What Is The Ideal Population For Corn & Soybeans?
By Paul Hermans
The No. 1 question I get as a seed agronomist is, “What is the ideal population for corn and soybeans?”
The usual answer is, “It depends.”
It depends on several factors – some of which are in our control and others which are not.
Corn
Many factors influence corn population, but the key criteria for setting a given seeding rate include:
- Hybrid response to population.
- Soil type and water/nutrient-holding capabilities broken down into yield zones on a per bushel basis.
- The price of corn seed.
- What is the commodity price of corn for the period you are selling it at?
I have said before that you should think about setting populations in the same way a realtor prioritizes criteria when selling a house. Location, location, location matters. In corn, the most important factor is water availability, water availability, water availability.
Corn planted at too high a seeding rate in water-limiting soils competes too much for nutrients and water. We can add more plants closer together in high-yielding areas because they can take the inter-plant stress more easily. More plants in higher-yield environments means more yield.
Every field has unique characteristics regarding water-holding capacity, not only at a field level but also at a crop management/soil zone level. This sets the basis for corn populations and the stress corn plants will go through in a growing season on a sub-fields basis.
At Corteva we look at decision zones (crop management zones) and what yield levels and corresponding populations are in these decision zones. For each decision zone, the zone can be broken down into yield “steps.”
Think of a set of stairs. For each step of stairs, yield can go up 10 bushels. Our research would say that for each step or yield level increment of 10 bushels, we have data that would recommend a certain population by hybrid for that yield step.
Every hybrid is unique in its own characteristics. Some hybrids flex down based on stressors thrown at it during a given growing season. For example, some flex down from 18 to 16 rows at V5-V6 if stress occurs at that growth stage. Others flex down during the critical pollination period and some others at the kernel depth/kernel weight phase, equating to the last 30 days of grain fill.
Corn tends to respond to higher populations in Northern climates. Sunlight hours and intensity changes the farther north you are than south. Capturing as much sunlight on a per-acre basis as early as possible is critical.
Researchers talk a lot about leaf area index and if we can utilize this to capture 95 per cent of the sunlight at the onset of reproductive stages. If we do that, we are set up for maximum yield. This is where corn populations come into play by increasing leaf area index and capturing solar energy to make grain.
Be careful when looking at broad corn population data that is not relevant to your corn maturity zone. Significant differences exist between seeding rates of 85 to 95 to 105 for comparative relative maturity hybrids.
Learning your hybrids and how they react to these environments is vital. Then taking the four key factors I talked about above and combining all this information equates to the ideal corn population for a given field.
Across North America populations have increased on average 280 seeds per acre per year over the last 20 years.
Over the past three years, Pioneer has conducted corn population trials in Eastern Ontario, looking at planting rates of 26-30-34-38 thousand seeds per acre. These rates are set up to determine the ideal kernels per acre corn can produce.
Research has shown that kernels per acre equates to about 85 per cent yield potential. The remaining 15 per cent of yield comes from the kernel weight (depth) of the kernels each year.
In 2023, there was no yield limit to our population trials. Across 14 trials the optimum agronomic population rate was 38K seeds per acre. The 2023 growing season reflected this with ample rain during the grain fill period, an extended fall grain fill growing season and ideal daytime and nighttime temperatures that did not stress the corn crop.
Looking at individual hybrids, there were differences in hybrid genetics to population. Knowing these differences assists in setting the correct population rate. Maximum economic seeding rates (which considers yield, commodity price and seed cost) in the 2023 trials was within the range of 34K-36K seeds/acre.
To sum up, in general, the higher the yield level is at a field or crop management zone, the higher the corn population recommendation would be.
In 2024 economics are different than previous years based on corn commodity prices. Carefully look at population rates and make sure you are using realistic yield goals. Changes in commodity prices changes population levels downward slightly from an economic standpoint.
If you are looking for population recommendations for your farm, check out seed company websites. Most allow you to plug in a hybrid corn, see price and commodity price, and get the latest research data at various yield levels to give you not only the highest agronomic yield but economic yield as well.
Soybeans
Setting soybean population rates are the opposite to corn. As soybean yield levels increase, soybean populations decrease. This reflects the soil’s capability of growing a larger, more robust plant that will have more nodes, flowers, and subsequently pods/beans at harvest time.
I am just over six feet in height. When talking about soybean populations, I make a few reference points to my height. If you have soybeans in a crop management zone that are above my waist in height compared to soybeans that are at my knee height, this is a fantastic way to talk about soybean populations.
Scouting fields will help you get a sense of these height differences at critical growing periods in soybean development stages. The lower-height soybeans will not have as many nodes and subsequent pods/beans, so you need to offset this with more plants/acre to have the same number of nodes/acres. Scouting fields at key times will help give you a better sense of these height differences within your fields.
Keep in mind water-holding capabilities of your fields, just like with corn. The key to soybeans is to set a large pod load, maintain it, and harvest it.
In a soybean crop, well over 50 per cent of the soybean flowers will abort and not make grain. There is a balance needed to get the right populations based on a yield zone’s potential.
Other factors that should be reviewed for soybean seeding rates include:
- No-till or minimum-till soybeans in high residue situations calls for higher seeding rates.
- Planting soybeans into high-residue corn rotations versus soybeans calls for seeding rate differences, especially if row cleaners are not used at planting time.
- For ultra-early planting dates, look at increasing soybean seeding rates.
- Treated seed versus none. Bump rates up when not using seed treatments.
A few concluding thoughts
Setting seeding rates is an art and a science all rolled into one.
Utilize harvest maps to look at variability of yield across a given field.
If corn and soybean yields vary less than five per cent yield from one side of the field to another it is easy to set just one planting rate.
If you have variability of 10 to 20 per cent or even higher across a field, the use of variable rate seeding (VRS) will improve your field profitability immensely.
Reach out to equipment vendors and seed suppliers and your local agronomy advisor to explore the benefits of VRS on your farm. BF
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