Agronomy Guide For Field Crops – Corn – Hybrid Selection

Hybrid Selection

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 producers to select hybrids that have a high probability of reaching maturity before a killing frost occurs.

Ontario Crop Heat Units

CHU calculations require a start date, a formula for calculating CHU based on daily temperatures and an end date. Starting in 2009, Ontario began recording CHU on May 1, regardless of location or temperatures experienced up to that date. The CHU system uses a calculation to arrive at a daily CHU total and employs the following trigger to mark the season end: when average temperature falls below 12°C, or the first occurrence of -2°C. The current CHU system and map (sometimes referred to as CHU-M1 because of the May 1 start date) are based on data from the 1971–2000 time period. The CHU map for Ontario is found in
Figure 1–1, Crop heat units (CHU-M1) available for corn production. 

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 1–7, 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.



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.


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 CHU for their own farm.



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 50 years. Yield increases of approximately 1.5% per year have been achieved. To remain competitive, producers must introduce new hybrids to their acreage on a regular basis. The following are a few key considerations intended as general guidelines for selecting hybrids. Fine-tuning
hybrid selection for an individual farm should be done in consultation with seed company representatives.

Maturity and CHU-M1

Physiological maturity (black layer) is achieved when all the kernels have reached their maximum dry matter accumulation and there is no additional moisture or nutrient transport from the plant. Using crop heat unit ratings, hybrids can be selected to reach black layer before traditional season-ending frosts occurs. 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. Producers must be able to sort through information from several key sources: public performance trial data, strip trial data (seed company or farm organization) and on-farm comparisons.

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. This data is available at and can be viewed in several formats to allow for stakeholders to carefully examine the results. These trials give a good indication of yield potential; however, they are
limited to a few locations and therefore do not adequately evaluate hybrid adaptability over a wide range of conditions. For this information, producers need to turn to strip trials that are conducted on a larger number of sites across a wide range of environments. Seed companies summarize these strip trial results which are made available through their seed guides.

Many producers find it valuable to have corn hybrid strip trials or comparisons 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 producer’s own farm. Producers should look for 2‐year data that originates from many sites (preferably more than 30) before making decisions about hybrids that will be
planted on a significant portion of their acreage.

One way to look at hybrid selection is to define two groups of hybrids for a farm operation. The first group is “New Hybrids” and is comprised of the most promising new hybrids in the market place. This group will represent hybrids that are grown on a relatively small acreage and that are tracked carefully for their performance on a given farm, in strip trials and in public performance trials. The goal is to quickly identify the top hybrid in this group and move it into the second hybrid group which is called “Tested Hybrids”. The Tested Hybrids group represents hybrids that have proven their performance and are grown on a large percentage of a given operation’s corn acres. Producers who make the most accurate and quickest decisions to move new, higher performing hybrids into their operations will achieve maximum competitive advantage and yield increases.


Producers who make the most accurate and quickest decisions to move new, higher performing hybrids into their operations will achieve maximum competitive advantage.


Hybrid Positioning

Corn hybrids have often been classified with various terms such as “workhorses” or “racehorses”, having offensive versus defensive natures. Hybrids that produce above-average yield under good conditions, but perform below average under poor conditions are considered racehorses (offensive). Hybrids that have relatively consistent yields in both low- and high-yielding conditions are considered workhorses (defensive). As site-specific management gains popularity, many producers will chose racehorse varieties in the most productive areas of their field and workhorse varieties where soil or weather conditions are less favourable. Trends within the seed industry indicate that hybrids will be increasingly defined for their ability to fit into certain management strategies and/or environments. Precision agriculture technologies can better define the potential for hybrids to exploit site specific resources more effectively.

Producers should be aware of the possibility of selecting hybrids that will respond more effectively to higher or lower input strategies. Producers can avoid some of the risk associated with hybrid selection by taking time to investigate a hybrid’s past performance. Select hybrids that complement each other, because they have different weaknesses for specific characteristics. For example, when selecting two long-season hybrids with high yield potential for earliest planting, ensure that they do not both score
relatively low for resistance to the leaf diseases.


Select hybrids that have suitable maturity ratings and outstanding yield potential. Selecting for hybrid standability is also 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 above average late-season plant health. Plant intactness or late-season plant health ratings also indicate better harvestability ratings.

One of the most significant advancements in improved standability has been the introduction of Bt hybrids that are resistant to a range of corn feeding pests. All producers using Bt hybrids are required to plant a refuge which contains corn plants that are not genetically modified in order to prevent a build-up of resistant pest strains. Producers can now purchase refuge incorporated blends that contain both Bt and non-Bt seed in the same bag, eliminating some of the issues with having to plant separate refuge.
For further information on Bt corn refer to Chapter 15 as well as the Canadian Corn Pest Coalition website at

Harvest Moistures and Drying Costs

Hybrid selection may also be influenced by the producer’s target harvest moistures. In situations where corn is stored as high moisture grain (e.g., 28% moisture), producers have an opportunity to maximize returns by growing full-season, high-yielding hybrids. If corn is dried during 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 has shown that when corn is planted early, aggressive hybrid selection (i.e., full-season and beyond) often results in yield advantages over hybrids that mature in less days (shorter-season hybrids). The increased yield from full- or long-season hybrids more than compensates for the increased drying costs due to higher harvest moistures. Producers should evaluate net returns for hybrids after dryings costs. Depending on drying costs a 2–3 bushel per
acre increase in yield often more than compensates for an additional 1% increase in harvest moistures.

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 offer grain harvest as an option, providing more flexibility when the silo is full. Without sufficient independent data, it is very difficult to compare and select corn silage hybrids between 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 the time it reaches 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 reasonable alternative from a grain yield perspective if earlier maturing hybrids can produce within 10% of the full-season hybrid’s yield. Generally, this is a more favourable proposition in longer-season areas.

Growing full season 3,000 CHU-M1 hybrids 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.

Extensive research across the northern cornbelt defines the optimal date when producers should switch away from full-season hybrids. Some of this data is summarized in Table 1–8, 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 stresses at silking time and end-of-season risks. However, making significant adjustments to shorter season hybrids should not be considered until May 30–June 1 for areas in southwest (>3,200 CHU-M1); until May 20–25 for the mid- maturity corn growing areas (2,800–3,200 CHU-M1) and until May 15–20 in the shorter-season areas (<2,800 CHU-M1).

A general rule has been to reduce hybrid maturity by 100 CHU 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 1–9, Grain corn test weights and potential dockage.


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, 2000 and 2014 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.


Remaining dedicated to 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 snow In November. The snow adheres to leaves and husks, delaying harvest until the snow melts from the corn plants.



This is an excerpt of the hybrid selection section from the corn chapter of Ontario Ministry of Agriculture, Food and Rural Affair’s “Agronomy Guide for Field Crops – Publication 811”. The whole corn chapter can be viewed as PDF here. All chapters are online, and can be viewed by searching the internet for “Agronomy Guide for Field Crops – Publication 811”. Special thanks to Andrew Priest, OMAFRA summer student, for producing these Field Crop News excerpts.