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Soybean acreage is likely to increase in 2018 because of economic factors. As a result, 2017 soybean fields may be planted again with a soybean crop. Though not a recommended practice, repeating a soybean crop can be successful if agronomic concepts are followed.
Soybean fertility is an important consideration. Reasonably high amounts of potassium, phosphorus, zinc, and sulfur are used by soybean plants during the growing season. A soil test to help determine available nutrients should be a consideration. Potassium helps reduce the potential for certain diseases and potassium-deficient plants are generally more attractive to soybean aphids.1 During the growing season, foliar testing can help determine if deficiencies are present.
Product selection is a key element of continuous soybean production. Products should be selected based on yield potential, maturity, and resistance to diseases. Product rotation is a recommended practice to help reduce the potential for disease development. Disease inoculum can build up on crop residue and in the soil during each growing season when in the constant presence of a host crop.
Pests such as white mold, sudden death syndrome, brown stem rot, certain root rots, and soybean cyst nematode (SCN) can remain in the soil or in/on crop residue for several years after a host plant is removed. Therefore, it is of great importance to evaluate and select products with inherent disease and nematode resistance to help protect against these pests. Seed treatments such as Acceleron® Seed Applied Solutions can help minimize disease, insect, and nematode infestations.
If a soybean field was infected with white mold last year, considerations should include the selection of seed products with the most white mold tolerance and a range of maturities (early maturity products may have less infection), reduction of planting rates, use of wider rows, and proper use of foliar fungicides.1 Soybean cyst nematode resistant seed products should be used to help reduce the potential for SCN infection. Seed treatments for SCN protection are also available.
To help reduce the potential for pythium and phytophthora root rot (PRR), seed treatments should be utilized and in the case of PRR, resistant seed products are available. Planting after soil temperatures reach 60 °F can help reduce the potential for problems with pythium.1
Foliar fungicides should also be considered to help protect plants from frogeye leaf spot, cercospora leaf blight, brown spot, and soybean rust. Scouting for signs of initial disease development is a fundamental step for the successful use of foliar fungicides.
Stable yields from nearly 20 years of continuous soybean production in the same field have been reported.2 Successes like these tend to occur in areas where soils are less prone to moisture retention and where disease and soybean cyst nematode problems are minor. Low lying fields and fields with poor drainage are not ideal for continuous soybean production because of the increased potential for disease development.
Continuous soybean production can potentially reduce soil quality and increase wind and water erosion because of the limited amount of residue compared to corn or wheat. Cover crops may be a consideration to help offset these situations.
Independent studies have shown that continuous soybean yields can be good; however, they are generally less than rotated acres. In one 2017 study, a grower in Illinois split a 78 acre field. The north portion was rotated and the south portion was continuous soybean. The rotated portion of the field showed a yield advantage of more than 5 bu/acre (Figure 1).
Sources: 1Stateon, M., 2017. Recommendations for planting soybeans after soybeans. Miichigan State University.http://msue.anr.msu.edu/news/recommendations_for_planting_soybeans_after_soybeans.2Howe, D. 2006. Risky business or good business? Corn and Soybean Digest. http://cornandsoybeandigest.com/rsky-business-or-good-business.Web sites verified 1/24/18. 180206113017