Effects of Fungicide and Seeding Rate on Soybean Productivity


  • A critical agronomic decision in soybean production is seed product selection and seeding rate. Although soybeans can compensate well at different seeding rates, soybean products can respond differently to seeding rates.

  • It is important to protect established soybean plants from stressors that can reduce yield potential. The application of a fungicide can help protect soybean plants from foliar diseases and increase overall plant health for increased grain yield potential.

  • Yield response from the application of a foliar fungicide can depend on seed product selection, as individual products respond differently to fungicide applications. 

  • The objective of this trial was to evaluate the impact of fungicide application and seeding rate on soybean productivity. 



LocationHuxley, IA  
Soil Type Clay loam  


Strip Till 

Planting Date5/11/20  
Harvest Date10/13/20 

Potential Yield
Seeding Rate

100K, 140K, 180K



  • Eighteen soybean products ranging from 1.8 to 3.5 maturity group (MG) were used for this trial.
  • Each product was planted at: 
    • 100,000 seeds/acre (100K)
    • 140,000 seeds/acre (140K) 
    • 180,000 seeds/acre (180K)
  • Soybeans were planted in 30-inch rows with two rows per treatment in 200-foot long plots. 
  • Delaro® Complete fungicide was foliar applied at a rate of 8 oz/acre at the R3 growth stage. 
  • Tillage and weed management were the same for all treatments. 



  • Averaged across the fungicide treatment, soybean yield response was variable across seeding rates and across the products tested (Figure 1).
    • The difference in average yield between the seeding rates for the 2.31, 2.41, and 2.81 MG soybean products was minimal, and average yields were highest at the 100K seeding rate for the 2.20 and 3.50 MG products.
    • Highest average yields were achieved at the 100K seeding rate with 11 of the 18 soybean products, 3 of the 18 products at the 140K seeding rate, and 4 of the 18 products at the 180K seeding rate.
    • Across all soybean products, average yields were highest at the 100K seeding rate and nearly the same at 140K and 180K seeding rates.
  • All soybean products tested had a positive yield response to fungicide application with exception of the 3.50 MG product (Figure 2). The earliest product tested (1.80 MG) had the highest average yield response of 15 bu/acre and across all soybean products test, the fungicide application had an average yield response of 9 bu/acre (Figure 3).
  • The fungicide application increased average yields at all seeding rates tested (Figure 4).
  • There was not a disease outbreak at the trial site. However, there were minor incidences of sudden death syndrome (SDS), frogeye leaf spot, Septoria brown spot, and Cercospora leaf blight in both the sprayed and unsprayed plots. The test site was also in the path of the derecho that hit Iowa in August of 2020. 
image Figure 1a.
image Figure 1b.


Figure 1.  Average yield responses of 18 soybean products to three different seeding rates. Data represents yields averaged across fungicide applications. Average represents the average yield of all products at that seeding rate. 

image Figure 2a.
image Figure 2b.


Figure 2. Effects of fungicide application on the yields of 18 soybean products. Data represents yields averaged across seeding rates.

image Figure 3. Effects of fungicide application on the yields of 18 soybean products. Data represents the yield difference between fungicide sprayed and unsprayed treatments averaged across seeding rates. Average represents the average yield difference of all the products.
image Figure 4. Summary of soybean yield response to fungicide application at three different seeding rates. Data represents yields averaged across 18 soybean products. Average represents the average yield of all products for that treatment.


  • An average yield increase of 2 to 3 bu/acre is required to justify a 40K seeds/acre seeding rate increase in the current soybean commodity market of around $10/bushel. Under this scenario, 17 out of the 18 products tested were most profitable at the 100K seeding rate. Only one product was most profitable at the 140K seeding rate, and no product was most profitable at the 180K seeding rate. 

  • With the current soybean grain price around $10/bushel, a 2 to 3 bu/acre yield increase is needed to cover the cost of a fungicide application. Therefore, a fungicide application was profitable in all but one of the products evaluated in this testing. The 3.50 MG product did not respond to the fungicide application because it was a late maturing product that did not reach physiological maturity before a frost affected its yield potential. 

  • Crop yield response to production inputs can be highly variable and impacted by environmental conditions during the growing season. Farmers are advised to consult with their trusted crop advisors when making production input decisions. 


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