Using Climatological Information for Corn Hybrid Selection (E2471)

Using Climatological Information for Corn Hybrid Selection (E2471)

Since production of corn hybrids became a major agricultural enterprise in Michigan, there has been a push to plant "full-season" hybrids.

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Since production of corn hybrids became a major agricultural enterprise in Michigan, there has been a push to plant “full-season” hybrids. If planted early, a full-season hybrid can take advantage of more of the growing season and thus produce higher yields. Recent experience with cool, wet springs and cool growing seasons (e.g., 1992) brings the advisability of planting full-season hybrids under question. Even if higher yields can be obtained, higher kernel moisture may cause unacceptable harvesting delays and/or excessive drying costs. The purposes of this publication are to:

  • Define full-, medium-, and short-season hybrids.
  • Explain growing degree-days and how they relate to corn hybrid maturity.
  • Present information on growing degree-day accumulation and dates of fall frost for various Michigan locations.

Growing degree-days (GDD) and frost dates can be used to help select corn hybrids for a particular location. Planting hybrids that are “out of synch” with climatological normal has certain consequences, which we will point out.

Which corn hybrid to plant is one of the most important decisions made each year in a corn management system. This bulletin will assist decision making based on the normal or average climate for your location. Corn hybrids may not differ much in the cost of seed, but selecting a hybrid with the proper maturity for your location can return huge dividends at harvest.

What is a full-season hybrid?

In general, a full-season hybrid is one that uses (or requires) the entire growing season available at a particular location and reaches physiological maturity before the first killing frost in the fall. Which varieties are full-season hybrids for a particular location will depend on air temperature patterns throughout the growing season. Variations in what is considered a full-season hybrid may also occur as a result of any factor that influences soil temperature early in the season, such as topography, residue cover, tillage system and drainage. For example, if a 110-day relative maturity (RM) hybrid is considered full-season for  a particular location under a fall moldboard plowed system, a 100- or 105-day RM hybrid may be considered full-season on the same field under a no-till system in which soil warming and spring corn planting are delayed because of heavy residue cover.

It is important to understand that the “days” designation used to identify a maturity a group of hybrids may not reflect the actual number of days that it takes for a specific hybrid to reach maturity. This classification is used to indicate when hybrids mature in relation to other hybrids of known mature. Mid-, short-, and ultra-short-season hybrids could be considered those that are approximately 5, 15, and 20 days RM, respectively, earlier than a full-season hybrid for a particular location. For example, a 95-day RM hybrid does not necessarily mature in 95 days. However, under similar climatic conditions and cultural practices (planting date, soil type, tillage, fertility, etc.), a 95-day RM hybrid should mature approximately 10 days earlier than a 105-day RM hybrid.

Table 1 gives approximate relative maturities for full- to ultra-short-season hybrids for four regions in Michigan. Adjustments would need to be made to take into consideration local climatic conditions (e.g., proximity to Great Lakes, topography, etc.) and cultural practices. These estimates are based on a planting date of early May.

Not all companies use the same standards for determining relative maturities of their hybrids. Though most seed companies categorize their corn hybrids according to relative maturity (days), another way to categorize hybrids is by growing degree-days. Your seed company representative can clarify that company’s practice. Following a discussion of growing degree-days, we will present information about the relationship between these two methods of categorizing corn hybrid maturity.

What are growing degree-days?

The growth rate of corn depends largely on the relative amount of heat in the environment in which it grows. Knowing this, we can easily monitor the development of a corn crop as well as select hybrids that are climatologically suitable for your area by learning how to calculate and use growing degree-day statistics. A growing degree-day unit (GDD) is a representative index of accumulated heat, normally derived from temperatures at a given location. GDD are calculated daily and summed for all or a portion of the growing season.

To calculate GDD for corn on a gizen day, take the day’s minimum temperature and, if it is lower than 50 degrees F, set it up to 50 degrees F. if the maximum temperature is higher than 86 degrees F, set it down to 86 degrees F. We do this because corn growth doesn’t begin until temperatures warm to about 50 degrees F and growth slows significantly at 86 degrees F and higher. Next, calculate the average of the day by dividing the sum of the maximum and the minimum by 2. Finally, subtract the base temperature of 50 from this average to get your GDD for the day.


Endless methods and reference temperatures may be used for calculating “degree-days”. Be very careful when using data from different sources that the methods are consistent – direct comparisons with other methods will likely not be possible. Most growers, businesses and scientists involved in corn production use the above methodology. Also, applying the numbers here to other crops that do not share the 50 degree F base temperature would be inappropriate.

