Setting Soybean Populations

Wednesday, April 23, 2025

An art or a science?

By Paul Hermans

Setting soybean population is both an art and a science.

There are a lot of things to consider, including soybean variety choice, crop rotation, tillage practices, yield goals and return on investment, disease management and much more. In 2024, across Central/Eastern Ontario and the Maritimes, we conducted a soybean population trial to answer grower questions on what the best soybean population rate is. As any agronomist would tell you, it depends.

In this article, I will review the 2024 study, how it was set up, the results, and how you can use this data to fine-tune your own soybean populations.

When we first set up these trials back in the winter of 2024, we had just had one of the worst white mould years (2023) in a long time. Growers were looking for answers and, more so, solutions to help reduce white mould yield loss. A common question was, ‘Can I reduce my population without sacrificing yield?’ Fast forward to the 2024 growing season, and we had zero to low levels of white mould. We did, however, learn a lot, not only about seeding rates but also about where the majority of yield components for soybeans come from and how the plant architecture changes with population.

The population study included planting rates at 80-120-160K seeds per acre (KSPA). We had 19 trials in total. In 14 trials, we used plant counts to compare percentage emergence to the target planting rate.

During the growing season, we looked at three plots and their respective yield components (branches, pods, seed per pod). We looked at seeds per pound in 14 locations as well.

So, what did the study tell us?

Yields, respectively, for the 80-120-160 KSPA were as follows: 59.3, 62.6 and 62.8 bushels per acre.

When we looked at emergence, we had an overall 89.5 per cent emergence rate across the trials. In Canada, to sell Certified No. 1 seed, you must guarantee a minimum warm germination rate of 85 per cent or higher. Unlike corn, soybeans are more pernickety in their emergence. They require twice as much moisture on a seed weight basis to germinate than corn. Having an 89 per cent emergence rate was amazing. A lot of this was attributed to ideal planting conditions and available soil moisture.

On a seeds per pound basis, there was no statistical difference in population rates. Comparing 80-120-160 KSPA rates, seeds per pound were 2,354, 2,392 and 2,370.

This makes one wonder – for the yield to be within 3.3 bushels comparing the 80-120 seeding rates, where did the yield come from?

When we looked at three plots more in depth, it came from added branches (nodes), which equated to more pods per plant. The number of seeds per pod was not significantly different.

So why more nodes/branches on the lower populations? At these lower rates, there is less competition between plants, and less stress from plants being crowded within the row. This allowed the soybeans to branch more, adding more nodes and pods.

This was also evident when we looked at plots during leaf drop and the final stages of reproduction. We could visually see healthy plants and less leaf drop on the lower plant populations than the higher ones.

We looked at stem diameter at a few locations. From low to higher populations, we saw stalk diameters ranging from 11 mm to 7 mm to 6.5 mm (+-). If lodging is an issue in high-fertility, high-yielding fields or compacted soils, population settings are an alternative to be considered.

Fast forward to setting populations for 2025 and beyond. The three biggest factors I would look at include: Yield environment, disease pressure and tillage/crop rotation.

I also conducted a social media survey on X (Twitter) to get a sense across Eastern Canada what populations growers were planting at. Out of 197 respondents, three per cent plant at 80K, 25 per cent at 120K and 44 per cent at 160K. Interesting enough, responses from some growers in heavier clay soils and low white mould areas ranged from 180K to 220K seeds per acre. This confirmed that there is no exact answer to population rates across Eastern Canada based on varying management, yield environment and other management factors.

This is where I add a bit of a disclaimer. Research studies are great for learning and adapting. Take the information and see if it is feasible for your operation on a small acreage basis before drastically changing any agronomic practice. Local experience pays off, and talking with local growers and agricultural advisors/Certified Crop Advisors can give you another opinion to look at.

When looking at soybean replant decisions I am confident plant stands at 75 to 80K are the best economic choice. There is no need to replant unless populations dip below these levels. Other things to consider with rates: For ultra-early planting, consider bumping up rates for less-than-ideal conditions. Conversely, for late planting (Ontario June 7 to 10 or later), bump up populations. Growth, height wise, during this later planting will be less. This results in fewer nodes and fewer pods, leading to lower yields. Like lower yield environments, offset this by adding more plants per acre.

More growers are implementing variable rate seeding, which has shown tremendous value, especially in fields with yields that can vary across the field by five bushels or more. Unlike corn, as yield increases, VRS rates decrease for soybeans. In lower-yielding soybean areas, they increase. Using soil data maps from SWAT, VERIS or SoilOptix machines can fine-tune zones even more.

I chatted with Peter Archer, owner-operator at Maizeing Acres in Northumberland County. They have rolling hills with variable soils and historical white mould in certain pockets. On their farm operation, VRS is a key management tool they use. In historical mould areas, they plant soybeans at 100 KSPA. In lower yielding zones, 160 KSPA. Overall, their average population is 140 KSPA.

In summary, the 2024 population study we did sheds light on several questions. Keep in mind, though, it is a snapshot of one year and one growing environment. Conduct your own trials, varying populations by plus or minus 40 KSPA to determine the optimum rate for your farm.

Happy soybean population planning. I hope this article helps as we head into the final stretch before plant 2025. BF