The ideal planting time is only a few days long. In most years, each day’s delay past this period reduces yields up to 1 bushel per acre per day (see Table 1.4-8 and Figure 1.4-1). Consequently, corn planting should begin early enough to ensure that the bulk of the crop can be planted during this optimum period.
Normally, corn can be planted safely 10 to 14 days before the average date of the last killing frost. This date ranges from April 15 in long-season areas to May 15 in cooler regions of the state. Ideally, soil temperatures in the seed zone at 8:00 a.m. should be 50°F or above, and the 5-day extended weather forecast should indicate continued warm, or warmer, conditions. Planting before a forecast cool rainy period can result is poorer emergence due to imbibitional chilling injury, or injury caused by the seeds imibibing cold water. Soil physical conditions for good seed coverage are equally important in deciding to plant.
When planting is delayed past the optimum dates, or if a crop needs to be replanted, it may be necessary to switch hybrid maturities. In most areas, switching to shorter than adapted hybrid maturities should not be considered until at least the last week of May. Very full-season hybrids should be planted by mid-May in most areas.
An important factor is to determine the approximate number of growing degree days left in the season before a killing frost in your area. If you are growing corn for grain, use only hybrids that require fewer than that number of growing degree days to mature. Hybrids may reduce their GDD requirement by 100 to 150 GDDs in late planting situations. That is, they adapt to the later planting by maturing earlier. Consult “Latest Planting Dates for Various Hybrid Maturities in Pennsylvania” on Penn State’s Corn and Soybean Web site for more information on growing degree days and hybrid requirements. For silage, full-season hybrids still can be planted and can yield more than shorter-season hybrids. If soils on these late-planted fields are poorly drained, though, earlier hybrids probably also are justified for silage, since they help avoid the risk of wet soils at harvest.
When stands are less than desired, you may consider replanting the field. Base this decision on the anticipated costs of replanting compared to the potential yield gain. In some cases, it may be advisable to leave a reduced stand and forego replanting, since the yield potential of the replanted corn may not be high enough to justify the cost. Use Tables 1.4-8, 1.4-8A, and 1.4-8B as guides for estimating the yield of replanted corn. Table 1.4-8B is most appropriate for fields where yields average more than 175 bushels per acre. Recent research in Illinois has shown that high-yielding fields are particularly responsive to early planting and higher plant populations. For many Pennsylvania fields with moderate yield potentials between 125 and 175 bushel per acre, the responses in Table 1.4-8B are most appropriate. Note that in Table 1.4-8 the response to plant population is often limited when there are more than 24,000 plants per acre. Internet tools are also available to assist in replant decisions.