Now that we have entered the month of May, you might start rethinking your planting populations for soybeans.  As a seed sales agronomist, a lot of growers continue to be shocked when I tell them they can lower their planting population for soybeans. For the most part, planting 140,000 seeds per acre has been adopted in the countryside. In Central Illinois, when planting in early April, it is not uncommon for growers to pull the population back and plant 120,000 seeds per acre. I think shooting to achieve a final stand of 100,000 seeds per acre is a good place to be.  If we are evaluating replant situations, having a stand of 80,000 is still okay, but shooting for that in an average year is a little drastic, in my opinion.

As we move into the middle of May (or later) in Central Illinois, we start thinking about increasing our soybean planting population. The reasoning for this is to help increase plant growth due to the later planting date and help achieve maximum yield. Here’s some correspondence I had with AJ Woodyard to explain this concept.

“As the calendar moves into the middle of May, the soybean plant will more dramatically reduce the total number of nodes per plant it can achieve.  Due to this physiological response, our management response is to increase the number of plants per acre to make up for the reduced number of nodes per plant,” says AJ Woodyard, Research Agronomy Lead with Advanced Agrilytics.

A few things you should consider are row spacing and soybean plant type. Are you planting a thin line soybean or a bushier soybean? This information can be found in the product guide if you are unaware of what the plant type is. Thin line soybean will not produce as many branches so it will have more pods on the main stem.

Woodyard explains, “We know some varieties have a propensity to build more yield on the branches, while other varieties tend to build more yield on the main stem. Interestingly, the mechanism on how soybeans produce their yield also has influence over their population response. Soybeans that tend to build more yield on the main stem, for example, need higher population density to achieve maximal economic return versus soybeans that build more yield on branches.

“So, it stands to reason that as planting date gets later and later, understanding this characteristic of your soybean variety can also be crucial in determining how to adjust populations. If you have a known main stem yield variety, it’s even more critical to increase population, as these varieties are reducing total nodes per plant while also not compensating with additional yield on the branches.”

In Central Illinois, our general guideline is to bump up our planting population every 10 days after May 5th. We recommend to our growers on or after May 5^th to be at the 140,000-planting population, if you were below that before May 5th. On May 15th, we want to increase 3-5 percent or 2500-5000 seeds per acre population. On May 25^th, we would increase again by the 3-5 percent.

If you were going to attempt to estimate your soybean yield prior to harvest, one piece of the formula is to count the amount of pods/plant. As AJ explained, the plant is going to reduce the number of nodes the plant can achieve; so, you want to consider the soybean type you have and where your node/pod count will be decreased. If the number of nodes per plant is going to be reduced and you are planting a thin line soybean, you may want to go to the 5 percent increase to make sure you make up for the lost number of nodes due to later planting.  Whereas if you are planting a bushier soybean with branches, you may only need to increase by the 3 percent. Sometimes the increase depends on where your planter can be set!

Also, increasing your population will help with row closure. When soybeans are planted later, they will not grow as tall and they can struggle to close the rows early, which we all know, can lead to potential weed issues.