The technology for variable rate seed has been around for about 15 years. Planters have hydraulic drives and row shut-offs and are coupled with GPS, monitors and controllers. You even can adjust the seeding rate on-the-go as long as you have a map to plant by.

Growers have been planting corn variably by increasing seed rates in the high-yield zones with the expectation of yield gain. However in soybeans, with a stand of 100,000 plants at harvest optimizes yield, so increasing seed rate doesn’t necessarily increase yield.

So is this the year to finally adjust soybean seeding rates on-the-go, and can you use this technology to actually cut seeding rates and save on seed cost? Or can you use this technology to lower seeding rates in high-yield zones while increasing seeding rates in low-yielding zones with tougher soils to optimize yield? The results aren’t as clear cut with corn, but are worth considering.

Sid Parks, manager of agronomy information services at Growmark, said that they have evaluated variable rate seeding of soybeans and have not seen a yield response to changing population. “We looked at the University of Wisconsin variable population studies and they never saw a response to a population of about 100,000 final, as long as there is good distribution and coverage. And we looked at drilled, 15-inch or 30-inch, with good seed soil contact and coverage, and there was no real economic advantage to variable seeding.”

Parks added that growers can plant at a high rate in tough areas at 180,000 to 200,000, and cut back the population to about 150,000 in better soils “However, we have concluded that variable rate seeding isn’t justified and profitable for soybeans.”

It has long been established that 100,000 plants at harvest optimizes yield. So the question I ask as an agronomist is what population do I need to plant to end up with 100,000 plants evenly distributed across the field at harvest? If you are planting 140,000, 160,000 or 180,000 seeds you have the opportunity to cut back that rate 20% to 30%. That means you can save 20% to 30% on seed costs. But you have to understand soil type, soil conditions and soil temperature at planting, as well as whether you are using seed treatments to protect the seed and seedling.

Brien Fell, soybean product agronomist with DuPont Pioneer in Illinois, said they have been experimenting with variable seeding in soybeans. “In our seeding rate trial, we realize there are opportunities to capture more yield in low-yield zones by increasing population and saving seed costs in high-yield zones by reducing population.”

He explained that their main focus is to plant more seed in variable, drought-prone, timber soils. “We need to close the canopy before it begins to get dry and hot. We like to seed a spread 20% to 25% between high- and low-yield areas. For example, we like populations to be 140,000 to 145,000 on prairie soils and up to 175,000 to 180,000 on poor soils.

Changing population depends on identifying appropriate management zones. “We identify management zones based on soil, water-holding capacity and yield. We like to see multiyear yield history on a particular soil,” said Fell. “Regarding the number of zones, today we feel two zones are enough to manage until we learn more about varieties and their response to soil type.”

Does variable rate seeding for soybeans in order to target a higher population in margin areas and a lower population in high-yield areas appeal to you?

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com.