Based on one replicated field trial in one location in one year, if you set your planter to drop 32,166 seeds per acre in 30-inch rows, you would be on target to achieve maximum agronomic yield. Based on 97 field-scale trials in Indiana since 2008, if you dialed in your seeding rate at 33,840 seeds per acre and achieved 32,150 plants per acre, with minimal to moderate stress conditions, you should be able to reach maximum yield if other parameters are met, such as supplying enough nutrients.
“That’s what we see when we look at the 2021 Indiana Prairie Farmer/Purdue University Agronomy trial at the Throckmorton Ag Center, and then look at data collected over the past dozen years or more,” says Dan Quinn, the Purdue Extension corn specialist.
“What we found this year in the Throckmorton Ag Center field-scale trial was close to what we would have expected, based on those long-term results,” he says. “The maximum agronomic seeding rate in this trial was within a couple thousand seeds of what the long-term data would suggest.”
Quinn notes that results in any one season at one location can vary. In 2021, for example, the Throckmorton field, planted in late April, performed well into late July, when a dry spell began that covered several weeks during grain fill. It also set up favorable conditions for stalk rot and stalk lodging. The field was harvested Nov. 1, and Quinn says lodging was much worse on the strips planted at 38,000 seeds per acre. That’s likely why yield was lower at 38,000 seeds per acre than 30,000 and 34,000 seeds per acre, he says. In fact, it was second-lowest in the trial, at 205.6 bushels per acre, and was less than 6 bushels higher than the 22,000 seeds per acre rate, which averaged 199.8 bushels per acre across both hybrids.
Yields for 38,000 and 22,000 seeds per acre were not significantly different from each other when statistics were applied to the results, Quinn says. However, both were significantly lower than 30,000 and 34,000 seeds per acre, at 221.8 and 222.9 bushels per acre, respectively. The rate of 26,000 seeds per acre yielded 211.9 bushels per acre.
This field-scale trial was replicated three times. Plant populations were counted in mid-June (see table). The long-term data summarizing 97 trials assumed 95% germination and emergence to arrive at an agronomic optimum seeding rate of 33,840 seeds per acre based on optimum final populations of 32,150 plants per acre. Except for the seeding rate of 30,000, where the actual plant count was slightly under 95% of the seeding rate, actual plant counts in the Throckmorton trial were higher than 97%.
“It’s important to distinguish between seeding rate and plant population when discussing corn seeding rates,” Quinn says. “In the results for this trial, we talk about seeding rates because that was the factor we compared in the trial.”
Five rates were included: 22,000; 26,000; 30,000; 34,000 and 38,000. Two hybrids were planted at each of the five seeding rates. Beck’s provided seed for the trial. Twelve-row strips were planted for each seeding rate of each hybrid, and the middle six rows were harvested. The field where the trial was conducted consists of timber soils with average organic matter levels.
Here are observations and takeaways:
Hybrids responded the same to seeding rates. There was not a significant interaction between the two hybrids in their response to varying seeding rates. “That means that the agronomic optimum and economic optimum seeding rate values do not differ between the two hybrids,” Quinn says.
Low rate performed well. The average for 22,000 seeds per acre was right at 200 bushels per acre, Quinn observes. While that may seem surprising, note that the actual population was 21,583 plants per acre, 98% of what was planted, and weeds were controlled well.
High rate lodged more. One pass at the 38,000 seeding rate was actually the highest-yielding pass in the trial, but overall, lodging on most of the high-rate strips cut yield.
Moisture differences. All yields are reported on a dry-corn basis. One hybrid was significantly higher in moisture than the other at 19.1% vs. 17.5 %. The highest-moisture hybrid was the highest-yielding hybrid. There was no significant difference in moisture based on the different seeding rates, Quinn says.
Bottom line. Corn yield response to seeding rate was not significantly greater beyond 30,000 seeds per acre, Quinn says. There was about 1 bushel per acre difference between 30,000 and 34,000, with 34,000 being 1.1 bushels higher. However, that’s why you include statistics, Quinn notes.
“That’s well within the margin of error,” he says. “Statistically, they are the same, with no difference between those yields.”
Maximum economic yield
Purdue Extension agronomist Robert Nielsen has emphasized the point of considering maximum economic yield over the past several years. In the long-term summary by Nielsen and Jim Camberato, also a Purdue Extension agronomist, the maximum economic seeding rate is often a couple of thousand or more seeds per acre below the agronomic optimum rate for maximum yield.
“When you talk maximum economic seeding rate for corn, you must look at seed cost and selling price of corn,” Quinn says. “The more you pay for corn seed, the lower the maximum economic seeding rate based on yield results in trials over time. But the higher the price of corn, the higher the seeding rate.”
Quinn prepared a table for the economic optimum seeding rate with corn prices and possible seed costs for seed corn. This table is based solely on the 2021 Throckmorton trial. Note that you’re generally talking changes of only a few hundred to several hundred kernels per acre for each change in corn price or seed cost.
To learn more about selecting seeding rates, including the economic optimum seeding rate across many trials, refer to Yield Responses of Corn to Plant Populations in Indiana by Nielsen, Camberato and Jason Lee.