If you order 105-day spaghetti squash for your garden from Rupp Seeds and plant the seeds on May 1, you can expect to harvest spaghetti squash on Aug. 13. In fact, Rupp’s guidelines give a maturity range from as early as Aug. 4 to as late as Aug. 24. Not all fruit will ripen the same day. Plus, weather conditions, including temperature and rainfall during the season, impact growth and maturity.

So, if you plant a 105-day corn hybrid on May 1, why can’t you expect to start shelling 25% corn on Aug. 13, 105 days later? Experience tells you that corn likely won’t be ready to harvest until mid-September, more or less.

That’s because numbers assigned to seed corn maturities don’t actually represent days from planting to harvest or from emergence to harvest. Because farmers think in terms of calendar days, the term “days” became loosely associated with maturity ratings, but it’s a misnomer, says Mark Jeschke, Pioneer agronomy manager.

“At Pioneer, CRM stands for comparative relative maturity,” Jeschke says. “It’s a system which allows us to compare one product to another in terms of maturity, but it doesn’t correspond to days on the calendar.”

Maturity methods vary

While CRM or RM numbers are useful if comparing hybrids within a single company, they aren’t meaningful when comparing Hybrid A from Company A to Hybrid B from Company B. That is because there is no standardized system used by all companies.

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“Numeric ratings seem less intuitive and sometimes almost mysterious,” quips Bob Nielsen, retired Purdue Extension corn specialist. “Some mystery arises because not all companies and agronomists define maturity the same way.”

To many agronomists, physiological maturity is when the black layer develops at the tip of the kernel, Nielsen says. Grain moisture at black layer typically ranges from 25% to 35% but can be 40%. Yet some refer to “harvest maturity,” when corn can be handled with less damage, which is usually around 25%.

Most companies also rate maturities based on heat units needed to mature, referring to growing degree days, or GDDs, or growing degree units, or GDUs. However, there are different ways to calculate GDDs, and again, no standardized system.

One saving grace is that when most companies assign CRM or RM values, there is a linear relationship, Nielsen says. For Pioneer, for example, Jeschke notes that 1 point difference in CRM means moisture on the same day at harvest will differ by about 0.5%. That’s equivalent to the average field-drying rate in mid-September per day. At silking, a one-day difference in CRM equals about 25 GDUs, Jeschke says.

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Using CRM, GDD information

Here’s how to use knowledge about hybrid maturity while avoiding pitfalls at the same time:

Spreading silking risk. Don’t rely just on CRM numbers, Jeschke says. You will need GDUs or GDDs to silking, which most companies list in agronomic information about hybrids. “Suppose Hybrid A and Hybrid B are both rated at 100 CRM, but Hybrid B silks 50 GDD units earlier,” Jeschke says. “At silking, that’s about two days earlier.”

Irrigation decisions. Hybrids that silk earlier usually require irrigation sooner if conditions warrant, Jeschke says. Knowing silking times can help plan irrigation scheduling.

Judging drydown. Nielsen offers this example: “I find two hybrids I like with nearly identical GDD ratings, but one is RM 113 and the other is RM 106. That implies both will reach kernel black layer at about the same time, but RM 106 apparently dries faster after maturity. If true, choosing RM 106 will save money in drying costs.”

Using GDUs. Now, flip the script. Suppose two hybrids are both rated at 111 CRM, but GDUs vary. “One hybrid requires 2,580 GDUs to physiological maturity, but the other requires 2,730,” Jeschke says. “At physiological maturity, 15 GDUs equals about one day, so they would reach physiological maturity about 10 days apart.”

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Late-planting scenario. Since GDD ratings are reasonably accurate predictors of how many GDDs are necessary to reach kernel black layer after planting, that knowledge can be useful if faced with late-planting or replant decisions, Nielsen says. For help on selecting a maturity that will likely mature before a killing freeze, see Hybrid Maturity Decisions for Delayed Planting.

Note that late-planted corn typically requires 200 fewer GDDs to mature than if planted on time, Nielsen adds. The only time that didn’t hold true was in 2019, when an unusually wet spring delayed planting, followed by a cool summer.

Scheduling harvest. If one hybrid rates at 100 CRM and another at 110 CRM and both are planted the same day, the 100 CRM should be 5 points drier at harvest, Jeschke says. “That’s based on 0.5% in-field drying on an average day in September,” he explains. “Other environmental factors could affect actual maturity in the field.”