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Over the past few years, the Illinois Soybean Association has been working with the Iowa On-Farm Network (OFN) to evaluate promising production practices. The two most common concepts tested were population and row spacing, which are easy to implement in an on-farm scenario. One of the requirements for participating is collecting as-planted and yield maps. Along the way data is collected on plant stand after emergence and an aerial image of the trial site is captured during fill in August.

In one 2017 trial in Whiteside County a grower compared seeding rates of 100,000 and 160,000 using strip trials and four replications. Soybeans were planted on May 20 and harvested on October 18. 2017 Soy Envoy Stephanie Porter assisted with the project, as the cooperator is a Burrus® customer. 

Researchers have been studying soybean population for many years. It is probably fair to say that over the last two decades planting populations have dropped drastically across Illinois and the Corn Belt from 180,000 – 200,000 to 140,000 – 160,000. And with early planting and seed treatments, rates can drop to 120,000.

The other key discovery is that soybean yield is generally maximized at harvest at 100,000 plants per acre when plants are evenly distributed across the field. The key trick is planting enough seed to end up with 100,000+ plants at harvest. 

In the Whiteside County study, the grower planted 100,000 and 160,000 in 30-inch rows. On June 16, I visited the trial site and did three plant counts in 36-inch lengths for each treatment and replication. The average stand densities were 93,047 and 132,260 plants per acre. It is interesting to note that there was significantly greater average seed or seedling loss at the 160,000 population (27,740 seeds/A) compared to the 100,000 population (6,953 seeds/A).

But it all comes down to final yield and the analysis shows no significant difference in yield, with yield across both treatments averaging 60 bu/A. And when you look at the data across replications, sometimes yield was greater at 100,000 and sometimes greater at 160,000. As an OFN analyst wrote, “A randomization test suggested no evidence of a significant yield difference.” 

There were no yield differences, but there were big seed savings. At a treated seed cost of $73 per seed unit (140,000 seeds), it cost $52.14 and $83.43 per acre to plant 100,000 and 160,000 seeds, respectively, a savings of $31.29 per acre at the 100,000 rate. No change in yield, but a big increase in profitability per acre by dropping seeding rate.

These findings shouldn’t be a real surprise. We are seeing more support all the time for the idea that maximizing soybean yield doesn’t require as big a population as we once thought. With better genetics, inherent vigor, early planting and seed treatments, planting less seed should be our goal.

Figure 1. Image courtesy of Iowa On-Farm Network

Soybean agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.