by David Morris, Ontario Corn Committee Secretary & Greg Stewart, OMAFRA Corn Specialist

Recent changes to the Crop Heat Units System

THE CORN HYBRIDS of Ontario span a wide range of maturities. Crop Heat Units (CHU) provide a system to assist farmers in selecting hybrids with maturities suitable for their area.  The CHU system was originally developed in Ontario for field corn and has been successfully used in this province for over 45 years. 

In 2009 the Ontario Corn Committee introduced changes to the CHU system and a new Ontario CHU map was created.  Because the map has significant changes compared to the old system, seed companies, corn producers and other stakeholders will need to understand these changes in order to continue to work together effectively.

HOW IS THIS MAP DIFFERENT FROM THE PREVIOUS EDITION?
CHU ratings for most regions of Ontario appear much higher (150 to 300 CHUs) on the revised map (see Figure 1-1) than on previous versions, primarily because of a fundamental change in the procedure to determine the date to start recording heat units in the spring.  In this revised system CHU ratings for all areas of the province are now based on the average heat unit accumulation for the period from May 1 to the fall cut-off date, using weather records for the period 1971 to 2000. 

WHY ARE THESE CHANGES BEING MADE?
The primary reason for the change is to bring the method for accumulating heat units in line with current corn growing practices in Ontario.  Ideally, the CHU rating should reflect the number of heat units that can be expected from the time when corn is typically planted until the end of the growing season.  This should correspond to the way in which hybrids are rated (i.e. CHUs required from planting to black layer).  

Under the previous system, heat unit accumulation did not start in the spring until the third of three consecutive days with daily mean temperatures (average of max and min) over 12.8 Celsius.  This method may have been appropriate when the CHU system was introduced, but now, it results in starting dates much later than when Ontario growers typically plant corn.   Thus, the previous CHU map significantly underestimated the number of heat units available throughout the growing season with today’s earlier planting dates.

IS THE NEW SYSTEM MORE COMPLICATED?
No.  Adopting a simplified approach for determining the starting date will also eliminate the confusion that existed under the previous system and make it possible to compare the results from the various agencies, organizations and individuals who compile and report seasonal heat unit totals. The time period from 1971 to 2000 was used so that heat unit ratings would accurately reflect any recent changes in weather patterns.   However, the increase in heat unit rating resulting from using more recent data was quite small (about 50 CHUs on average).

WERE CHANGES MADE TO THE FORMULA FOR CALCULATING CHUS OR THE SEASON END DATE DETERMINATIONS?
No.  CHUs are still based on the daily maximum and minimum temperatures using the same formula as in the previous system.  Season end dates are also calculated the same way as in the past, which is, the date when the average daily temperature falls below 12 degrees Celsius or an occurrence of -2 degrees Celsius, whichever comes first.

WHAT IF MAY 1 IS NOT A REALISTIC STARTING DATE FOR MY AREA?
In choosing a standard starting date of May 1 for all of Ontario, it was assumed that growers in all corn growing regions (at least in southern Ontario) would plant corn in the first week of May if soil conditions were suitable.   It is recognized that growers in areas where planting in early May is not suitable may need to make appropriate adjustments based on planting dates.

WHAT EFFECT DO THESE CHANGES HAVE ON HYBRID SELECTION?
For the most part this revision reflects what is already happening, most growers who target May 1 plantings will still be using hybrids of the same relative maturity as they have been growing recently.   However, it should be noted that we have gone from a map that underestimated the number of heat units available to one that fairly accurately reflects the average available from May 1 to the end of the season.

For example, on the old CHU map an area that was identified with a certain CHU value often received significantly more than that value; particularly if the producer started totaling CHUs from a relatively early planting date.

Guelph, for example is rated in the old system at 2682 CHU. I can assure you that very few producers who get corn planted in the April 25 to May 5 window in the Guelph area are planting hybrids rated at 2682 CHU!

The new CHU (May 1) system now lists Guelph at 2828 and is more reflective of the actual heat units to work with over a full season.

WILL SEED COMPANIES NEED TO CHANGE HYBRID RATINGS?
We appreciate the support that the seed industry has given OCC in this initiative.  We also understand that most seed companies sell seed in many jurisdictions outside of Ontario and are faced with a range of maturity indicators and now face a revised system in Ontario. 

We anticipate that this simplified CHU approach can be adopted fairly rapidly; in some cases seed companies have suggested that very few if any of their hybrid ratings would need to change. 

The reminder to all stakeholders is that the new system runs from May 1 to season end and that this should most readily equate to a hybrid rating that runs from planting to maturity (black layer).