Part 1, Section 4: Corn

Section 4 Table of Contents

A mature ear of corn

Corn

ESTABLISHMENT

Planting date

Figure 1.4-1.

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.

Table 1.4-8. Estimated grain yields for various planting dates and population rates. Figures shown are most applicable to fields with less than 125 bushels per acre.
Planting date Plants/A at harvest
12,000 14,000 16,000 18,000 20,000 22,500 25,000
Optimum yield (percent)
April 20 72 78 83 87 90 93 95
April 25 75 81 86 90 93 96 98
May 1 77 83 88 92 95 98 100
May 6 78 83 88 92 95 98 100
May 11 77 83 88 92 95 98 99
May 16 75 81 86 90 93 96 98
May 21 73 78 83 87 91 94 95
May 26 69 75 80 84 87 90 92
May 31 64 70 75 79 82 85 87
June 5 59 64 69 73 77 80 81
June 10 52 58 63 67 70 73 75

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 Corn Hybrids 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.

Table 1.4-8A. Estimated grain yields for various planting dates and population rates. Figures shown are most applicable to corn fields with greater than 125 bushels per acre.
Planting date Plants/A at harvest
10,000 12,500 15,000 17,500 20,000 22,500 25,000 27,500 30,000
Optimum yield (percent)
Source: University of Illinois
April 10 62 70 76 82 86 90 92 94 94
April 15 65 73 79 84 89 92 95 97 97
April 20 67 74 81 86 91 94 97 98 99
April 25 68 75 82 87 92 95 98 99 100
April 30 68 75 82 87 92 95 98 99 100
May 4 67 75 81 86 91 94 97 99 99
May 9 65 73 79 85 89 93 95 97 97
May 14 63 70 76 82 86 90 92 94 95
May 19 59 66 73 78 83 86 89 90 91
May 24 54 62 68 74 78 82 84 86 86
May 29 49 56 63 68 73 76 79 80 81

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 and 1.4-8A as guides for estimating the yield of replanted corn. Table 1.4-8A is most appropriate for fields where yields average more than 125 bushels per acre.

Internet tools are available to assist in replant decisions.