushels per acre on highly productive soils or irrigated ground.”

But ideal plant population is very dependent on the local growing environment, he adds. “On low-yielding dryland acres in Nebraska, for example, 32,000 is usually too many plants.”

 “Knowing your target yield goal is key,” says Cook. “Once you hit a certain plant population within certain conditions, in your target yield environment and depending on the hybrid, yields will level off. Beyond that you’ll start to lose yield because even corn plants become weeds at some levels when they compete with each other.”

 “One reason we put so much attention on plant populations is we can control it,” Nafziger adds. “But you also can’t beat yourself up when you choose an appropriate seeding rate to manage your risk, then get a great growing year and decide you should have used more plants. It’s just as likely that plant populations could have been too high for the conditions.”

Do the math

The “sweet spot” of plant populations is where the yield increase from adding plants is just enough to pay for the seed needed to add them. Calculate how many seeds 1 extra bushel of yield will “buy,” says University of Illinois Agronomist Emerson Nafziger, At a seed cost of $3.75 per 1,000 seeds ($300 per 80,000-seed unit) and corn at $4.50 per bushel, one bushel of yield will buy 1,200 seeds. So adding that many plants would have to increase yield by 1 bushel per acre just to break even. That might not sound like a lot, but Nafziger’s data shows that if you’re already in the mid-30,000s for plant population, getting that is less likely than many believe.

Across 20 University of Illinois trials under dry conditions in recent years, yield leveled off at only about 32,000 plants per acre. Under lower-yielding conditions responses to population tended to flatten out at populations in the high 20,000s or low 30,000s. Under higher-yielding conditions there were some responses up to the low 40,000s.

Syngenta seeding rate study

Variable-rate seeding:  Make plant populations pay

With the right tools, growers can tailor plant populations to the varying growing environments within a single field. Knowing if, where and by how much you should change rates can make variable-rate (VR) seeding  challenging. And the jury is still out on whether or not it pays.

“If your field has some sand pockets among deep black soils, it only makes sense that planting different populations across that field can pay off,” says Syngenta’s Chris Cook. “However, it’s critical to calibrate your planter correctly for the hybrid you have and to select the right hybrid to maximize the populations you’re planting. It’s a balancing act.”

 “Variable-rate seeding is not a bad idea, but it’s not going to be incredibly profitable if you are already managing populations correctly for a field,” adds University of Illinois’ Agronomist Emerson Nafziger.

Southern Illinois grower Matt Bowlby, however, believes he has seen significant results. The fifth-generation row-crop grower who considers himself a “number cruncher” has been collecting data on his fields for nearly a decade and has been variable-rate seeding all his corn acres for 3 years. His rates vary from 28,000 to 36,000 seeds per acre across the fields due to a wide variance in soil types.

“We always felt like we were leaving yield behind, so we traded planters for one with variable-rate seeding capability,” Bowlby says. “We’re trying to maximize the yield potential of different soil types and field conditions.”

A couple years ago, Bowlby received the prescription maps from his consultant and something about them just didn’t seem right, so he switched off the VRS. However, the yield map clearly showed where he should have been planting at a lower population in the middle of the field. “I’d calculate that we lost about $75 an acre in that area by seeding too high,” he says.