Quinn Talks Soil, Corn Dynamics
Published: Friday, March 7, 2025
Dan Quinn, Purdue University corn specialist, presented the latest research regarding variable seeding rates in corn during the annual Lake County Winter Breakfast Meeting last Tuesday in Cedar Lake.
Quinn and his team of researchers are examining the optimum corn seeding rates for various conditions, with an eye on reducing seed costs. He pointed out that seed, at 20% of variable cost, is the second largest expense behind fertilizer. With corn prices being much lower compared to a couple years ago, saving on seed is even more important.
Adjusting your seeding rates according to soil type, slope and other factors is "easy," Quinn said, "as long as you have the equipment." Planters equipped with precision controls make it possible to vary your seeding rates across the field.
The overall yield trend for corn is going up in Indiana, with five out of the past seven years posting record yields. The average annual increase is between 1.8 and 2 bushels per acre, Quinn said.
"So, the big question that I have is, in terms of both fertility rates and seeding rates, do those need to keep getting bumped up as yields keep getting higher?" he asked.
According to Quinn, the answer is not necessarily.
According to USDA data, Indiana farmers dramatically increased plant populations from 1980 to 2012. However, plant populations reached a plateau from 2013-24. Yields still increased while plant populations were relatively flat.
Quinn said this data points to genetics as the main reason for higher yields during the past 12 years.
"I think the big thing with hybrids today that we have noted is that they are able to obtain higher yields and have better tolerance at higher plant populations," he said. "They also do pretty well at lower plant populations as well. They often have a much wider optimum seeding rate range in hybrids today."
He said farmers can get the same yields from 32,000 seeds per acre compared with 38,000.
"In many cases, we don't see much of a difference in yield between those," he said. "It's largely due to the genetics and the way they behave."
Quinn and his research team are looking at spatial responses "outside the extremes." For example, higher seeding rates respond well under irrigation, but what is the response to variable seeding rates with normal weather and no irrigation?
Other factors, such as soil type, drainage class, slope and elevation can influence the optimum seeding rate.
Farmers who want to build a seeding-rate prescription for their fields can access a free service from the University of Illinois, called DIFM (Data-Intensive Farm Management Program). Quinn said farmers can visit the website (https://difm.farm/), answer some questions, and the service will spit out a prescription in the exact file format that they need for their planters.
"There are a lot of things you can build in to try and understand the variability in yield responses in the field and potentially how that tracks to the optimum seeding rates," he said.
Quinn used DIFM for the field trials he conducted last year. He worked with seven farmers across the state, including operators in Marshall, Porter, White and Carroll counties. At one farm, the seeding rates varied from 26,000 to 44,000.
"For this data, we've largely looked at soil type, elevation and slope but understand that we're also looking at all these others as well," he said, "and there's probably more out there that we're trying to understand."
The farmers used DIFM to build a prescription for variable seeding rates. They planted and harvested without having to do anything out of the ordinary, the only exception being wider plot widths. Quinn and his team then looked at the harvest data to see how the field performed.
At the Porter County trial, the optimum seeding rate for the whole field was 33,000 seeds per acre, but Quinn said there were "drastic" differences within that field.
"When you break it apart by different management zones—something like soil type is a prime example —on sandier areas, sandy-loam soils, primarily, his optimum seeding rate is around 27,000," Quinn said. "But when you get into more loam areas, more productive areas of the field, I actually see where he was still gaining yield with a little higher seeding rate. So, his optimum seeding rate (on the loam soil) was pushing about 40,000."
At the Marshall County trial, the optimum seeding rate for the entire field was 31,000, Quinn said. Broken down by soil type, the farmer had higher productivity on loam soils, where the optimum seeding rate was 31,500. But, as with the Porter County trial, the optimum seeding rate on sandy soil in Marshall County was between 27,000 and 28,000, he said. Seeding rates can be higher on more productive ground.
With respect to elevation, the loamy, finer-textured soils on flat areas of the fields had an optimum seeding rate of 31,500. The sloped areas with sandy soils had a lower seeding rate of 27,000, he said.
The above figures reflect the best agronomic seeding rates. Regarding the economics, Quinn looked at seeding rates, yield and the price received for the corn.
"Looking at the entire field, his most economical seeding rate ranged from about 26,000 to 30,000," Quinn said. "The most economical was about 28,000, very similar to what the optimum yield was."
Above that seeding rate, the farmer lost between $30 to $50 per acre.
"His yield actually went down in some cases," he said. "But there aren't many differences in yield, but that profitability is going down."
Not all the farms produced the same results. In some situations, the optimum seeding rate was much higher. In Carroll County, the test farm has a variety of productive soil types and uses hog manure.
"His optimum seeding rate (agronomically) was about 38,000," Quinn said, while more poorly drained soils were around 35,000.
The most economical seeding rate for the Carroll County farm was about 35,000, a little lower than the agronomic rate. Like other farms, the higher seeding rates produced slightly higher yields, but "his profitability was actually going down."
The preliminary results of the study show that the agronomic optimum seeding rate differs across fields, depending on soil type, drainage, slope, etc. Well-drained silt loams had the highest seeding rates (over 38,000 per acre) while well-drained sandy loams—typical in northern Indiana—had the lowest seeding rates (under 30,000 per acre). Poorly drained silt and clay loams were in the middle (31,000 to 35,000 per acre).
"One shoe size does not fit all," Quinn said, referring to the varying results of the study. "Just because one assumption holds true in one location or field, doesn't mean it holds true in others."
He said this underscores the importance of localized data as farmers build variable rate prescriptions.
Quinn said the study will continue for two more years, and he's hoping to add more farms to the study, with more parameters. His goal is to have farms that reflect all the different soil types and farming practices.
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