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1/11/2010 - 1/17/2010

Using Yield Trial Data to Make Variety Selections

By Jim Rouse, Department of Agronomy

Each year I receive several questions about how best to use yield trial data to make hybrid and variety selection decisions. This article will cover the most common issues that are discussed.

As always, variety selection is about much more than just yield. Growers also need to evaluate the various combinations of maturities, defensive traits and herbicide traits that are important to them. Even with all that, there is still a high priority on yield potential and it’s easy to see why. The rest of the selection process is relatively easy—the candidates for selection either possess the desired trait or they don’t. They are either within the desired maturity range or they’re not. But among those that meet your desired criteria, how do you choose those with the greatest yield potential?

Use proper data reports
Remember, variety selection is not about identifying which lines did best over the past year—it is about predicting which lines will do best in the future. This is not dependent upon how you use data reports. Instead, it depends on the proper selection of data reports to use in the first place.

Predictive information for yield potential should come only from multi-environment trial averages. If your favorite data report does not include district or regional yield averages, you should not use it to make selection decisions.

Why are multi-location averages more predictive? Consider this: The data from a single location is a measure of the yields produced by the interactions of the varieties (genetics) with the environment (everything else). In these experiments, the environment is comprised of soil type(s), soil conditions, weather, nutrients, pests, pathogens, and any other factor that can impact the expression of genetic yield potential during that season. But the only factors that you can know for next season will be the soil type(s) where you plant and the varieties you choose. Because of this, you cannot expect the results from a single-location trial in one season to be duplicated in another season.

Be aware that varieties will perform differently at different locations, even when steps are taken to choose similar environments. In most yield trials researchers attempt to test in as many different environments as possible. If these data are not averaged across locations, how then does one evaluate the results?

Many people ignore this and continue to use other criteria to choose a single location from which to select. These criteria include, but are not limited to, the location that:

• Is closest to your farm
• Had the same heat units you had
• Had the same crop rotation you use
• Had the same tillage method you use
• Had the soil type most similar to yours

Remember that all of these criteria will interact in various unknown and unpredictable ways to impact the final data measurements in each field. Thus, for these results to be predictive, your field next year must experience conditions essentially identical to the yield trial field where the data were collected.

Since it is highly unlikely that next season’s conditions will be the same as those in any single-location report, you will increase your probability of success by selecting a variety that can perform well in many environments. And you can identify these varieties only in test reports that display averages over locations.

Understanding the data
The most important aspect of reviewing data involves understanding the data that are provided. Use information like the least significant difference (LSD) to help you sort entries. Any entries that differ by less than the reported LSD for a trait (i.e. yield, maturity, disease rating, and pest resistance) should be considered equal for that trait. Measurements within a LSD could be due to a number of different factors, including measurement error or random chance. These differences are not considered to be significant and are not likely repeatable in your field.

Do not rely on summary tables or diagrams to determine if one variety is better or worse than another—look at the data. All data provided without LSD values should be considered unreliable and should not be used to make variety decisions. This point cannot be overstated: using test results without the accompanying statistics will lead to conclusions that are not supported by the test results.

Using the data
Now that you know how to evaluate reports, the next step is to sort through the data to make your selections. Variety selection is composed of two distinct but related components. The first is selecting high-yielding varieties for your operation. The second is risk management, as defined by the number of varieties you select, their mix of maturities, defensive traits, seed treatments, and their acreage allocation.

Even though the risk management aspect of variety selection can instill some variability in methodology, there are certain characteristics that should remain consistent among all users of yield trial data:

1) Only multiple-location data should be used to make predictive selection decisions.
2) Yield trials do not have to be performed on your farm, on your soil type, or even under your crop rotation scheme to provide relevant data.
3) Sort the data by yield. Make initial selections based on yield and appropriate maturity.
4) Once you have a pool of candidates, sort among these to identify lines that have the desired mix of defensive traits.
5) More information is better information, so use all reliable sources of data.

Because variety selection is a multi-step process the most effective approach will incorporate several sources of information. At Iowa State University, the most comprehensive source of information for corn and soybean yields and several defensive traits can be found at Iowa Crop Performance Testing at

Jim Rouse is a program manager with research and extension responsibilities in corn hybrid and soybean variety testing, and the Executive Director of Iowa Crop Improvement Association. He can be reached at or by calling (515) 294-5604.

