Corn: Hybrid Selection
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811: Agronomy Guide > Corn>
Hybrid Selection
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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
| Sites |
1961-1990 CHU
(starts after 3 days of 12.8°C)
|
1971-2000 CHU-M1
(starts May 1st)
|
|
Ridgetown
|
3342
|
3462
|
|
London
|
2899
|
3120
|
|
Walkerton/Hanover
|
2680
|
2759
|
|
Guelph
|
2682
|
2828
|
|
Belleville
|
3139
|
3369
|
|
Ottawa
|
2885
|
3099
|
|
Thunder Bay
|
1789
|
2058
|
|
Earlton
|
1934
|
2228
|
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
|
Location
|
Ontario
Crop Heat Units
(CHU-M1)
|
Corn Relative Maturity
(CRM) |
Growing Degree Days (Base 10)
(GDD or GDU)
|
|
Walkerton
|
2,759
|
84
|
2,000
|
|
Guelph
|
2,828
|
84
|
2,012
|
|
Ottawa
|
3,099
|
91
|
2,174
|
|
London
|
3,120
|
92
|
2,203
|
|
Simcoe
|
3,190
|
94
|
2,268
|
|
Belleville
|
3,369
|
98
|
2,353
|
|
Ridgetown
|
3,462
|
104
|
2,511
|
|
Harrow
|
3,702
|
111
|
2,673
|
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
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 www.ontariocorn.org.
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.
Standability
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
|
Grade1
|
Test Weight Minimum (kg/ha) |
Test Weight Minimum
(lb/bu) |
Potential Dockage
$/t
|
|
1
|
68.0
|
55.6
|
.00
|
|
2
|
66.0
|
54.1
|
.00
|
|
3
|
64.0
|
52.6
|
0.79
|
|
4
|
62.0
|
51.0
|
3.94
|
|
5
|
58.0
|
47.7
|
7.88
|
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.
Harvesting
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
