Soybean [Glycine max (L.) Merr.] yield increases per hectare have been slow in the past decade, raising the need to evaluate management decisions and identify yield limiting factors. Time of planting, row spacing, seeding rate, variety selection, and management of soilborne pathogens, specifically soybean cyst nematode (Heterodera glycines, SCN), were identified as critical components for achieving high yields. Studies were conducted between 2004 and 2006 near De Witt, Nevada, and Whiting in eastern, central, and western Iowa, respectively. At each location 4 ha were fumigated in the fall with the fungicide/nematicide Telone C-35 and 4 ha were not fumigated. In the spring, four studies that consisted of the variables planting date, row spacing, seeding rate, and variety, were established in different combinations in both fumigated and non-fumigated soil conditions.

Soil fumigation was used as a tool to study interactions between agronomic practices and soilborne pathogens. Soil fumigation increased yield 200 to 300 kg ha^-1 and increased the yield responses from early planting and for specific varieties. Agronomic decisions lay the foundation for achieving high yields. High-yielding varieties will not improve yields if agronomic practices are not managed correctly. Planting the last week of April rather than the second week of May increased yield 87 to 342 kg ha^-1 depending on location. Narrow row spacing (38 cm) increased yield by 302 kg ha^-1 compared to wide row spacing (76 cm). Yield of varieties susceptible to SCN were competitive in fields with low SCN population densities, but were quickly outpaced by SCN-resistant varieties as SCN population densities increased. Modern varieties are extremely productive, but must be matched to field conditions and properly managed throughout the season. Modern varieties must be continually challenged with proven agronomic decisions to ensure future yield advances. The outcome of this work can be summarized very simply. To increase soybean yields: (1) select a high-yielding variety to match field soilborne pathogen populations, (2) plant that variety early, (3) plant that variety in narrow rows, and (4) reduce the seeding rate for that variety. These simple practices will increase yield, reduce production costs, and improve profitability.