The availability and performance of superior hybrids essentially have made the practical use of open-pollinated corns mostly obsolete. Open pollinated corns often suffer from low yields and poor standability. Four open-pollinated varieties—Early Butter, Reids Yellow Dent, Golden Surprise, and Lancaster Surecrop—were evaluated in Pennsylvania hybrid corn tests over several years. Of 59 county tests, the average yield for the open-pollinated varieties was 82.9 bushels per acre with 29.4 percent down stalks. The average yield of the hybrids was 132.9 bushels per acre with 7.8 percent down stalks—a 50-bushel or 60 percent yield advantage.
Hybrids are available for all types of corn, and include double crosses, three-way crosses, single crosses, and modified crosses (or sister line crosses). These terms simply describe the number of inbred parent lines used in producing a hybrid and do not guarantee that one is superior to another. The merits of a hybrid rest with its genetic potential to produce grain and/or silage.
Many corn hybrids containing transgenic traits are resistant to a number of herbicides and insecticides. Refer to Part 2, Section 2 for pest-resistant traits. Hybrid corn performance results for both grain and silage are published in the Pennsylvania Commercial Grain and Silage Hybrid Corn Tests Report. See Figure 1.4-1. This publication is available at local Penn State Cooperative Extension offices and on the Web at cornandsoybeans.psu.edu.
The performance of a particular hybrid may vary from location to location and from year to year. Silking dates for the hybrids being grown on a particular farm will vary, and hybrid maturity differences will increase on farms that frequently encounter drought stress. This variability has two implications for corn growers. First, you should use more than one hybrid each year. Reserve part of your acreage for evaluating new hybrids, but grow the bulk of your acreage to hybrids with which you have experience and that have a known performance record on your farm.
The second implication of the variability of hybrid performance is that hybrids should be selected on the basis of performance in a number of environments. Average performance over a wide range of environments is usually the best predictor of hybrid performance on your farm next year. As a result, two-year performance averages or average performance results from several test locations within a year increase the reliability of data on hybrid performance.
Hybrids with wide ranges of adaptability have less variation and consistently rank as superior. Although specific hybrids may adapt particularly well to some environments, these environments often are difficult to identify and usually are found after growers have had several years’ experience growing a specific hybrid.
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
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. To monitor growing-degree-day accumulations throughout the season, visit the Pennsylvania Pest Information Platform for Extension and Education (PA-PIPE) Web site at pa-pipe.zedxinc.com/cgi-bin/index.cgi.