New Iowa Performance Information Available on SCN-Resistant Soybean Varieties

By Greg Tylka, Department of Plant Pathology

The soybean cyst nematode (SCN) is a serious yield-limiting pest of soybeans in Iowa and the Midwest. SCN-resistant soybean varieties are critical for managing SCN. There are hundreds of soybean varieties available to Iowa soybean growers that are marketed as being resistant to SCN (see Soybean cyst nematode-resistant soybean varieties for Iowa – PM 1649).

The Iowa State University SCN-resistant Soybean Variety Trial program has been evaluating the yield and SCN control offered by SCN-resistant soybean varieties for 15 years. The work is supported by fees paid by seed companies entering varieties in the experiments and also by soybean checkoff funds from the Iowa Soybean Association. The program conducts field-plot testing of SCN-resistant varieties at numerous locations throughout Iowa. Every plot is tested for the presence of SCN in the spring, and SCN population densities are measured from soil samples collected from every plot in the fall to assess how SCN population densities were affected through the growing season by the different varieties. Both yield and SCN control must be considered when evaluating SCN-resistant varieties because high-yielding SCN-resistant varieties don’t always control SCN population densities well and it is very difficult to reduce SCN numbers in a field once they develop to high levels (see ICM News article So Many SCN-Resistant Varieties: Which Should You Use?).

The ISU SCN-resistant Soybean Variety Trial program results for 2009 were finalized recently.  The results currently are available online at A print copy of the report can be obtained at no charge by contacting Carla Harris, ISU Department of Plant Pathology, at (515) 294-1741.

Experiments were conducted at nine locations throughout Iowa in 2009 (see map). Thirty-eight Roundup Ready® SCN-resistant varieties were evaluated at each northern Iowa location, 24 Roundup Ready® SCN-resistant varieties were assessed at the three locations in central Iowa, and 23 Roundup Ready® SCN-resistant varieties were evaluated in the three southern Iowa locations. Also, at the central and southern Iowa locations, several SCN-resistant soybean varieties that are not Roundup Ready® were evaluated in experiments located adjacent to the experiments in which Roundup Ready® SCN-resistant varieties were evaluated. The non-Roundup Ready® soybean varieties included a few LibertyLink® varieties and a few soybean varieties not resistant to any herbicide.

Following is a summary of observations about results from the 2009 ISU SCN-resistant Soybean Variety Trial experiments. The summary statements pertain only to the Roundup Ready® SCN-resistant varieties, which comprise the bulk of the varieties evaluated.

• Initial SCN population densities or numbers at the various experimental locations were relatively low (below 1,500 eggs per 100 cc soil) except at Sutherland (NW Iowa), which had 3,155 eggs per 100 cc soil at planting. It is ideal to have an average initial SCN population density of more than 3,000 eggs per 100 cc soil at each variety trial location.
• The SCN populations in the fields at five of the nine experimental locations had greater than 10 percent reproduction on the PI 88788 source of resistance; the SCN populations were found to be HG type 2 or 2.5.7 or 2.7 (the number “2” in the HG type designation indicates >10 percent reproduction on PI 88788, which is HG type indicator line #2).  The SCN populations in the other four fields had less than 10 percent reproduction on PI 88788.
• Yields of the SCN-resistant soybean varieties were the best (above 60 bushels per acre for many of the top-yielding resistant varieties) at the three southern Iowa locations – Malvern, Oskaloosa, and Fruitland.
• The top-yielding SCN-resistant varieties yielded 55 to 59 bushels per acre in the three northern and three central Iowa district locations.
• The central Iowa experiment at Nevada had a fair bit of sudden death syndrome and the disease likely affected yields of the varieties. Also, SCN reproduction was relatively high on all SCN-resistant varieties at the Nevada location.
• The Oskaloosa location (south central Iowa location) had too many plots with initial SCN population densities of 0 to compare SCN reproduction on or yields of the different varieties. It is not known why the SCN population was particularly aggregated or patchy in this field.
• Despite relatively low initial SCN numbers and a cool, wet growing season that doesn’t typically lead to great damage from SCN:

- yields of SCN-resistant varieties were generally greater that yields of the widely-grown susceptible varieties at several of the locations, and  
- yields of the highest-yielding SCN-resistant soybean varieties were greater (although often not significantly greater) than the yields of the top-yielding susceptible variety in all but two of the nine locations.

