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 15 inches. Appropriate seeding rates for the first half of June are about 150,000 to 175,000 seeds per acre. For the last half of June, 175,000 to 200,000 seeds per acre is recommended, and in early July, the recommendation is 200,000 to 250,000 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 declines and there is concern about whether late maturing varieties will mature before frost.
When planting late, the rule of thumb is to plant the latest maturing variety that will reach physiological maturity before the first killing frost. The reason for using late maturing varieties for late planting is to allow the plants to grow vegetatively as long as possible to produce nodes where pods can form before vegetative growth is slowed due to flowering and pod formation. More nodes equates to more pods and more yield. Late-maturing varieties are needed that will mature before getting frosted, but because the first frost date is unknown, we use a narrow range of maturity that will not be damaged by frost occurring at the normal time.
The recommended relative maturity ranges in Table 5.3 assume normal weather and frost dates, so varieties with those relative maturities should mature before frost and produce maximum possible yields when planted on the dates indicated. Varieties with an earlier relative maturity will mature earlier but will produce reduced yields.
