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Ultra-Low Soybean Seeding Rates and Branching

Categories: PLANTING, SOYBEANS
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  • Branching differences across soybean varieties at ultra-low seeding rate trials were not enough to justify managing seeding rates differently.
  • Ultra-low seeding rates, which are seeding rates less than 100K seeds/A, did not provide any yield or economic benefits.
  • Seeding rate decisions should be based on achieving a final stand equal to or greater than 100K plants/A.

Introduction

Corn seeding rates are commonly adjusted for specific hybrids to maximize yield and returns. Conversely, soybeans, which flower and develop pods over a broader time frame than corn, are considered less responsive to increased seeding rates. However, soybeans are known to respond to increased seeding rates when planting is delayed, largely due to a shortened growing season which limits the number of nodes a plant can produce. Seeding rate responses are also observed in lower yielding environments where soybean growth is limited, making more plants/A advantageous.

The primary reason increased soybean seeding rates are not usually considered is because of the crop’s ability to compensate by increasing the number of pods per plant at lower populations. This occurs partly through branching, or creating additional stems, to produce more nodes and pods. The ability of a soybean plant to increase seeds per plant with lower populations has piqued the curiosity of many farmers to wonder how low you can go with seeding rates and if varieties respond differently. To study these questions, the Golden Harvest® Agronomy in Action team conducted ultra-low seeding rate trials in the 2021 growing season.

Figure 1. Branching and pod development differences when planted at 60K (left) and 140K (right) seeds/acre

Ultra-low Seeding Rate Trials

Ultra-low seeding rate trials were planted at 9 Midwestern locations to evaluate the response of 4 soybean varieties at 60K, 100K and 140K seeds/A (Figure 1). Varieties were chosen based on having uniquely different branching scores. Golden Harvest soybean varieties GH2788X and GH2610E3 each have a branching score of 4, meaning they have a lower tendency to branch at normal populations. Golden Harvest soybean varieties GH2523E3 and GH2505E3 have higher branching scores of 6 and 7 respectively, indicating that they tend to produce more branches per plant. At each location, emergence, branching differences and yield were collected.

Seeding Rate Influence on Branching

Reducing seeding rate consistently increased plant branching and number of pods per plant (Graph 1). There were differences in branching observed between varieties, although differences were less distinct than anticipated. Prior branching ratings for Golden Harvest soybean varieties GH2788X and GH2610E3 indicated they should branch similarly, however GH2788X branched less than its counterpart in trials. Likewise, Golden Harvest soybean varieties GH2523E3 and GH2610E3 branched similarly, although prior ratings predicted GH2610E3 should branch less. The relatively small and inconsistent differences between varieties observed indicate that branching rating differences may not be substantial enough to justify modifying management strategies.

Graph 1. Changes in variety branching due to seeding rate

Yield Response to Ultra-low Seeding Rates

Of the 9 trial locations, yield potential was decreased at 4 locations when seeding rates were reduced to 60K seeds/A and at 2 locations when reduced to 100K seeds/A (Graph 2). Averaged across trials, there was only a 4.3% and 0.7% yield loss for 60K and 100K seeding rates, respectively, when compared to 140K seeding rates. Varieties generally responded similarly to 60K seeding rates, and yield loss ranged from 1-6% across varieties. Golden Harvest soybean varieties GH3927LG was more sensitive to the ultra-low 60K seeding rate than all other varieties, reducing yield by 11% (Graph 3). The lower branching score of GH2610E3 Golden Harvest soybean variety suggests it would respond to higher seeding rates more than other varieties, but it maintained yield potential at lower populations better than all other varieties tested at the same locations (Graph 4). Almost no differences in variety or seeding rates were observed at early relative maturity locations (Graph 5). 

Graph 2. Individual location response to ultra-low seeding rates

Graph 3.

Graph 4.
 
Graph 5.
 

Economics of Reduced Seeding Rates

Lowering soybean seeding rates has the potential to decrease seed costs enough to offset lost yield potential. Assuming a seed cost of $60/unit (140K seeds/unit), seeding costs would be $25.71, $42.86 and $60.00/A for 60K, 100K and 140K seeding rates, respectively. Using these assumptions, in combination with the average yield of each seeding rate across trial locations, an economic cost analysis indicates that the 100K seeding rate has the potential to be $11/A more profitable than the 140K seeding rate (Table 1). Reducing seeding rates lower than 100K did not provide any yield or economic benefits in these trials.

Table 1. Economic comparison of ultra-low seeding rates based on 2021 trial yields

Soybean response to reduced seeding rates is highly dependent on achieving a minimum stand establishment at or near 100K plants/A. Trials conducted in 2021 had good emergence and seed spacing that was critical to achieving these results. It is interesting to note that under good planting conditions experienced in 2021, percent plant establishment rates improved from 91% at 140K to 97% at 60K, respectively. Although establishment was good in these trials, there will be less of a safeguard when emergence challenges occur and seeding rates are significantly reduced. The best seeding rate will be the one that takes all the following factors into consideration to achieve a minimum final stand of 100K plants/A.

Considerations of Ultra-low Seeding Rates

  • Soil productivity: Drought-prone or poorly drained soils that limit plant growth and development have limited ability to support soybeans to aggressively branch and compensate for lower populations. Maintain or increase seeding rates under these conditions.
  • Canopy closure: Reduced seeding rates will be more prone to delayed canopy closure, resulting in less effective weed control and soil moisture loss. There is also a potentially negative aesthetic component to having fewer plants in a field.
  • Stand establishment: Poor emergence caused by wet soils and soilborne disease may be more of a risk with reduced seeding rates and potentially result in more replanted fields. It is critical to protect seeds with a highly effective fungicide and insecticide seed treatment, such as Golden Harvest Preferred Seed Treatments.
  • Late planting dates: Delayed planting or double-crop fields have a reduced growth window and less ability to maximize branching and node development. Responses to increased seeding rate are commonly observed in these types of situations.
  • Early planting dates: Planting dates are being pushed earlier in many areas. Early planted soybeans are more prone to being in the soil for longer periods of time before emerging and have higher risk of injury due to cooler soil temperatures. Lower seeding rates will have less margin for lost seed before needing to replant. Ultra-low seeding rates are not well suited for these planting conditions.
  • Planter accuracy: As seeding rates are lowered, seed singulation and proper planting depth become much more important to ensure as many seeds as possible can germinate and emerge evenly. Closing wheel adjustment will help ensure good seed-to-soil contact.
  • Seed quality: Golden Harvest strives to deliver 90% soybean germination rates. However, seed quality can be challenging in hot and dry seed production years making it possible to see soybeans ranging from 80­-85% germination occasionally. It is important to calculate “live” seed rates when planting at lower rates.
  • Plant phenotype: Although branching scores had minimal impact on yield in our trials, it is important to select bushy plant type soybeans to help better close the canopy with lower seeding rates. 

To learn more about our 2021 soybean branching trial, watch this video.

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