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Corn populations for ’13 all depend on weather

Should we drop our corn populations this spring since it looks like the drought may stick around?” If I had a dollar for every time I fielded that question, I could put a hefty down payment on a new pickup.

Corn populations for ’13 all depend on weather

Should we drop our corn populations this spring since it looks like the drought may stick around?” If I had a dollar for every time I fielded that question, I could put a hefty down payment on a new pickup.

My daughter Maggie is out and about enough with me to notice that crop questions come up often when we run into growers, so she asked “Dad, do you ever get tired of answering farming questions?” Pretty simple answer: “No.” Agriculture is what I love, it’s how I was raised, and it is what I do.

The hardworking men and women who feed our world spend anywhere from $600 (soybeans) to nearly $1,000 (corn on corn) per acre to raise our feed and fuel. One word for that: respect. I’ll answer questions until the cows come home, including “What do we do with our corn populations this spring?” every time it is asked. But be prepared to talk for a few minutes, and you can already guess that the answer includes “It depends.”

My agronomy education started in 1988 in Lenox, during one of the worst agronomic droughts on record. As recent high school grads with no agronomy courses under our belts yet, the team at the local retail dealer trained three of us to scout corn and soybeans for their customers. We got an education that summer that went well beyond basic agronomy.

It was a rough year for growers; the drought started at planting time and never let up. We learned a lot from those farmers and the guys at the retail plant about perseverance, stress management and just plain “guts.” Statewide, corn yields were 29% below trendline at 84 bushels per acre, and some of the innovators who pushed corn populations up paid a heftier price.

Hard lesson to forget

While I recall average populations at the time being in the low 20s, a few guys were pushing the upper 20s to nearly 30,000 plants per acre. Barren stalks, lodged corn, extra seed expense and lower yields were the reward for many of the innovators. A lesson like that is hard to forget.

Although 2012 was not a very pleasant year by any measure, it gave us some insight on how far we have come since 1988. Our 2012 drought was more severe than in 1988; yet, we were able to average 137 bushels per acre.

We didn’t like our corn yield results last year, but being 7% closer to trendline and 53 bushels above the 1988 drought show that genetics and overall crop management have advanced greatly the last couple of decades.

Another lesson learned: Pushing corn populations in 2012 wasn’t as penalizing as it was in 1988. While we did see yield penalties in some fields where farmers pushed populations into the upper 30s to 40,000 plants per acre, for the most part they were small. The larger issue was that given the corn market and the cost of seed, profits took a sizable hit even if the yield penalties were small. Where yields were a push between high and average populations, the extra seed cost was up to $15 to $20 per acre.

While results were all over the board given the wide range of yields across the state, a pretty common number discussed was about a “5- to 10-bushel-per-acre hit” for pushing plant populations in many areas. Add that to the extra seed cost and we are talking about getting tagged for around $50 to $80 per acre in fields that had a relatively small yield penalty for high corn plant populations.

Having sold seed for years and being aggressive with population recommendations, in droughty 2012 I got a lesson in the difference between maximum yield vs. maximum net income.

Yield, hybrid and weather

Where does all this leave us for 2013? Here it comes again: It depends. The great work done every season by universities and industry tells us that collectively we are on the right track with populations, and for most of Iowa the corn population recommendations are pretty basic — mid-30s. I know, you could drive a truck through that set of numbers, but when you dig deeper into the data, it is clear why the range is pretty wide.

Optimum plant populations depend on dozens of factors, some we can manage and some we cannot. Since a discussion of those factors would fill a library, I’ll cast a wide net and talk about yield level, hybrid and weather.

Yield level typically means breaking our acres into broad-yield potential categories like high, medium and low, and adjusting populations based on the varying potentials and on experience and agronomic recommendations. I’ve seen plot data where the optimum populations for a group of hybrids varied from the upper 20s in low-yield potential fields to close to 40,000 in high-end ground.

There are also some significant differences in optimum populations between individual hybrids. A great example is a large set of results from Iowa trials conducted by a major seed company over the last several years.

A selection of its hybrids showed an optimum planting rate for most was about 34,000 to 36,000 plants per acre, but there were a few that preferred closer to 32,000 and a few that did best closer to 38,000.

While this data set gave specific numbers for each hybrid’s optimum, typically the population recommendations are given in a range of a couple thousand plants wide that each hybrid will perform very well within.

Weather factor is elusive

If we knew how to better predict that third factor (weather), we could tell you which end of a hybrid’s population range to shoot for. Since we can’t forecast our growing-season weather with pinpoint accuracy, we have to make the best decisions we can with the info we have.

In 2012, crops drained our soil profile of plant-available moisture; as I write this, 90% of Iowa has a substantial moisture deficit. While we could get timely rains to alleviate the continued drought, it is hard to bank on that, and I recommend we get aggressive with seeding rates.

So where should we go with corn populations? You already guessed right: If we were assured of plentiful moisture, we’d aim for the high end of the recommended ranges. If we saw a continued drought, we’d go to the low end of the recommended ranges for our hybrids and yield levels. One small but important factor is that we can make population decisions each day when we head to the field with the planter.

Between now and planting time, a lot can change or not change in regard to drought. I can’t say it any better than ISU’s corn specialist, Roger Elmore, does.

“Normal seeding rate or plant population recommendations are based on full soil profiles at planting. If 2013 soil moisture conditions do not improve, if soils are dry at planting, be conservative on seeding rate changes. Although planting to achieve high plant populations is a good approach in most years, 2013 may be the year to not push seeding rates to higher levels especially in drought-prone soils,” he says.

Combine Roger’s insight with your field-by-field experiences and the hybrid by environment recommendations from your seed dealers. In the meantime, we’re all hoping for the timely rains and soil moisture recharge that could lead to a good 2013 crop year.

McGrath is an ISU Extension field agronomist, co-leader of the crops team and author of this Corn-Soybean Insight column. Contact him at

Visit for more corn population information.


This article published in the March, 2013 edition of WALLACES FARMER.

All rights reserved. Copyright Farm Progress Cos. 2013.

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