Corn: Hybrid Selection


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Pub 811: Agronomy Guide > Corn> Hybrid Selection

Order OMAFRA Publication 811: Agronomy Guide for Field Crops


Table of Contents

Maturity Ratings

Corn development is driven primarily by temperature, especially during the planting-to-silking period. Unlike soybeans, day length has little effect on the rate at which corn develops. The Ontario Crop Heat Unit System has been developed to calculate the impact of temperature on corn development. Ontario crop heat units (CHUs) are calculated based on daily maximum and minimum temperatures and allow for a numerical rating of growing seasons, geographical locations and corn hybrids. This system allows growers to select hybrids that have a high probability of reaching maturity before a killing frost.

Revised Ontario Crop Heat Units

CHU calculations require a start date, a formula for calculating CHU based on daily temperatures and an end date. Traditionally, the start date was triggered by the first occurrence of three consecutive days that reached an average temperature of 12.8°C. Starting in 2009, Ontario is modifying the CHU start date to be May 1, regardless of location or temperatures experienced up to that date. To avoid confusion, these new CHU designations will be referred to as CHU-M1. The new CHU-M1 system will use the same calculation to arrive at a daily CHU total and will use the same season-ending date (30-year daily average temperature falls below 12°C, or first occurrence of -2°C)). The other change included in the CHU-M1 system is that the 30-year normal temperatures used to identify averages have been updated from the 1961-1990 to the 1971-2000 period. The numerical changes between the new CHU-M1 and former CHU system are summarized in Table 1-12, Old and New CHU Comparisons for Various Locations in Ontario, and Figure 1-1, Crop Heat Units (CHU-M1) Available for Corn Production

Table 1-12. Old and New CHU Comparisons for Various Locations in Ontario
Accumulated crop heat units over the course of an average growing season
at various locations across Ontario
1961-1990 CHU
(starts after 3 days of 12.8°C)
1971-2000 CHU-M1
(starts May 1st)
Thunder Bay


It takes approximately 75-80 crop heat units to produce each corn leaf. Therefore, at temperatures of 30°C during the day and 20°C at night, there is one new leaf every 2-3 days. At 20°C during the day, and 10°C at night, one new leaf appears every 5-6 days.

Other jurisdictions use different systems for quantifying the effect of temperature on corn development and for rating corn hybrid maturity. Unfortunately, these systems are unique, and true mathematical conversions from one to the other are not possible. Table 1-13, Approximate Conversions Between Three Systems of Measuring Heat Accumulation in a Growing Season, provides values to assist in making reasonable comparisons between the different systems.

Table 1-13. Approximate Conversions Between Three Systems of Measuring Heat Accumulation in a Growing Season
Crop Heat Units
Corn Relative Maturity
Growing Degree Days (Base 10)
(GDD or GDU)


Producers who record daily high and low temperatures can use Table 10-4, Daily Crop Heat Unit Accumulations Based on Maximum and Minimum Temperatures, to calculate CHUs for their own farm.

Figure 1-1. Crop Heat Units (CH-M1) Available for Corn Production

igure 1-1. Crop Heat Units (CH-M1) Available for Corn Production

This map is based on weather data from 1971-2000 with a common season start date across the province of May 1.
Source: Weather Innovations Inc. (WIN)

Text explanation for Figure 1-1. Crop Heat Units

Selecting the Most Profitable Hybrids

Hybrid selection is probably the single most important management decision in determining cropping profitability. Corn hybrids with superior yield potential have been continuously introduced into the market place over the past 40 years. Yield increases of approximately 1.5% per year have been occurring. To remain competitive, growers must introduce new hybrids to their acreage on a regular basis. The following are a few key considerations intended as general guidelines. Fine-tuning hybrid selection for an individual farm should be done in consultation with seed company representatives.

Maturity and CHU-M1

Using crop heat unit ratings, select hybrids that will reach maturity (black layer) before season-ending frosts traditionally occur in your area. Figure 1-1, Crop Heat Units (CHU-M1) Available for Corn Production, or farm records will provide the heat units normally accumulated in a given area.

