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.
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.
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