If you can hit 100,000 soybean plants/acre at harvest, you're well on your way to maximizing economic returns, says Palle Pedersen, Extension soybean agronomist at Iowa State University (ISU).
“When farmers plant high populations they reduce the productivity per plant,” Pedersen says. “The soybean plant is very sensitive to light interception and the photosynthetic rate. So if you have too many plants, each plant is going to run at reduced capacity, competing for light, and will produce less yield. If you have fewer plants they're going to intercept more light and are going to produce more yield.”
Taking this approach means using less seed and that means more money in your pocket, Pedersen adds.
“Beans have gone up to $60-70/bag,” Pedersen says. “We simply cannot afford to use the seeding rates that we used a decade ago.”
Shawn Conley, soybean specialist at the University of Wisconsin, agrees. “I'd like growers to know that what you plant is not as important as what you get up,” he says. “You can get more plant stands through better equipment, using a seed treatment and by taking your time and driving a little slower. There are lots of things that help with getting a more uniform stand.”
Farmers need to know the capabilities and age of their equipment and soil type to determine how many seeds it takes to result in at least 100,000 final plant stands/acre, Conley says. In his research plots, for example, he drops 150,000-165,000 seeds to ensure a base stand of 100,000 plants in 15-in.-row beans. Pedersen says he can often get 100,000 plants using 10-20% fewer seeds than Conley in Wisconsin, simply because of Iowa soil types and higher soil temperatures.
“Some farmers tell me they can drop 140,000 seeds with a 15-in. row and get 125,000 plants up,” Conley adds. It's all about knowing what works in your field and with your equipment to achieve success, he says, citing heavy clay fields that take 250,000 seeds/acre to get the required 100,000 soybean plants up.
Sheila Hebenstreit, grower and agronomist with West Central Cooperative in Ralston, IA, says she also thinks growers should have a minimum of 100,000 plants/acre. “It's not what you plant, it's what you end up with,” she says.
PEDERSEN SAYS THE changes in looking at seed to plant stand ratio began after the introduction of Roundup Ready soybeans. It was the beginning of a slew of changes in the industry, while also marking the end of soybeans selling for a mere $15-20/bag.
“There weren't good herbicide programs at the time,” Pedersen says. “So if you had higher plant populations, it helped out-compete the weeds and it didn't really have a negative impact on yield. But after 1996, when we first started using Roundup Ready soybeans, suddenly we had a herbicide program that was very efficient.”
How can growers figure out the seed rate they need in order to achieve 100,000 plant stands/acre?
Ease into the process, Pedersen says. “I don't want farmers to do it in every field. Do it in one field and see if you can get it to work. Not every farmer will be able to do it right and it varies from soil type to soil type. You have to manage weeds well, too. A low population is very sensitive to weeds compared to higher plant populations.
“When you're cutting seeding rates, stands are going to be closer and closer,” Pedersen says. “You don't want to cut it too much if your final population at harvest is below 100,000 plants per acre. Then there's a good chance you'll lose yield.”
Conley shares Pedersen's sentiments, pointing out the importance of growers doing their own farm research before taking on this new procedure.
“Take baby steps,” Conley says. “Growers should see what they need to do, based on their equipment, soil type, environment and go on that idea.”
And if growers decide to give it a go in one of their fields — start monitoring. “Either have a yield monitor or access a weigh wagon,” he says. “Determine what yields are in those plots.”