• With the exception of the Nevada location, the end-of-season SCN egg population densities on SCN-resistant varieties were nearly always less than the SCN numbers on susceptible varieties, even in locations where SCN-resistant varieties did not yield greater than susceptible varieties (like at Farnhamville, Urbana, and Malvern, for examples).

These results illustrate that there is wide variation in the yield and SCN control provided by SCN-resistant soybean varieties and that SCN-resistant varieties can provide good soybean yields and SCN control (relative to susceptible varieties, in particular) even when SCN is not very damaging due to low population densities, cool temperatures, and excess rainfall.

trial location map

Locations of the 2009 ISU SCN-resistant Soybean Variety Trial Program experiments.



Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes. Tylka can be contacted at or by calling (515) 294-3021.

So Many SCN-Resistant Varieties: Which Should You Use?

By Greg Tylka, Department of Plant Pathology

Soybean varieties that are resistant to the soybean cyst nematode (SCN) are a critical management tool for the pest. In general, SCN-resistant varieties produce greater yields and result in lower SCN numbers at the end of the season than non-resistant (susceptible) varieties.

Resistance to SCN is conferred by several genes that are transferred into soybean varieties from breeding lines with names like Peking, PI 88788, and Hartwig. There are hundreds of SCN-resistant soybean varieties available to Iowa growers (see figure below and the publication, Soybean cyst nematode-resistant soybean varieties for Iowa – PM 1649, for a listing of individual varieties).

Not all SCN-resistant varieties yield equally well in SCN-infested fields, nor do they suppress SCN reproduction to the same extent. SCN resistance is conferred by several genes. Soybean varieties bred from a single resistance source, like PI 88788, do not necessarily possess all of the SCN resistance genes that are in PI 88788. Also, the SCN resistance genes from the different breeding lines vary in effectiveness in controlling the different SCN populations that infest Iowa fields. And no doubt, yield and SCN control provided by the SCN-resistant soybean varieties can vary in response to many other biotic and abiotic factors in field environments.

There's discussion about how to select SCN-resistant varieties. Should you consider SCN reproduction data from the field as well as yield data? Should you look for single-site yield data from locations near you, yield data averaged from across the state, or yield data averaged across multiple years? And, should you consider yield data from small plots or large strip trials or both?

One way to go about selecting high-yielding SCN-resistant soybean varieties that keep SCN numbers in check is to look for data from as many different reliable sources as possible, including university variety trials and strip trials conducted by co-ops, grain elevators, and seed companies. High-yielding varieties don’t always control SCN population densities the best, so pay attention to information about SCN reproduction in the field as well as yield. It is very difficult to reduce SCN numbers in a field once they develop to high levels, so it is important to consider how well SCN-resistant varieties control SCN numbers in order to maintain the productivity of fields for soybean production for years to come.

Iowa State University conducts SCN-resistant variety evaluation experiments throughout the state that measure both yield and control of the nematode. The work is supported by fees paid by the seed companies and also by soybean checkoff funds from the Iowa Soybean Association. Results of the experiments are published online and also in print. Print copies of the results are available by contacting Carla Harris in the Iowa State University Department of Plant Pathology at (515) 294-1741.

No matter what sources of information you consider when picking SCN-resistant varieties, be sure to look for SCN-resistant varieties that yield consistently well in numerous SCN-infested fields (yield data from noninfested fields are not useful). Also look for varieties that consistently decrease SCN population densities or keep the SCN numbers in check in multiple fields. Growing resistant varieties with these characteristics should ensure that soybeans can be grown profitably in SCN-infested fields for many years to come.

scn resist varieties

Number of SCN-resistant soybean varieties available to Iowa growers 1991 – 2007.


Greg Tylka is a professor of plant pathology with extension and research responsibilities in management of plant-parasitic nematodes. Tylka can be contacted at or by calling (515) 294-3021.

This article was published originally on 1/18/2010 The information contained within the article may or may not be up to date depending on when you are accessing the information.

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