Research Highlights

Finding the Best Soybean Seeding Rates for Optimum Yield in Maryland

By Carol Brown

Planting seeds and expecting them to grow is the nature of a farmer’s business. But do more soybean seeds in the ground translate to higher yields? That’s where Maryland Extension agriculture agent Kelly Nichols is focusing her research.

“The goal of this project is to see how soybean plant populations affect yield,” Nichols said. “Can farmers plant at lower populations and still achieve optimum yield? We’re looking at a range from 80,000 plants per acre up to 160,000. Most farmers in Maryland plant in a range around 140,000 plants per acre.”

Nichols’ research project, supported by the Maryland Soybean Board, began in 2019 and continues this cropping season, data will be coming in after harvest. Last year’s results showed there wasn’t really a yield difference between the population levels on full-season soybeans.

“At one site, results were a stairstep: as plant populations increased, yield slightly increased as well,” she said. “At another site, it was like a bell curve: at 80,000 plants per acre the yield went up; 120,000 had the highest yield and then as we increased to 160,000 plants per acre, the yields dropped.”

Based on Nichols’s study, there is an indication that soybean plant populations could be reduced without a yield penalty. Fewer soybean plants per acre reduces seed costs, saving farmers money. But Nichols said she’s not ready to recommend that farmers drop down to planting at the lowest seeding rates. There are other factors to consider, which she and her research team explored as well.

Alan Leslie, a fellow ag agent for Charles County, is studying the Dectes stem borer, an invasive soybean pest. Leslie measured the soybean stem diameters in each of the different plant population acres in Nichols’ plots. The larger soybean stems, which were in the lower plant population sections, may be more attractive to the Dectes stem borer, Nichols said.

“Because soybeans tend to compensate, the plants will get bushier if there’s not another plant close to them. The stems will get larger, providing more food for the Dectes stem borer,” Nichols said. “The data showed a significant trend that thicker stems had higher populations. If farmers are dealing with the Dectes stem borer, they may not want to lower their plant population, which will help deter the pest.”

Doing the math

Nichols did some number crunching of the 2019 data, considering the seed costs, which did not include any seed treatments or other discounts, and the value of harvested soybeans.

At the Thurmont farm location, the 100,000-planted population had the highest net per acre at $598.19. At the Tuscarora farm, the most profitable was the 120,000-seeding rate (Table 1). Yields ranged from 61-64 bu/acre at the Tuscarora farm and 67-70 bu/acre at the Thurmont farm.

For Maryland farmers, seeding rates could be altered to bring down input costs without making a large impact on yield. But farmers need to be aware of other issues in their fields to determine if they can, or should, reduce seeding rates.

Published: Dec 7, 2020