Part 1, Section 4: Corn
Corn
VARIETIES
Hybrid maturity
Along with yield potential, maturity is of primary importance in selecting a hybrid that will produce mature, high test weight grain during a normal growing season for a particular area. Relative maturity among hybrids in performance reports is best shown by grain moisture content. Hybrids are ranked from earliest to latest in many test reports. Days to maturity and growing degree days (GDD) are two methods of expressing the energy required for corn to mature.
The relative maturity rating of hybrids is just that, a relative value; an 85-day hybrid is not ready to harvest in 85 days. One can be confident, however, that an 85-day hybrid will reach a particular moisture content earlier than a 95-day hybrid. Once a grower establishes the rating for his or her farm, most hybrids of similar relative maturity should mature. Maturity ratings may vary slightly among seed corn companies. The best way to compare the maturity of hybrids from different companies is to compare grain moisture contents of the two hybrids in the same trial.
Growing degree days are a daily accumulation of heat for crop growth. Corn does not grow when temperatures are below 50°F, and temperatures above 86°F do not increase plant growth rate. Most seed-corn suppliers include GDD ratings on their seed tags or in their hybrid descriptions. The hybrid GDD ratings are determined from planting to black layer formation. The formula for calculating growing degree days is to add the daily high temperature (86°F maximum) and daily low temperature (50°F minimum); divide this figure by 2 and subtract 50. The remainder represents the heat units for one day:
| GDD = (daily high + daily low) ÷ 2 - 50 |
| Example: |
| 84 (daily high) + 62 (daily low) = 146 |
| 146 ÷ 2 = 73 |
| 73 - 50 = 23 GDD |
A minimum-maximum thermometer enables producers to determine and record growing degree days for their farms. We have found that GDD ratings vary somewhat for hybrids depending on where the rating was done. Hybrids typically use about 200 fewer GDDs to mature in Pennsylvania compared to some midwestern locations, so companies that use GDD ratings based on midwest data may overestimate the GDD requirements of their hybrids in the east. Our data indicate that a 105-day hybrid matures in about 2,350 GDDs when grown in Pennsylvania, despite the fact that some 105-day hybrids are rated at 2,600 GDDs. Hybrids with similar GDD ratings may vary in moisture content at harvest because of different drydown rates following physiological maturity.
The Pennsylvania corn maturity zones shown in Figure 1.4-1 are based on long-term weather data, including frost-free growing days and growing degree days. See Table 1.4-1 for relative maturity and growing degree ratings for Pennsylvania corn maturity zones. Growing degree days also are valuable for estimating the appropriate maturity for delayed planting. “Latest Planting Dates for Various Hybrid Maturities in Pennsylvania” on the Penn State Corn and Soybean Web site can provide information on hybrid maturities for different planting dates and locations in Pennsylvania.
| Maturity zone¹ | Approximate relative maturity rating | Growing degree days | Planting dates |
|---|---|---|---|
| ¹ See map, Figure 1.4-1. | |||
| 1 | 90–95 | 1,600–1,824 | May 15–25 |
| 96–100 | 1,825–2,024 | ||
| 2 | 101–105 | 2,025–2,350 | May 1–15 |
| 106–110 | 2,350–2,499 | ||
| 3 | 111–115 | 2,500–2,724 | April 25–May 7 |
| >115 | 2,725–2,949 | ||
| 4 | >115 | 2,950–3,174 | April 15–May 1 |
| >115 | 3,175 or > | ||
To monitor growing degree day accumulations throughout the season, visit the Pennsylvania Pest Information Platform for Extension and Education (PA-PIPE) Web site.