Conserve Seed, Push Soybean Yield

Conserve Seed, Push Soybean Yield

Our Test Plots show benefits to using variable-rate technology

The concept of variable-rate technology (VRT) in corn is no stranger to fields across the Midwest. Farmers continue to reap the agronomic and economic benefits of VRT in corn. Although using the technology in soybeans is not as common, some new planters are equipped to vary seeding rates based on management zones. 

In the final year of a two-year Farm Journal Test Plots study, Farm Journal Associate Field Agronomist Missy Bauer and crew evaluated VRT soybean populations as a tool to better manage seed costs. 

The foundation of the work lies in the theory that highly productive soils might produce adequate yields with lower plant populations and medium to poor soils might need higher plant populations to optimize yield. 

“Our goal for VRT is to gain confidence on where to dial back population when we know we improved the plant’s yield components,” Bauer says. “This lays the foundation to conserve seed and push yields.”

The first step to achieve the concept of VRT populations is to understand how yield components can be 
influenced, Bauer says. Plant density affects how plants branch, set nodes and pods and fill pods. Thinner densities have the potential to hold more nodes per plant and pods per node. Plant density also impacts canopy development, weed strategies and the risk of disease. 

Like all Farm Journal Test Plots, yields from these tests in southern Michigan were captured with a calibrated yield monitor before being validated by a weigh wagon with scales. 

Good management zones are the foundation for variable-rate seeding. “The management zones were created by in-field evaluation of changing soil type and elevation; aerial imagery; and yields,” Bauer says. “The goal of each management zone is to manage the variability of the field.” 

The effort included three plot fields each year for a total of six locations, each with different soil types. Each field was divided in three to five zones based on soil variability. 

The six plot fields included five treatments—two methods of variable-rate seeding and three uniform rates, including low, normal and high seeding rates. The study used a randomized block design, with five treatments and three replications in the field-sized plots.  

The five treatments included: 

  • VRT 1: Increase populations in poor-producing management zones and decrease populations in highly productive zones.
  • VRT 2: Similar to VRT 1 but with a bigger range in populations.
  • Three uniform rate populations: 125,000, 170,000 and 215,000.
The samples, pictured above, show the average pods per plant pulled from three different management zones all within the same field. The table above supports the photo showing you can pull back populations and maintain soybean yield components, specifically beans per plant and three-bean pods.  

For example, in one location in 2014, VRT 2 included five populations—90,000, 115,000, 150,000, 180,000 and 200,000—across the management zones. 

Stand count data was collected shortly after emergence to determine planting rates and populations in the field. The low populations tended to be slightly overseeded while the high populations were often slightly underseeded. It was a challenge to nail the specific populations on the head. 

The standard deviation for stand counts was about 17,000 plants per acre. This suggests it might be difficult 
to fine-tune seeding rates by less than 15,000 or 20,000, Bauer adds. 

To further monitor yield components, Bauer took pod and bean counts prior to harvest. Lower plant populations resulted in more nodes per plant and more nodes with pods in comparison to higher populations—specifically, there was an increase in the number of three-bean pods. For example, in 2014 in the first plot location at 125,000 to 170,000 populations, the number of pods per plant dropped from 42.9 to 34.4. At lower densities, plants also had more beans per plant. In the same plot, from 125,000 to 170,000 populations, the number of beans per plant dropped from 94.1 to 75.1. The number of one- and two-bean pods was comparable across populations. 

“The pod data gives us the confidence that seed rates can be lowered in some management zones as plants compensate for the decrease in population by adding three-bean pods,” Bauer says. “Bean size was not influenced by plant population.”

Across two locations in 2014, the VRT 2 plot with the widest range in populations resulted in higher yields compared with the uniform seeding rates. For example in 2014, at the first and third plot site, the VRT 2 prescription yielded 1.4 bu. higher than the normal uniform rate. 

“Looking at the uniform seeding rates, the third plot leaned more toward the higher populations,” Bauer explains. “We only received 87% of target population—it wasn’t quite the emergence we wanted due to cold and wet conditions.”

In the uniform seeding rates plots, the optimum population to maximize yields ranged from 125,000 to 215,000 and varied based on plot location. In 2013, all three sites resulted in one of the VRT prescriptions being the best agronomic performer. For example, at the second site, the VRT 2 prescription yielded 5.8 bu. higher than the highest uniform rate. 

Averaging both years, VRT 2 net $30.40 per acre more than the traditional seeding rate of 170,000. The second highest economic performer was VRT 1 with a $19.45 per acre advantage. When it comes to the straight rates, although the highest yield varied by location, the lowest seeding rate of 125,000 was economically better than the normal (by $14.34) or high uniform rate (by $38.46). This indicates population rates in general probably need to be reduced in 15" rows. 

In times of tight margins, VRT can reduce overall seed cost without sacrificing yield. “Overall, we found the biggest gains in the better ground where we are able to maintain yield components and pull back populations,” Bauer adds. 

The key takeaways to remember:

  • Soybeans adjust to the environment and sunlight more than agronomic management practices. 
  • As shown in the yield components table at left, when populations decrease you see soybean plants shift or adjust due to the amount of sunlight captured. Once 97% of sunlight is captured, you can’t change yield. 
  • In lighter soils, increase populations to close the rows quicker and conserve water. In heavy soils, pull back populations so plants receive more sunlight.
  • The serious money to be made is in seed costs, not yield boost.


Thank You to Our Test Plot Partners

Case IH, Bill Hoeg, Luke Gazaway and CJ Parker; Unverferth Manufacturing and Jerry Ecklund; Wells Equipment; Michigan Soybean Promotion Committee; AirScout and Brian Sutton; GeoVantage; Trimble, Sid Siefkin and Brian Stark; OmniStar and John Pointon; Massey Ferguson, Conor Bergin, David Webster and Lindsey Pettyjohn; Williams Farm Machinery and Dave Gloor; New Holland, Mark Hooper, Gary Wojcik and Paul Canavan; Burnips Equipment and Carl VanderKolk; LDK Farms and Leon Knirk; Scott Simington; B&M Crop Consulting, Bill Bauer, Amanda Anderson, Jared Haylett and Lauren Mezo


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