Western Research and Demonstration Farm
Past research by Iowa State University has shown that the optimum planting date for soybeans, assuming favorable soil conditions, is the first week in May for the northern third of Iowa. The optimum date for the southern two thirds of Iowa is the last week of April. Given that rapidly changing soybean genetics have shown improvements in both yield and disease resistance, these two trials were designed to demonstrate the planting recommendation under local conditions.
Iowa State University
Roush, Wayne B., "Optimum Soybean Planting Date" (2013). Iowa State Research Farm Progress Reports. 1995.