In Ohio, most soybeans are planted in narrow rows (7.5- to 15-inch). Soybeans grown in narrow rows produce more grain because they capture more sunlight energy, which drives photosynthesis. Within limits, as sunlight interception increases, so does yield. Researchers have learned that the peak demand for the products of photosynthesis occurs during the reproductive stage. Therefore, the row width should be narrow enough for the soybean canopy to completely cover the interrow space by the time the soybeans begin to flower (June 20 to July 10). The row widths that will accomplish that goal will vary with soil type, planting date, weather conditions and, in some cases, variety. The later in the growing season soybeans are planted, the greater the yield increase due to narrow rows. The response to narrow rows is also greater for short varieties and when growing conditions cause plants to grow slowly or be short. Planting systems using precision seed
metering to achieve uniform seed spacing within the row plus uniform depth of seed placement usually produce higher yields than planting systems with less uniform seed spacing and variable depth of planting.
The effect of row spacing on the yield for soybeans planted in May can be seen in Figure 5-3. Soybeans planted in 7.5- to 15-inch row widths yielded similarly while soybeans planted in 30-inch row widths yielded 14 percent less. The yield reduction associated with 30-inch row widths would be magnified when planting in June or July.
Figure 5-3. Effect of row spacing on yield for soybeans planted in May.