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.
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.
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.
- 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.