Once upon a time, not that long ago, if you asked a specialist for a recommendation on soybean seeding rate, he or she likely would ask you what row width you were planting in before answering the question. Ask Shaun Casteel that question today, and row width won’t figure into his answer.
“We’ve found, based on numerous Midwest studies, that soybean yields are maximized at 100,000 to 120,000 plants per acre, regardless of row width,” says the Purdue University Extension soybean specialist.
Casteel notes that when he first arrived at Purdue in 2009, agronomists were still breaking out recommendations by row width, and generally recommending higher rates in narrower rows — especially 7-inch rows.
“The difference was really more about differences in planting equipment than anything to do with the soybean itself,” he says. “If you were in 7-inch rows with a drill, seed placement could be anywhere from nearly on top to deeper than desired. Extra seed basically compensated for variable seed placement.
“Today, when many people are planting with planters in 15-inch rows, seed placement is precise,” Casteel says. “There is no need to up rates in those cases.”
Air seeders also tend to do a good job of seed placement, Casteel notes. Seed placement also refers to distance between seeds within the row, in addition to vertical depth placement. Even today, he still recommends increasing seeding rate 10% to 20% if you’re using a typical no-till or conventional drill without modern planting units.
In years past, many recommendations also compensated for 90% germination and 90% emergence. Today, Casteel often still assumes 90% germination. However, he doesn't typically discount an additional amount for emergence. That’s because of the shift to planting soybeans with planters vs. drills, and improvement in equipment.
If you’re still drilling or planting into poor field conditions, you would need to account for reduced emergence as well as 90% germination, he notes.