April 25, 2008

Trials Show Value of Resistant Varieties

In 2007, soybean cyst nematodes (SCN) lowered U.S. soybean yields by an estimated 125 million bushels. With today's prices, that means soybean producers lost $1.5 to $2 billion in production.

If you raise soybeans and haven't tested for SCN or taken steps to manage it, you may be contributing to these losses. While soybean rust has attracted more attention in recent years, SCN causes more yield losses in the U.S.

Since 2005 the Nebraska Soybean Board has sponsored a program to help identify the distribution and frequency of SCN in Nebraska. It provides sample bags to conduct a free SCN analysis (a $20 value). These bags have been distributed through UNL Extension offices, extension programs and by crop consultants.

With increased testing, SCN has been identified in 18 new counties which produce over one-fourth of Nebraska's soybeans. In 2007, SCN was detected for the first time in Holt, Knox and Wheeler counties and as far west as Red Willow County near McCook. Now SCN has been identified in 45 counties which represent over three-fourths of Nebraska's soybean production. Over the project's three years, 25%-30% of the soil samples have tested positive for SCN. Since it is difficult to detect, in many cases farmers were not aware that SCN was present . Yield losses up to 30% can occur without any above-ground visual symptoms. Frequently, the first sign of an SCN infestation is when soybean yields level off or start dropping and other factors, such as soil type, weather, insects, disease, or herbicide injury aren't indicated.

SCN samples can be taken throughout the year; however, two optimum times are during the growing season if soybeans aren't looking healthy or right after harvest when you're aware of areas where yields weren't up to expectations. If you're sampling for soil fertility, consider taking a larger soil sample and submit half for fertility analysis and half for SCN analysis.

Soil Samples

To sample for SCN, pull a minimum of 10-15 cores from an area. Sample bags and instructions for submitting them are available from your local extension office.

SCN-Resistant Varieties Pay Off

Between 1999 and 2004, 12 research trials comparing susceptible and resistant varieties were conducted on SCN-infested sites in eastern Nebraska. The same varieties also were planted on six non-infested sites.

Resistant varieties outyielded susceptible varieties by an average of 5 bu/ac on the infested sites. On the noninfested sites, there was less than a bushel difference between the resistant and susceptible varieties. This shows resistant varieties yield significantly better than susceptible varieties in fields where SCN is present and just as well as susceptible varieties in the absence of SCN.

In these same trials, SCN egg counts were taken in the spring as the crop emerged and in the fall after harvest. Resistant varieties reduced SCN reproduction by an average of 20% while susceptible varieties allowed an average 300% egg increase during one growing season. This could have a huge impact on losses from SCN the next time soybeans are planted in the field.

The best news is that SCN-resistant varieties do not cost any more than susceptible varieties and there are no tech fees. For more information on SCN or to get your free sample bag, contact your local UNL Extension office.

John Wilson
Extension educator, Burt County
Loren Giesler
Extension Plant Pathologist