Cut back plant populations
Another surprise is that Shaun Casteel, state soybean specialist at Purdue University, and his colleagues have determined that soybean yields do not effectively increase at populations higher than 120,000 plants per acre, or ppa. Populations in the 100,000 to 120,000 ppa range deliver the best return on investment, he notes.
“This is lower than traditional thoughts of soybean plant stands, and certainly lower than the target of many farmers,” Casteel points out.
Fine-tuning plant populations requires several straightforward but important steps:
- Check your work. Scout fields and count stands so you know if you’ve planted accurately.
- Compensate for seed germination rates. Check the seed tag and increase seed numbers to account for the percentage that won’t germinate.
- Adjust planter based on seed size. Each year’s growing conditions impact the number of seeds per pound — which means you need to adjust your planter based on seed size and number, not weight.
Adjustments can help
Researchers at Ohio State University recommend increasing seed numbers at the end of May and first couple of weeks in June. They note that stands of 125,000 seeds per acre and 225,000 seeds per acre planted April 25 and May 11 yielded similarly, but yields significantly favored higher populations when planted in June.
Dial back to thinner stands in fields where white mold has been a problem. Shawn Conley, University of Wisconsin Extension state soybean and small-grains specialist, recommends populations not much more than 100,000 ppa in those fields to alleviate the heavy canopy and tight spacing that favor white mold.
When it comes to soybean planting, it looks like thinking big starts by thinking small — lower populations and narrow rows.