Late planting reduces the cultural practice options for row spacing, seeding rate, and variety maturity. The row spacing for June planting should be no greater than 7.5 inches. Appropriate seeding rates for the first half of June are about 200,000 to 225,000 seeds per acre. For the last half of June, 225,000 to 250,000 seeds per acre is recommended, and in early July, the recommendation is 250,000 to 275,000 seeds per acre.
Relative maturity (RM) has little effect on yield for plantings made during the first three weeks of May, but the effect can be large for late plantings. During the first half of June, a four-day delay in planting delays physiological maturity about one day. In the last half of June it takes a five-day planting delay to delay physiological maturity one day. As planting is delayed, yield potential goes down and there is concern about whether late maturing varieties will mature 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.