Is variable-rate seeding worth the investment?

In most cases, no. That’s the verdict so far from Purdue University after three years of 100 field-scale trials with multiple hybrids across Indiana.

The thinking behind variable-rate seeding is to nix field variability by varying what’s calculated to be an optimal seed population to specific field areas.

“I admit I’m a wet blanket when it comes to the widespread benefits of using variable-rate seeding in corn,” says Bob Nielsen, Purdue Extension agronomist. “But most of the time, we haven’t seen a response.”

Nielsen reported findings that Purdue scientists have conducted on variable-rate corn seeding in 30- to 100-acre fields with many soil types that are large enough to accurately reflect field variability at this month’s Top Farmer Conference at Purdue University.

Exceptions exist, such as dryland fields with areas perennially at risk for drought.

“In those situations, our data suggests it makes sense to back off populations in those areas of the field,” Nielsen says. “In other zones that have fairly good water-holding capacity with a low risk of droughty conditions, just one common optimal population is needed.”

Optimal agronomic populations are lower than what many farmers would initially expect.

“The final harvest population is no more than the mid-20s on drought-risk fields,” says Nielsen. “For almost everything else – and for a yield ranging from 140 to at least 250 (bushels per acre) – the common harvest plant population per acre (ppa) that optimizes grain yield is in the low 30,000s.”

Harvest populations that maximize dollar return are even lower. If $240-per-bag seed costs and approximate $3.50-per-bushel corn prices are considered, the optimal harvest population is closer to 26,000 ppa, says Nielsen. This harvest population would typically require a seeding rate close to 29,000 ppa.



More than sky-high populations

Farmers who want to crack the 300-bushel-per-acre level and above – as winners in the National Corn Growers Association do – need to do more than plant stratospheric seed levels, says Nielsen.

“There are quite a few contest winners into the 300s (bushels per acre) who report harvest populations in the neighborhood of 30,000 plants per acre in the National Corn Growers yield contest,” he says. “They aren’t all seeding 45,000 and 50,000 plants per acre. By itself, high populations do not guarantee sky-high yields, in my opinion. Up to 240 and 250 bushels per acre (yields), there is no evidence that it (yield) takes off. Yield plateaus at 30,000 to low 30,000s population."

Nielsen adds that the Purdue researchers do not have enough data on optimal plant populations for yield levels above the 250-bushel-per-acre range.

Farmers planting lower agronomic and economic plant populations need to make sure every seed counts each year and every field, though.

Nielsen advises farmers to do planter stand counts in every field each year to assess their planter’s seed drop. “If you don’t do that, then you are throwing darts,” he says.

Doing on-farm trials – and Indiana farmers are welcome to join the Purdue data collection – can help farmers be more comfortable with their own seeding rates. Indiana farmers who wish to participate in the seeding trials can contact Nielsen at rnielsen@purdue.edu.

“We welcome any and all farmers for more on-farm trials,” says Nielsen. “One of the reasons we are confident in our data is we have a lot of trials to back up what we are saying. The more trials we have, the better (our data) gets.”

For more information, farmers can download this online summary of corn yield response to population.