Highest Yield

In any given hybrid performance trial, there may be a 1.9-2.5 t/ha (30-40 bu/acre) difference in yield between the highest-and lowest-yielding hybrids. This emphasizes the importance of obtaining reliable information on hybrid yield potential and adaptability. Growers must be able to sort through information from two main sources: performance trial data and strip trial data.

The Ontario Corn Committee (OCC) conducts corn hybrid performance trials each year across the province. These performance trials include the majority of available hybrids. Generally, these trials are set up so that a given set of hybrids, for a certain heat unit range, are tested at three to four locations. These trials give a good indication of yield potential but because they are limited to a few locations, do not adequately evaluate hybrid adaptability over a wide range of conditions. For this sort of data, growers need to turn to strip trials that are conducted on a larger number of sites across a wide range of environments. Seed companies usually summarize these strip trials.

Many growers find it valuable to have a corn hybrid strip trial on their own farm. This allows new, high-yield potential hybrids to be tested against those proven performers in the farming practice. However, it is important to remember that reliable hybrid selections require more than one test site even if that site is on the grower's own farm. Growers should look for 2-year data that originate from many sites (preferably more than 30) before making decisions about hybrids that will be planted on a significant portion of their acreage.

Never purchase a corn hybrid without consulting performance data.

The Ontario Corn Committee publishes the Hybrid Performance Trial Report each December. This information is also available at the Ontario Corn Producers website at

Workhorses vs. Racehorses

Corn hybrids are often classified as "workhorses" or "racehorses." Hybrids that produce above-average yield under good conditions but perform below average under poor conditions are considered race-horses, while those that have relatively consistent yields in both low- and high-yielding conditions are considered workhorses. Most hybrids that are considered to be variable performers (racehorses) have specific defects that cause them to yield lower than average when exposed to certain conditions. Growers can avoid some of the risk associated with hybrid selection by taking time to find out as much as possible about a hybrid's past performance. Select hybrids that complement each other because they have different specific weaknesses. For example, when selecting two full-season hybrids with high yield potential for earliest planting, ensure that they don't both score low for stalk strength.


Select hybrids that have suitable maturity ratings and outstanding yield potential. A further selection based on hybrid standability rating is recommended. This trait is particularly important where significant field drying is expected. If drying facilities are available on the farm and harvesting at relatively high moisture levels (>26%) is an option, standability may be less critical. Traits associated with improved hybrid standability include resistance to stalk rot and leaf blights, genetic stalk strength (a thick stalk rind), short plant height, lower ear placement and high late-season plant health.

One of the most significant advancements in improved standability has been the introduction of Bt hybrids that are resistant to European corn borer. On a provincial average, Bt hybrids have generally resulted in enough yield increase over their non-Bt counterparts to pay for the additional cost of the Bt seed. Corn grown in areas of the province where corn borer pressure is traditionally high and where corn is being planted earlier or later than the majority of the surrounding corn often benefit significantly from having the Bt gene.

For further information on European corn borer management using Bt hybrids, see European Corn Borer.

Harvest Moistures and Drying Costs

Hybrid selection may also be influenced by target harvest moistures. In situations where corn is stored as high moisture grain (e.g., 28%), growers have more opportunity to maximize returns by growing full-season, high-yielding hybrids. If corn is dried for storage, evaluate the impact that high harvest moistures may have on net returns. For example, any potential gains in net returns from a hybrid that yields 0.31 t/ha (5 bu/acre) greater than another should be balanced against increased drying charges. OCC performance trial data have shown that when corn is planted early, aggressive hybrid selection (i.e., full-season and beyond) often results in yield advantages over shorter-season hybrids that more than compensate for increased drying costs.

Selecting Hybrids for Silage

When choosing hybrids specifically for whole-plant silage, a yield advantage can usually be obtained by selecting hybrids rated 100-200 heat units higher than those selected for grain. Select hybrids for high silage yields with improved digestible energy. Silage-only and dual-purpose corn hybrids are available on the market. Dual-purpose hybrids may provide some flexibility where grain harvest needs to be an option, such as when the silo is full.

Without sufficient independent data, it is very difficult to compare and select corn silage hybrids be-tween companies. Choose top hybrids that have strong ratings for silage yield and quality. Various models are used to compare the economic value of corn silage hybrids. The University of Wisconsin has developed "milk per acre" and "milk per ton" calculations using their Milk 2006 model to combine the traits of silage yield, digestibility, fibre, starch, crude protein and intake potential into single measures. Milk per ton measures quality, while milk per acre combines yield and quality.

