As more producers plant varieties with resistance to soybean cyst nematode, it is important to correctly use varieties to maintain resistance over time.Because there usually are no visible symptoms, SCN is a silent yield robber; producers do not know populations have reached economically damaging levels until it is too late. SCN populations can reduce yields of susceptible varieties up to 80 percent, and commonly 30 to 40 percent, said Don Hershman, Extension plant pathologistwith the University of Kentucky College of Agriculture Research and Education Center in Princeton.Hershman said producers traditionally managed SCN using a three-part strategy of alternating resistant varieties, susceptible varieties and non-host crops. Producers started using more resistant varieties, and the seed industry developed a smaller number of susceptible varieties.Few susceptible varieties now are on the market."All resistant varieties are not equally effective in managing this insidious pest because they vary in many ways including SCN resistance levels and yield potential," he said. "So if you plant 10 different varieties with seed tags stating they have SCN resistance to one degree or another, you likely will have many different yield results."SCN exists virtually everywhere soybeans are grown in Kentucky," Hershman said. "The best way to manage populations is to rotate varieties that represent different sources of resistance, based on several years' variety performance and yield data from different locations. Research has shown that alternating different varieties, based on a different source of resistance, will delay shifts to other types as well as problems with resistance."The worst management practice is to continually plant the same resistant variety.  In four-year and six-year rotation studies, UK scientists found that continually planting the same resistant variety resulted in resistance-breaking SCN populations in just three years. The resistant populations occurred in year six when soybeans were rotated with corn.Hershman advised soybean producers to seek assurance from seed dealers that each variety has been thoroughly tested under a range of conditions, both in university and company trials.County Cooperative Extension offices have information on UK soybean variety trials. These data also are available at http://www.uky.edu/Ag/Agronomy/Extension."I am concerned that very few producers periodically are testing fields for SCN population changes over time," Hershman said. "Many just assume that populations are being manipulated as desired, when, in fact, SCN is not being effectively managed. To maximize yields, and profits, it is important for producers to sample all SCN-infested fields at least every four years. More frequent sampling may be necessary for specific management situations."Instructions for sampling fields and submitting samples for SCN analyses are available at local Cooperative Extension offices. The fee is $8.50 per sample.