Recommendations for Planting Soybeans After Soybeans

By Michael Staton

Given the projected market prices and current production costs for corn and soybeans, some producers are altering their long-term rotation plans by increasing soybean acreage in 2021. In these situations, soybeans will be planted into fields that were planted to soybeans in 2020. Long-term crop-rotation research conducted at the University of Wisconsin showed soybean yields declined by 5% in the second year of soybeans and by 12% in the third year when compared to a corn-soybean rotation. However, plant stress caused by environmental conditions, diseases or insects can easily increase yield losses to 20% or more. This article will list the principal challenges and risks of planting second-year soybeans and provide some management recommendations for mitigating them.

Diseases including soybean cyst nematodes (SCN) present the largest risk to second-year soybeans. The best strategy is to avoid planting soybeans into fields that were infested with white mold, sudden death syndrome (SDS) or SCN in 2020. These are soil-borne pathogens having the potential to cause large yield losses in the 2021 crop as well as future soybean crops. However, if you plan to plant soybeans into fields that were infested with these pathogens, consider the recommendations provided below.

Variety selection is your first line of defense when planting into fields infested with white mold, SDS or SCN. Try selecting varieties that have the highest level of resistance you can obtain for the identified challenge. Seed companies typically use a scale of 1 to 9 when rating the disease resistance or tolerance of their varieties. Read the scale carefully as 1 is excellent and 9 is poor in some catalogs, while in others it is the opposite. These ratings are useful when comparing varieties from a given company. However, they should not be used to compare varieties from different companies.

Consider using ILeVO or Saltro seed treatments in combination with SDS and SCN tolerant varieties when planting soybeans into fields having a history of significant SDS. This recommendation is based on recent research conducted by Michigan State University.

If you must plant soybeans into a field that was infested with white mold in 2020, be prepared for the possibility of large yield losses if extended periods of cool and wet weather occur from late June to early August. These losses may be reduced by the following management practices:

• Select the most resistant/tolerant varieties available.
• Bury the sclerotia deeper than 2 inches with tillage operations. This practice will be less beneficial if sclerotia from previous infestations are present in the soil. It also increases the long-term survival of the sclerotia produced in 2020.
• Reduce planting rates.
• Increase row width to 20 inches or greater.
• Plant varieties from a range of maturity groups (early maturing varieties appeared to escape white mold infection or development in 2014).
• Select and apply foliar fungicides labelled for white mold to protect the flowers between the beginning of flowering (R1) and initiation of pod development (R3) growth stages.

Keep in mind additional tillage operations and foliar fungicide applications will increase production costs.

If you plan to plant soybeans into a field infested with SCN in 2021, yield losses, SCN populations and the risk of developing a resistant SCN population type are likely to increase. The potential for these outcomes occurring can be reduced by implementing the following management practices:

• Collect soil samples from each field and submit them to Michigan State University Pest & Plant Diagnostics for the standard SCN analysis and the SCN type test if the yields in the field have been declining or SCN numbers are high.
• Select SCN resistant varieties. Ideally, the variety planted in 2021 should carry a different source of resistance than the variety planted in 2020. This is especially important if Peking varieties were planted in 2020. When planting varieties with the PI88788 source of resistance two years in a row, it is important to plant a different variety each year as this has been shown to delay resistance development.

The potential for seedling diseases such as Pythium and Phytophthora may increase in second-year soybeans. Planting after soil temperatures reach 60 degrees Fahrenheit reduces the risk from Pythium and the use of resistant or tolerant varieties is the preferred management strategy for Phytophthora. Martin Chilvers, MSU plant pathologist, has shown that some of the Phytophthora resistance genes currently being used have lost much of their efficacy, so producers concerned about Phytophthora should plant varieties with very good field tolerance to this disease. Effective seed treatments for Pythium and Phytophthora are also available. When relying primarily on field tolerance for varietal protection from Phytophthora root rot, seed treatments must be applied at the labeled rate for Phytophthora to provide early-season protection.

Planting soybeans after soybeans also increases the development of herbicide-resistant weeds. The main reasons for this are that fields planted to corn are usually tilled more intensively than fields planted to soybeans, and two important herbicide groups (triazines and HPPD inhibitors) cannot be used in soybeans. The following steps will help delay the development of herbicide-resistant weeds in continuous soybean rotations.

• Use more intensive tillage operations.
• Prevent weed seed production by removing escaped marestail and pigweed plants prior to seed development.
• Use different seed technologies (LibertyLink, Roundup Ready 2 Xtend, LibertyLink GT27, Enlist E3 and Roundup Ready) each year to use herbicides with different sites of action.
• Include herbicides having different sites of action in all pre-emergence and post-emergence herbicide applications. This is especially important for post-emergence applications.
• Avoid using herbicides with the same sites of action each year.

Pay attention to soil fertility and base fertilizer applications on recent soil tests. Many Michigan soybean producers apply phosphorus and potassium fertilizers prior to planting corn and let the following soybean crop scavenge for these nutrients. This works well on finer-textured, mineral soils (cation exchange capacities greater than 5 meq/100g) when enough fertilizer is supplied to meet the needs of both crops. Don’t forget to apply the recommended potassium fertilizer when soybeans will be planted instead of corn in 2021 as potassium contributes to disease resistance and potassium-deficient plants are more attractive to soybean aphids.

Insect problems are not typically increased in second-year soybeans. Producers should scout all soybean fields for bean leaf beetles and soybean aphids.

There is potential for soil quality to be degraded by not staying with a corn-soybean rotation in 2021 as corn and wheat are the only two field crops that can build soil organic matter levels. The reduction in residue cover in second-year soybeans also increases the risk of soil loss due to erosion. Second-year soybeans should not be planted on sloping fields with low organic matter levels.

Changing your crop rotation, especially shortening it, is an important decision as it will have long-term effects on pest populations and soil quality. Make sure the benefits exceed the risks and manage the increased risk with proven practices.