Switching to Shorter-Season Hybrids

Field conditions may delay planting and necessitate switching to less than full-season hybrids. Factors to consider in this decision include yield potential of shorter-season hybrids, test weight concerns, drying costs and late-season harvesting capabilities.

Grain corn obtains 90% of its total grain weight by one-half milk line, a maturity stage that even late-planted, full-season hybrids reach in most years. Switching to shorter-season hybrids may be a reason-able alternative from a grain yield perspective if earlier hybrids can produce within 10% of the full-season hybrid's yield. Generally, this is a more favourable proposition in longer-season areas.

Growing 3,000 CHU-M1 hybrids as the full-season selections allows for switching to hybrids that are 100-150 heat units less without sacrificing excessive yield. If the full-season hybrids are in the 2,800 CHU-M1 range, the odds of dropping to a hybrid 100 heat units less without giving up more than 10% yield are low.

Table 1-14. Recommended Dates to Switch
From Full-Season Hybrids Across Various Heat Unit Zones
Heat Unit Zone (CHU-M1) Switch Date
>3,200+ May 30-early June
2800-3,200 May 20-25
< 2,800 May 15-20

Source: Adapted from R.Iragavarapu. Basing Hybrid Maturity on Long-Term Data. Pioneer Hi-Bred Ltd.

Extensive research across the northern corn belt defines the optimal date when producers should switch away from full-season hybrids. Some of this data is summarized in Table 1-14, Recommended Dates to Switch From Full-Season Hybrids Across Various Heat Unit Zones. This collection of long-term data took into account yields for hybrids of various maturity ratings as well as deductions for test weight and drying. The switch date indicates the planting date when earlier-maturing hybrids surpass full-season hybrids in terms of net returns (gross returns less drying and test weight deductions).

Growing hybrids with a range in maturity provides some buffer against both silking time stresses and end-of-season risks. However, making significant shifts to earlier hybrids should be reserved in the Southwest (>3,200 CHU-M1) until May 30-June 1; in the mid-maturity corn growing areas (2,800-3,200 CHU-M1) until May 20-25 and in the shorter-season areas (<2,800 CHU-M1) until May 15-20.

A general rule has been to reduce hybrid maturity by 100 CHUs for every week that planting is delayed beyond the cut-off date for full-season hybrids.

Test Weight Concerns

Lower test weights often result if end-of-season frosts occur before late-planted corn has reached maturity (black layer). Consider test weight potential when selecting hybrids for planting in a late spring. Potential dockage from delivering lower bushel weight corn to an elevator or end user is shown in Table 1-15, Grain Corn Test Weights and Potential Dockage.

Table 1-15. Grain Corn Test Weights and Potential Dockage
Test Weight Minimum (kg/ha) Test Weight Minimum
Potential Dockage
1 Based on 2001 market. Potential dockage may vary considerably depending on year and location.

Farming operations that handle and feed all of their own corn may be unaffected by test weight concerns and may choose to remain with full-season hybrids longer into the planting season. Experience and research from 1992 and 2000 indicated there was little or no correlation between test weight and livestock feed value. Producers who deliver all their corn to elevators or processors may want to switch to earlier hybrids to increase the potential for suitable test weights at harvest. Producers in shorter-season areas who fear significant yield losses by switching to earlier-maturing hybrids may consider staying with full-season hybrids but switching to hybrids that have higher test weight scores.


Sticking with high-yielding, later-maturing hybrids may present some logistical harvest issues. Fields planted to potentially delayed hybrids should be well-drained and have good load-bearing capacities to facilitate late-season harvesting in less than ideal conditions. Avoid planting later-maturing hybrids in areas of the province that are more prone to November snow. The snow adheres to leaves and husks, making harvest impossible until a change in the weather allows the snow to melt from the corn plants.


Updates on Corn: Hybrid Selection


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Toll Free: 1-877-424-1300
Author: OMAFRA Staff
Creation Date: 30 April 2009
Last Reviewed: 30 April 2009