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Though continuous soybean production is generally not recommended, some farmers have reported stable yields with no downward trend after nearly 20 years or more of planting soybeans continuously in the same field.1 Successes like these tend to occur in areas where soils are not prone to holding excessive moisture and where disease and soybean cyst nematode (SCN) problems are minor. Low lying fields and fields with poor drainage are not ideal for continuous soybean production due to the increased potential for disease development. Greater yield penalties and smaller profit margins, due to increased pest control costs, may be expected when soybeans are planted continuously in fields that have a history of insect and disease problems.
Disease problems and weed species shifts are two concerns when planting continuous soybeans. Crop rotation is important in all crops to help break disease and insect cycles and soybean are no exception. Diseases such as white mold, stem canker, brown stem rot, sudden death syndrome (SDS), and soybean cyst nematode (SCN) survive in soil or crop residue and can readily attack a successive soybean crop. Dramatic yield reductions can occur by the third or fourth year soybeans are planted into fields with disease or nematode issues.2
Disease Management. Without crop rotation to break pest and pathogen life cycles, other management approaches will be needed. Often, planting a product with disease or insect resistance is the most effective management approach.
Phytophthora root rot can be managed by selection of soybean products with race-specific resistance, designated Rps. Products with the appropriate Rps designations relative to the races in your field, combined with partial resistance, will provide the best control.3 No soybean products are completely resistant to white mold and SDS. Selecting soybean products with partial resistance to these diseases may help reduce damage.
If white mold occurred in the previous year, crop rotation is the most effective cultural practice for preventing future damage. With continuous soybeans, other cultural practices must be considered. Alternative control strategies for managing diseases include reduced seeding rates or wider row widths to increase air flow in the canopy for suppression of foliar diseases. Planting later in the season or planting soybean products with shorter relative maturities can help to avoid disease outbreaks during sensitive growth stages.
Nematode Management. Crop rotation and the use of SCN resistant products are the most effective methods of managing SCN. Soybean plants with SCN resistance will not provide complete control. Resistant soybeans limit reproduction of SCN, but are still attacked by these nematodes.
Seed Treatments. Seed treatments are a critical component in continuous soybean production. Broad-spectrum fungicidal seed treatments can protect soybeans from early-season diseases including Pythium, Phytophthora, Fusarium, and Rhizoctonia. Broad-spectrum insecticidal seed treatments can protect soybeans from damage by early-season insect pests such as bean leaf beetle, soybean aphid, seed corn maggot, wireworm, and white grub. Some seed treatments can also protect against early-season SCN damage by making young soybean roots unattractive to the nematodes.
Weed Management. Weed control can be challenging in continuous
soybean production systems. A weed control plan including residual herbicides
and multiple sites of action should be in place to ensure effective weed
Soil Fertility. Soil nutrient levels should be carefully monitored to ensure that adequate phosphorus (P) and potassium (K) are available to the crop. Because a soybean grain harvest typically removes more K than a corn grain harvest, this nutrient may need to be supplied more often in continuous soybean production. Nutrient inputs should be based on soil test recommendations, which should be conducted every three to four years.