Considerations when planting soybean back-to-back

December 13, 2021 11:45 AM
Blog Post

Planting soybean in the same field that just grew soybean is not recommended. Soybean yield will suffer even before factoring in environmental conditions, weather, and pest/disease pressures. 

Four different studies in Minnesota and Wisconsin have shown a yield decline of at least 5% and as much as 9% for second-year soybean compared to first year and a study in North Carolina shows a 5 bu/ac penalty for second-year soybeans.

However, high fertilizer costs and possible shortages this past fall or expected ones in the spring might make planting soybean back-to-back a viable option. If so, keep these considerations in mind.

Selecting the right fields

It is important to choose well-drained, highly productive fields. Try avoiding fields with heavy weed, soybean cyst nematode (SCN), and sudden death syndrome (SDS) pressures. These pathogens, plus others like white mold, could lead to yield losses of up to 20%. Highly productive fields under good timely management will maintain yield potential better.

Disease and pest pressures

Several pathogens can be problematic in second-year soybean production. These include SCN, Pythium and Phytophthora. Take soil samples to get an idea of SCN egg population densities and use scouting notes from past years to know where diseases have been most problematic.

Variety selection is important when planting into infested fields, look for varieties with the highest level of resistance possible for the identified disease. The ISU Extension publication, Soybean Cyst Nematode-resistant Soybean Varieties for Iowa, recently was updated and contains information about SCN-resistant soybean varieties from 22 companies that are appropriate for growing in Iowa. Also, check out High SCN population densities from 2021 make beans on beans in 2022 a risky proposition for more specific recommendations with regards to SCN.

Consider foliar fungicides targeted for white mold or seed treatments targeted for SDS, Pythium and Phytophthora. Another tactic is to wait for soil temperatures to reach 60°F and be increasing for best vigor to overcome the fungal seedling pathogens.

A bit of good news is insects tend not to be any more problematic in second-year soybean, but do pay particular attention to bean leaf beetle and Japanese beetle pressure.

Weed management

Develop a robust pre and post herbicide program that includes multiple modes of action with residual activity combined with applications at the right time, like avoiding applications to big weeds. Pre-emergence herbicides should be applied within two or three days of planting soybean. Your herbicide program should target specific weeds to obtain effective weed control and reduce costs.

But keep in mind that planting soybean in back-to-back seasons not only increases the potential for the development of herbicide-resistant weeds, but also creates problems with control of herbicide-resistant weeds.

Nutrients

There are no major fertility issues for a second-year soybean crop. Soybean crops do fix their own N and there is no need for inoculants at planting since Bradyrhizobium populations will be adequate for effective nodulation and N fixation. It is important to get field soil tested for P, K and pH (see publication PM 1688) with the primary focus on K. This is because high-yielding soybean harvest can remove more K from the field than corn, so this requires better awareness of what soil test K levels are needed to ensure no deficiencies exist.

Other considerations

  • Along with disease-resistant varieties, use a wide range of soybean crop maturities to spread out weather risks.
  • If white mold history exists, reduce seed rates and/or row width to 20” or greater for more airflow throughout the canopy.
  • If more tillage is used, for weed control or to bury white mold sclerotia deeper, it will increase the risk of diminishing soil health, and potentially cause more soil erosion.

Typically, soybean production has lower costs associated with it than corn, so pencil out the economics of it all. If soybean commodity prices remain strong, overall profitability can be achieved even with potential yield losses.

 

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Authors: 

Mark Licht Assistant Professor

Dr. Mark Licht is an assistant professor and extension cropping systems specialist with Iowa State University Extension and Outreach. His extension, research and teaching program is focused on how to holistically manage Iowa cropping systems to achieve productivity, profitability and en...

Daren Mueller Associate Professor

Dr. Daren Mueller is an associate professor and extension plant pathologist at Iowa State University. He is also the coordinator of the Iowa State University Integrated Pest Management (IPM) program. Dr. Mueller earned his bachelor's degree from the Univ...

Antonio Mallarino Professor of Soil Fertility and Nutrient Management, Extension Specialist

Dr. Antonio Mallarino is a professor of agronomy and nutrient management research and an extension specialist at Iowa State University. His programs focus on agronomic and environmental issues of nutrient management with emphasis on phosphorus, potassium, lime, and micronutrients. Issues addresse...

Greg Tylka Morrill Professor

Dr. Greg Tylka is a Morrill Professor in the Department of Plant Pathology and Microbiology at Iowa State University with extension and research responsibilities for management of plant-parasitic nematodes. The focus of Dr. Tylka's research program at Iowa State University is primarily the soybea...

Zachary Clemens Communications Specialist I

Zach Clemens grew up in Colorado, but migrated to Iowa as a young adult. He is an alumnus of Iowa State and is a former newspaper reporter and editor. He works to communicate agronomy news and real time updates for agriculture producers. He splits his time with the IPM program and runs the H...