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Make sense of hybrid maturity ratings.

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Several methods provide growers with a reasonable estimate of corn hybrids’ relative maturity, but differences among rating systems sometimes confuse growers. Are you familiar with each method?

Factors to Consider

  • Maturity
  • Minnesota relative maturity rating
  • Local growing conditions
  • Realtive maturity rating
  • Growing degree-days

Action Plan

  1. Define maturity. Kernels achieve maturity when maximum weight is reached at the end of the grain filling period. Kernel moisture content is usually between 25 percent and 35 percent at this point.
  2. Understand relative maturity (RM) rating. This traditional method compares hybrids near the time of harvest maturity, assuming that a hybrid will lose about one-half of a percentage point of moisture per day. RM ratings usually range from about 79 to 120. However, this figure offers little to northern growers at risk of fall frost because it does not reflect calendar time.
  3. Understand Minnesota relative maturity (MRM) rating. This method protects northern growers, ensuring that corn will likely reach its maturity within three days of the labeled RM classification. MRM ratings categorize hybrids into three relative maturity groups — full-season, mid-season and short-season — by comparing harvest grain moisture of a hybrid to the standard hybrids for each group, rounding ratings into five-day increments. Ratings typically fall between 85 and 115.
  4. Calculate growing degree-days (GDD). This method accumulates thermal values daily up to the formation of the kernel blacklayer, calculated by adding the day’s low and high temperature (50 and 86 are the default minimum and maximum values), dividing by two and then subtracting 50.

    • To demonstrate the equation, consider a day that has a low of 47 F and a high of 73 F would generate 11.5 GDD units.

    Commonly, short-season hybrids are grown to around 2,500 GDD units (corresponds roughly to 105 RM rating) whereas long-season hybrids require a little more than 2,800 GDD units (corresponds to about 115 RM rating). These hybrids include GDD units accumulated after planting. Bear in mind some companies calculate the requisite GDD units only after emergence. The difference is significant.


Growers who understand the various hybrid maturity rating methods are better prepared to purchase and grow hybrids for their geographic and operational needs. For more information, contact your local Mycogen Seeds customer agronomist or trusted agronomic adviser.

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