Ames, Iowa, USA
November 10, 2023
The soybean cyst nematode (SCN) is thought to cost soybean producers $1.5 billion annually in North America. A new online resource from the SCN Coalition uses data collected from Iowa State University research to estimate the cost of yield reductions from SCN in individual fields.
What is the online yield loss estimate calculator and how does it work?
The calculator, named the SCN Profit Checker, is available at scnprofitchecker.com and allows users to provide specific information about individual SCN-infested fields to gauge the possible economic cost of yield loss caused by the nematode. The tool is based on data from >35,000 ISU SCN-resistant soybean variety trial research plots in 180 studies conducted from 2001-2020 throughout all nine crop-reporting districts in Iowa. There was a wide range of SCN population densities in the plots, from very low to very high, and the experiments were conducted in fields rented from farmers in locations which had varying soil and local weather conditions. The research was funded primarily by the Iowa Soybean Association.
Details about the calculator
The calculator can be used for fields known to be infested with SCN and that have been sampled for SCN. Also, the calculator is meant to be used with fields where soybean varieties with PI 88788 SCN resistance have been grown.
The user provides the following about the SCN-infested field:
- the SCN egg population density (eggs per 100 cc of soil),
- the female index of the SCN population in the field on PI 88788 resistance (a percent),
- the percent sand content of the soil in the field, and
- the pH of the soil in the field.
The calculator asks only for the information above because these were the only factors that significantly affected yield in the data that serve as the basis of the calculator. If more than one SCN soil sample was collected from a field to determine SCN egg population densities, the average of the sample results should be entered into the calculator.
The calculator estimates relative yield reduction expressed as a percentage, which can be applied to soybeans grown in any SCN-infested field. Percent yield loss is converted into estimated economic yield loss based on the farmer’s yield expectation for the field and marketing sale price goal (see figure below).
Although the calculator is built from research results in Iowa, the percent yield loss can be used for a range of geographies. And users should be aware that the effects of weather, aspects of the specific soybean varieties grown elsewhere, local soil conditions, and other factors also may have significant effects on yield loss caused by SCN in an individual field. This is why the output from the calculator should be considered only an estimate of possible yield loss.
Figure: The results screen of the SCN Profit Checker reveal an estimated 19.1% yield loss, which represents $173.81 per acre loss based on a 70 bu./ac. yield expectation and $13.00 per bushel marketing price.
There are a few situations in which the calculator should not be used, namely if the SCN egg population density is 0 and if the soybean variety grown this season was susceptible or possessed Peking SCN resistance. Also, entering extreme values in the calculator, such as 80,000 eggs per 100 cc of soil, 100% sand, or pH of 10.0, likely will produce unreasonable yield loss estimates.
How is the female index determined and what if the female index is not known?
The female index of an SCN population on PI 88788 is determined by an HG type test. The index indicates how well an SCN population is controlled by the PI 88788 source of resistance that is commonly used to breed soybean varieties. Facilities that conduct HG type tests are listed in an article from the SCN Coalition here.
When soybean varieties with SCN resistance from PI 88788 initially are used, the SCN populations are controlled well and female indices typically will be between 0 and 10%, indicating 90 to 100% control. In situations where soybean varieties with PI 88788 SCN resistance have been grown repeatedly for numerous years (such as in Iowa), SCN populations evolve to develop high reproduction on PI 88788 resistance and female indices are 50% or greater, indicating that PI 88788 resistance offers reduced control of the nematode. The reduced effectiveness of resistance leads to greater SCN reproduction, increased egg population densities, and greater yield loss.
Once a user selects the state in which the field of interest is located, the calculator automatically provides a default female index for PI 88788 that was provided by a university SCN expert in that state. This allows farmers who do not not know the female indices of the SCN populations in their specific fields to use the resource. The default female index value is an educated guess that may be based on limited data. For Iowa, the default female index is 45, which is the midpoint of the range of female indices currently known for SCN populations in the state. The estimated yield loss of a field determined by the calculator will be most accurate if actual female index values are known and used.
Why was the calculator created?
The calculator was created by the SCN Coalition to illustrate to farmers and those who advise them the economic benefit from actively managing SCN. “Active” SCN management involves soil-sampling fields regularly to monitor SCN numbers, growing soybean varieties with differing SCN resistance genes (see ISU Extension publication CROPS 1649 here), growing soybeans in rotation with nonhost crops such as corn, and experimenting with nematode-protectant seed treatments when soybean are grown to see which provide a yield boost.
Getting the most from the calculator
Meaningful soil samples must be collected to assess SCN population densities in a field to get the most useful estimate of SCN yield loss from the SCN Profit Checker. A recent ICM News article here provides guidelines for collecting soils samples for SCN.
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