Tips to Compare Corn Maturity

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By Bill Mahanna and Ev Thomas

Corn hybrid maturity ratings help growers select and compare hybrids, manage agronomic risk and spread harvest timing. What is often misunderstood by growers is that there is no industry standard for these ratings, so comparing hybrids between companies can be challenging.

Hybrids are typically rated for CRM (comparative relative maturity) or RM (relative maturity). In the U.S., this is reported in calendar days such as a 105-day hybrid. In Canada, hybrid maturity is reported as CHU or corn heat units such as 3,100 CHU. Multiplying the day-length of a U.S. hybrid by 30 will approximate the CHU rating (100-day corn CR M roughly equals 3,000 CHU hybrid).

In-house comparisons
The most important word in the CRM acronym is “relative” because they are based on comparisons within each seed company’s own hybrids, not necessarily against competitive hybrids. For many companies, assigning a new grain hybrid CRM involves comparing grain harvest maturity (20 to 22 percent kernel moisture) to other hybrids in the company lineup.

This overall grain CRM is a function of when the plant reaches physiological maturity (blacklayer or zero kernel milk line) and the dry-down characteristics of the hybrid. With this approach, growers have a “relative” idea of how hybrids from the same company will advance through the various reproductive stages. Some companies also report silage CRM based either on comparing whole plant moistures to known silage check hybrids or regressing grain data to a silage kernel maturity standard.

Some benchmarking occurs
Seed companies may conduct research trials comparing their approach to assigning maturity to competitors and make subtle adjustments to how they are reporting hybrid maturities. For example, if a seed company observes a disadvantage in harvested grain moisture levels, they will want to be sure they are aligning their maturity ratings as closely as possible to key competitors.

It is important to read individual seed company footnotes to clearly understand the rating definitions. A grain hybrid will typically be given an overall CR M, a silking CR M and physiological CR M (blacklayer or zero milk line).

Physiological CRM can be particularly important for growers harvesting high-moisture corn or snaplage. Hybrids within the same genetic family, but containing different technology traits, will often be assigned the same maturity. However, depending upon level of insect infestation, these hybrids may differ by two to three days in maturity.

Most seed companies also report average GDU (growing degree units) to silking and GDU to physiological maturity (kernels 30 to 34 percent moisture). There are different methods of calculating GDU heat unit accumulation, but the most common is the Base 50 method. It is calculated as [(minimum temperature ≥ 50°F + maximum temperature ≤ 86°F)/2] – 50 reflecting that corn growth is most active between 50°F and 86°F.

For example, when daily high is 86°F and daily low is 65°F, the GDU accumulation for that day = (86 + 65/2 ) – 50 = 25.5. The Canadian CHU equation is [1.8 x (daily minimum temperature in °C – 4.4) + 3.3 x (daily maximum temperature in °C – 10) – 0.084 x (daily maximum temperature in °C – 10)²]/2.2. As with GDU, this calculation assumes limited corn growth with temperatures below 4.4°C or above 30°C.

Just like relative maturity ratings, GDU are also difficult to compare across seed companies. This is because of the use of different formulas, the calculation does not account for how long the maximum or minimum temperatures were held and the location of the research stations.

The best indicator
That said, GDU to physiological maturity (black layer) is probably the best overall indicator to determine if a hybrid can mature for grain harvest based on comparisons with longterm GDU accumulation records for that particular area. The length of time for the grain to dry down to 20 to 22 percent harvest moisture can further vary by hybrid dry-down tendencies (also typically given a relative score in seed catalogs).

Some growers like to reduce risk by spreading the pollination period between hybrids. However, planting hybrids with different overall CRM ratings such as 105-day corn may not always provide the desired effect because they could both have similar GDU to silking values. It is best to consult GDU to silk ratings to see the relative difference in timing of pollen shed and silk emergence.

To help determine if a new hybrid will adapt to local conditions, compare the silk rating to a well-known hybrid (from the same company). Research shows that earlier-silking hybrids generally move north of their adapted zone and more readily adapt to higher elevations. If moved too far north or up in elevation, late-silking hybrids may not reach physiological maturity before first frost or may have reduced grain yield potential if abnormally late silking exposes the crop to cooler temperatures during grain fill. Again, it is difficult to compare GDU to silk between companies.

The best source of information on hybrid maturity is the local seed sales professional or company agronomist. They will certainly know their hybrid lineup and have likely seen competitors in various plots to help put company differences in some perspective.