Not all soybean cyst nematode resistant soybean varieties are created equal

The soybean cyst nematode (SCN) continues to be a serious threat to soybean production in Iowa and the Midwest, occurring commonly throughout fields in all soybean-producing areas of the region. There are hundreds of SCN-resistant soybean varieties available to growers. The Iowa State University (ISU) SCN-resistant Soybean Variety Trial program evaluates many of these varieties for agronomic performance and for nematode control at numerous locations throughout Iowa each year. The results of the 2006 evaluations were released recently.

“In general, the SCN-resistant soybean varieties yielded 1.7 to 17.6 bushels per acre more than susceptible varieties in the trials in 2006,” said Greg Tylka, ISU research and extension nematologist who oversees the SCN-resistant soybean variety evaluations at ISU. “SCN population densities increased 2- to 15-fold on susceptible varieties but were kept in check or even decreased a little with the SCN-resistant varieties. But the results from our 2006 trials again show that not all SCN-resistant soybean varieties are the same - in terms of yield and also in terms of nematode control.”

The yield of SCN-resistant soybean varieties in fields without SCN does not indicate yield in SCN-infested fields, so ISU tests the varieties in SCN-infested fields. The presence of SCN is confirmed in each plot in all of the variety trial locations because the nematode can occur in patches and not uniformly throughout some infested fields. Soil samples also are collected from each variety trial plot at the end of the season to determine how well the resistant varieties controlled the nematode throughout the season. Results from previous years revealed that high-yielding SCN-resistant soybean varieties do not necessarily keep SCN from increasing in numbers.

“The yield and nematode control that occur in the field are the result of a season-long, complicated interaction of the resistance genes in the soybean varieties with genes in the SCN populations,” states Tylka. “Preventing increases in SCN population densities is necessary for continued profitable soybean production in Iowa because SCN is extremely long-lived in the soil. Once a field is infested with SCN, it is basically there forever, so we must manage SCN numbers as well as achieve high yields to ensure profitable soybean production for future years.”

Tylka also explains that combined data from multiple years and numerous locations often are used to select soybean varieties that have a high probability of yielding well in many different environments. “But SCN is not an environmental factor, like soil type or weather, that allows the data from SCN-infested research locations to be averaged with other locations, particularly non-infested locations,” says Tylka. “The individual location data reveal the amount of SCN reproduction that a resistant variety supports.”

Copies of the 2006 Iowa State University SCN-resistant Soybean Variety Trial results (ISU Extension Publication IPM 52) can be obtained at ISU Extension county offices or on the Web at www.isuscntrials.info or www.extension.iastate.edu/store. New in the 2006 report are color-coded bar graphs that complement the traditional data tables to illustrate, at a glance, the yield of the SCN-resistant soybean varieties as well as the extent of SCN reproduction on the varieties.

“Growers and agronomists are encouraged to look carefully at the data in the tables, including the statistical data,” says Tylka. But the color graphs give a first indication of the overall yield and nematode control of the numerous varieties evaluated at each of the nine variety trial sites. The ISU SCN-resistant Soybean Variety Trial program is funded, in part, by soybean checkoff dollars from the Iowa Soybean Association.