Seasonal growing degree-day accumulation in Michigan

The growing season in a climate such as Michigan’s is normally defined as the number of days between the last occurrence of 32 degrees F in the spring and the first occurrence of 32 degrees F in the fall. For corn, this is somewhat misleading, because the date of planting, when GDD accumulation should begin, varies from year to year depending on field conditions, and because the corn crop may not be entirely killed by the first freezing temperatures of the fall. GDD accumulations are, therefore, presented for a range of possible planting dates in the spring through the normal first occurrence of 30 degrees F in the fall. For this report, 20 representative stations were chosen from corn-growing areas of Michigan for calculation of GDD statistics for the period 1961-1990 period was chosen to be consistent with internationally established climatic practices. Using such statistics is probably the only realistic way to plan for future weather trends because forecasting weather-dependent events and processes (such as GDD accumulations) more than several weeks in advance is nearly impossible in our region of the world.

First killing freeze statistics

The 50th percentile or normal dates of first fall killing freeze (defined above as the first occurrence of 30 degrees F or lower are given in Figure 2.

Spatial patterns in these dates are fairly evident: the earliest dates are generally found at interior locations away from the moderating influence of the Great Lakes, especially in northern sections of lower Michigan and western upper Michigan.

Additional first killing freeze statistics are listed in Table 2. In this and subsequent tables, seven numbers representing different percentages of first freeze statistics (Tables 3-7 deal with seasonal GDD accumulations) are given for each station. The numbers in the tables describe the range and frequency with which events have occurred in the past.


In Table 2, the 30th percentile first killing freeze at Allegan is given as October 4. This means that for the period 1961-1990, the first killing freeze occurred on or before October 4 in 30 percent of the years.

Given the normal date of the first killing freeze in the fall, we were able to calculate accumulated GDD on a seasonal basis. Because planting dates differ markedly from year to year, sets of statistics were developed at each location for five hypothetical planting dates: April 20, May 1, May 10, May 20, and June 1. Seasonal accumulation for each year ended on the date of normal first killing freeze at each location. Percentile statistics of seasonal GDD accumulation are given for the five planting dates in Tables 3-7. The 50th percentile or normal seasonal GDD totals are illustrated in Figures 3-7.

Patterns for seasonal GDD accumulation are similar to those for the first killing freeze data, with statewide maxima in the southwest and southeast corners of the Lower Peninsula and minima in interior areas of northern lower Michigan and the Upper Peninsula. In the Lower Peninsula, seasonal totals drop off rapidly north of a line from Muskegon to Saginaw. Seasonal totals decrease only slightly (approximately 60 to 90 GDD) from the April 20 planting date to May 1 or May 10 but fall rapidly thereafter (approximately 140 to 290 GDD less for the June 1 planting date totals vs. May 20), following the raped seasonal rise in temperatures during May. This underscores the importance of early planting in Michigan, as potential accumulated GDD lost by planting after early May become increasingly difficult to make up later in the season.

Reading and interpreting the tables

The percentile statistics given for the seasonal GDD tables are similar to those for the first killing freeze. The normal or 50th percentile value indicates that, climatologically, 50 percent of the seasons in the sample (in this case 15 out of a total of 30) had accumulations at or below that value, while the other 50 percent had greater values. Though the normal value is useful as a reference for future seasons, use of the other percentiles allows an estimated range of GDD seasonal totals with climatological odds.


Over the past 30 years, for a planting date of May 10 at Allegan, 2338 or more GDD accumulated in 95 percent of the seasons (see Table 5). In 5 percent of the seasons, 2764 or more accumulated. Another way of looking at this is that 90 percent (the total percentage of years included in the 5 to 95 percent range), or 9 out of 10 years, would experience GDD totals within the range of 2338 to 2764. The same holds true for other ranges, so this information allows you to objectively estimate a given range of GDD accumulation at your location with some idea of the probability of occurrence. If your location is not close to any of those given in the tables, you can still pick the values of interest at the closest two or three listed stations and carefully average among them, perhaps with greatest weight given to the closest station or the one most similar in characteristics to your location.

A word of caution

Because of the wide variety of soils and topographical differences and the influence of large lakes, microclimate may play a significant role in determining seasonal GDD totals at your location. For example, if your crop lies in a low-lying area prone to cool air drainage or lies on muck soil, or if the closest reference station to you is near the lake while you are located farther inland, you probably need to consider the totals given here as to high, because nights at your location may be cooler and the first killing freeze may come earlier in the fall than at the reference location. The reverse would be true of locations tending to be warmer than the reference locations given in the tables.

