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Planning Soybean on Soybean?

With corn grain prices being down, some producers may be considering planting soybeans on soybeans because of economic reasons. Soybean does not require heavy inputs such as fertilizer and soybean grain prices have not taken such a big hit as corn. Should planting soybeans into a field that was under soybeans last year be the decision, producers need to be aware of disease risks and should plan ahead to minimize negative impacts of these diseases on soybean yield.

The majority of soybean diseases in South Dakota are either residue-borne or pathogens survive in the soil. This means that planting soybeans in a non-rotated field may increase the risk of these diseases to develop. If a soybean field has had a history of moderate to severe disease development, careful cultivar selection would be highly recommended. While resistance to many soybean diseases may not yet be available, seed companies do provide disease ratings for soybean cultivars for several pathogens including Phytopthora root rot, white mold, brown stem rot, sudden death syndrome, and soybean cyst nematode (SCN).

One particular soybean production constraint that needs to be assessed carefully before planting soybeans following soybeans is SCN. In non-rotated fields with a history of SCN this nematode problem can increase to reach damaging levels. Producers are encouraged to test their soils before planting soybeans on soybeans. Testing for SCN is free of charge courtesy of the South Dakota Soybean Research and Promotion Council and there is still time to test soils before soybean planting this year. Soil sampling for SCN can be done anytime provided the soil is not frozen or too wet. Knowing the status of SCN in a field will help in deciding the need for SCN-resistant cultivars. For fields with a history of SCN, it is important to keep testing soils to monitor SCN build up in the field. If SCN numbers keep on rising, this would mean SCN intervention methods being applied are not working. Therefore longer rotations out of soybeans and change in SCN resistant cultivars would be recommended.

Seed treatment may be another disease management strategy that producers planning soybeans on soybeans may want to consider especially for fields with a history of soybean stand establishment problems. Although pathogens causing seedling diseases in soybeans can survive in soil for several years, soybeans following soybeans increase the level of inoculum in the soil.

For foliar diseases where resistance is not available (e.g. brown spot, frogeye leaf spot, downy mildew) early scouting for disease pressure and applying a timely fungicide application will protect yield. Research done at SDSU and other universities in the region indicate a higher probability of return on investment when foliar fungicide application is done under significant disease pressure before or at R3 growth stage coupled with relatively wet weather conditions.

In making crop rotation decisions, producers may want to consider long term profitability of a crop rather than focusing on one season’s profit. Decisions made in one season may affect the productivity and therefore profitability of the crop in multiple upcoming seasons.

Source: Emmanuel Byamukama, South Dakota State University 

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