Editor’s note: The following was written by Mark Licht, Daren Mueller and Antonio Mallarino with Iowa State University for the university’s Integrated Crop Management blog Dec. 13.
Planting soybeans in the same field that just grew soybeans 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 soybeans compared to first year, and a study in North Carolina shows a 5 bu./acre penalty for second-year soybeans.
However, high fertilizer costs and possible shortages this past fall or expected ones in the spring might make planting soybeans back-to-back a viable option. If so, keep these considerations in mind.
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) pressure. These pathogens, plus others like white mold, could lead to yield losses of up to 20%.
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.
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 degrees 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 soybeans, but do pay particular attention to bean leaf beetle and Japanese beetle pressure.
If white mold history exists, reduce seed rates and/or set row width to 20 inches or greater for more airflow throughout the canopy.
Along with disease-resistant varieties, use a wide range of soybean crop maturities to spread out weather risks.
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 soybeans. Your herbicide program should target specific weeds to obtain effective weed control and reduce costs.
But keep in mind that planting soybeans 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.
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 soil tested for P, K and pH, 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.