Finally (and maybe most importantly), because we are dealing with statistics and because climate is not static over long periods of time, all values listed in this bulletin (derived from 30 recent years of data) are not a forecast of GDD totals in a given season, nor are they intended to be used for the next several decades. These GDD totals are odds-based estimates derived from climate data of the past 30 years and represent what may be expected in a given season during the next decade or so.

Using GDD information to select corn hybrids

Some seed companies collect information on the required number of GDD for specific hybrids to reach specific stages of development (e.g., silking, black layer). If this information is available, you may use it to match the normal GDD accumulation for your location, given your anticipated planting date, with hybrid requirements. Normally, if a hybrid reaches physiological maturity before the first killing freeze in the fall, it will have time to dry down so it can be harvested as grain with a minimum need for artificial drying. It is also desirable for corn grown for silage to reach black layer (physiological maturity) by the time of the first killing freeze in the fall, but loss of silage dry matter or nutrient yield may not be significant even if the crop is frosted before physiological maturity. Therefore, it may be desirable to select for corn grain production in the same location. (For additional information on the effect of early fall freezing temperatures on corn yield, refer to Extension bulletin NCH-57, “Handling Corn Damaged by Autumn Frost.”)

It is common for seed companies to provide information on relative maturities (in days), and there may be times when you would like to estimate GDD requirements from this information. One of the best sources of information is your seed company technical representative. Table 8 provides estimates of the relationship between GDD requirements and relative maturity for corn hybrids.

When to switch hybrids in relation to planting date

The information on GDD accumulations at progressive planting dates can be used as a guide when deciding to switch to a shorter season hybrid as planting date is delayed. If, for example, you had planned to plant corn by May 1 in Caro, you can expect between 2335 and 2715 GDD to accumulate prior to the first killing frost in 9 out of 10 years (Table 4). In a “normal” year, you would expect approximately 2472 GDD to be available for corn growth. According to Table 8, a hybrid with a relative maturity of 90 to 95 days should have adequate GDD to mature before the first fall frost. If planting is delayed until May 20, however, the normal GDD available decreases to 2283 (Table 6). In this case, a 95-day RM hybrid may not mature before frost and you might consider switching to an 85-day RM hybrid (Table 8). The decision to switch to shorter season corn hybrids as planting date is delayed will depend on:

  • The extent of the delay.
  • Your particular location.
  • Availability of seed.
  • Your individual assessment of the odds.
  • Your desired or anticipated harvest schedule.

Remember, the tales and figures in this bulletin are long-term averages and likely will not correspond precisely to any individual year. However, they can be of great value if you are willing to manage your corn hybrid selection according to long-term average climate statistics for your location rather than according to your guess of what the next growing season is likely to offer.

Economic importance of selecting the proper hybrid maturity

The major tradeoff to consider when selecting corn hybrid maturities is yield vs. grain moisture. Full-season hybrids will normally yield more because they take advantage of the entire growing season’s GDD accumulation. However, because they reach physiological maturity later than mid- or short-season hybrids, they will likely be higher in grain moisture and may require drying.

Results from the Michigan State University corn hybrid comparison trials, 1988-1990, were used to estimate this yield-moisture tradeoff and to estimate the economic benefit or cost of planting a full-season hybrid. The years 1988-1990 were used because those were years more like the long-term normal than either 1991 or 1992.

Four hybrids tested in central lower Michigan, with an average RM of 112 days (full-season hybrids for that location), were compared with four hybrids tested in the same location but with an average RM of 101 days (considered mid-season hybrids). The full-season hybrids averaged 23 bushels per acre higher in yield and 5.5 percent higher in moisture at harvest. Even after drying costs were calculated (estimated at 2 cents/point), the full-season hybrids gave an $18.88 per acre advantage. In years that are not typical or normal, such a comparison would give different results. Planning for the average year is still the most reasonable approach, however. Given that approach, planting full-season hybrids for your location and planting date makes the most economic sense.

The drying charges of 2 cents per percentage point of moisture per wet bushel used in the comparison in Table 9 may not accurately represent your actual drying charges. It’s important to consider your actual cost of drying corn when selecting the hybrid maturity range for your farm. The difference in net income per acre between the full-season and the mid-season hybrids will decrease significantly as the cost of drying increases.


The GDD tables and figures in this bulletin should help you decide which corn hybrids to select for your farm with respect to maturity. The hybrids that you should consider full-season will depend on a number of factors, including:

  • Your location.
  • Your cultural practices.
  • Your planting date.
  • Your soil characteristics.

Once you have determined the normal GDD accumulation for your location, you can select hybrids that fit those normal and you can adjust hybrid selection if planting is delayed.

Though few individual years will look like the 30-year normal, it still makes the most economic sense to plan for the normal year and then make adjustments as possible if and when the normal does not occur.


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