Estimating Date of Grain Corn Maturity from Silking Stage

The 2015 season has seen relatively normal accumulation of corn heat units (CHU), with a range of 92% of normal to upwards of 111% of normal as of July 19th. According to Issue #12 of the Manitoba Crop Report, grain corn ranges in development from late vegetative stages to silking (R1).

Silking marks the start of the reproductive phase of development and begins when the silk becomes visible outside the husk and pollination occurs. Each silk is attached to an ovule which will become a kernel if pollinated. The CHU accumulation from planting to silking is about 50 to 55% of that required for the plant to go from planting to physiological maturity.

While this could be used as a general guideline, temperature and relative maturity of the hybrid must be taken into consideration. Plus, the duration of each stage during grain fill can also be influenced by soil fertility, cultural practices (plant populations), and moisture.

If we consider CHU accumulation and maturity rating of the hybrid, we can calculate the number of corn heat units required for a crop to pass from silking to physiological maturity. As mentioned above, the period from planting to silking takes approximately 50 to 55% of the total heat units required for the crop. Therefore, the remaining 45 to 50% would be needed to carry the crop from silking to physiological maturity. The table below identifies the approximate CHU requirements to bring a corn crop from silking to physiological maturity based on a range of CHU maturity ratings.

Table 1: Approximate Corn Heat Unit (CHU) Requirements from Silking to Physiological Maturity for Various Hybrid Maturities.

CHU Rating of the Hybrid Approximate CHU Required from Silking to Physiological Maturity
2100 945 to 1050
2200 990 to 1100
2300 1035 to 1150
2400 1080 to 1200
2500 1125 to 1250
2600 1170 to 1300


Once a crop’s CHU requirement from silking to physiological maturity is determined, the next step is to establish the number of CHU that can reasonably be expected from the date of silking until the end of the season.  Referring to Tables 2 and 3 (where dates of expected additional CHU accumulation from two silking dates in the season), we can estimate the approximate date when a given accumulation of CHU past silking is reached.

For example, if the silking stage of a 2200 CHU hybrid grown near Morden occurred around July 18, the crop would require approximately 990 to 1100 CHU to go from silking to physiological maturity (see Table 1 and use 1100 for simplicity).  According to Table 2, the accumulation of 1100 CHU starting July 18 would occur by approximately September 5 in Morden.  It is important to keep in mind that these numbers are estimates based on historical observations.  Some years will have temperatures above or below average, causing the dates to shift forward or back.

Table 2: Date of Expected CHU Additional Accumulation from July 18 at Various Manitoba Locations (Source: Environment Canada averages 1971-2000).

From July 18 +900 +1000 +1100 +1200 +1300
Brandon 31-Aug 07-Sep 14-Sep 25-Sep 10-Oct
Elm Creek 28-Aug 03-Sep 09-Sep 17-Sep 27-Sep
Emerson 26-Aug 31-Aug 06-Sep 12-Sep 19-Sep
Morden 26-Aug 30-Aug 05-Sep 10-Sep 17-Sep
Portage 28-Aug 03-Sep 09-Sep 17-Sep 28-Sep
Selkirk 26-Aug 31-Aug 06-Sep 12-Sep 21-Sep
Starbuck 29-Aug 04-Sep 10-Sep 17-Sep 28-Sep
Steinbach 28-Aug 03-Sep 09-Sep 16-Sep 26-Sep


Table 3: Date of Expected CHU Additional Accumulation from July 25 at Various Manitoba Locations (Source: Environment Canada averages 1971-2000).

From July 25 +900 +1000 +1100 +1200 +1300
Brandon 12-Sep 22-Sep 05-Oct 01-Oct
Elm Creek 08-Sep 15-Sep 25-Sep 08-Oct
Emerson 05-Sep 11-Sep 18-Sep 28-Sep 12-Oct
Morden 04-Sep 10-Sep 17-Sep 26-Sep 09-Oct
Portage 08-Sep 16-Sep 26-Sep 12-Oct
Selkirk 05-Sep 12-Sep 20-Sep 01-Oct 24-Oct
Starbuck 08-Sep 16-Sep 25-Sep 09-Oct
Steinbach 08-Sep 15-Sep 24-Sep 08-Oct


Remember that this is only estimating time from silking to physiological maturity, not when harvest can start.  Field dry down rate from physiological maturity to start of harvest is influenced primarily by weather factors and, to a lesser degree, by hybrid characteristics.  In simple terms, warmer temperatures and lower humidity encourage rapid field drying of corn grain.  Because moisture loss is greatest just after physiological maturity, both because the weather is usually warmer and because wet kernels lose water more easily, it stands to reason that a corn crop that matures earlier in the season will dry down faster than a crop that matures later in the season.

Submitted by:  Pam de Rocquigny, Provincial Cereal Crops Specialist, MAFRD

For more information on corn production, please visit MAFRD’s webpage at
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Cooler Temperatures Prevailing So Far in 2014

In the June 16th Issue of the Manitoba Crop Report, it was noted that cooler than normal temperatures have slowed crop development slightly.  So how cool has it been in Manitoba to date?

Mike Wroblewski, MAFRD’s Ag Meteorologist who runs the Manitoba AgWeather Program, has created the following maps showing Accumulated Growing Degree Days (GDD) and Corn Heat Units (CHU) to date, as well as Percentage of Normal Accumulation for GDD and CHU, up to June 15th.

Accumulated Growing Degree Days (GDD) – May 1 to June 15, 2014

Percentage of Normal GDD – May 1 to June 15, 2014

Accumulated Corn Heat Units (CHU) – May 1 to June 15, 2014

Percentage of Normal CHU – May 1 to June 15, 2014

More information on the Manitoba Ag-Weather Program can be found at


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Yellowing Corn & Corn Heat Unit (CHU) Accumulation

Last week yellowing corn was being reported in the Eastern Region of Manitoba.  It is suspected that the yellowing is more likely associated with cooler temperatures more than lack of nutrients.   The return to warmer temperatures will go a long way in allowing those yellowing corn fields to resume normal growth and to latch onto the starter fertilizer that’s been applied.

How cool has it been?  Mike Wroblewski, MAFRI Ag Meteorologist, has generated Corn Heat Unit (CHU) accumulation maps showing total accumulation and percent of normal accumulation from May 1 to June 9, 2013.  As the maps illustrate, CHU accumulation is behind compared to the 30-year normal.  However, with that being said there is no need to worry at this early stage.  There remains a lot of growing season left to come and the return to warmer temperatures definitely is a good start! 

Total CHU Accumulation – May 1st to June 9th

Percent of Normal CHU Accumulation – May 1st to June 9th

For further information on MAFRI’s Ag-Weather Program, visit

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