Corn Maturity and Heat Units
Relative maturity is an important criterion in the selecting of corn hybrids for your farm. Yields may be greater for later maturing hybrids than for earlier maturing hybrids in relatively warm years, but the situation may be reversed in relatively cold years. In addition, the lower bushel weight of later maturing hybrids in relatively cold years could result in lower grade corn. To complicate matters, hybrids of similar relative maturity may have slightly different developmental patterns; hybrids with similar grain moisture percentage at grain harvest can have different silking dates and, consequently, grain filling can occur at a later date. Can this phenomenon have an impact on yield and grain quality in relatively cold growing seasons if a killing frost occurs in early September?The effect of a killing frost on differences in grain yield and grain quality between hybrids of similar harvest maturity will be minimal if the frost occurs after half milk line (see below). Any assessment of the impact of corn maturity on yield and grain quality should be based on an appreciation of the developmental stages of corn and the effect of temperature on corn development.
Stages of Corn Development
The three most important stages of development in the determination of corn maturity are (1) silking, (2) physiological maturity, and (3) harvest maturity. The period between silking and physiological maturity is called the grain-filling period.Silking occurs usually in Ontario at the end of July; heat unit accumulation (see below) from planting to silking is about 50 - 55% of that for the period from planting to physiological maturity.
The grain-filling period. About 80% of the grain is deposited during the 3- to 4-week interval of the "linear period of grain filling", which starts about 16 days after silking. Rate of grain filling during this period per day is 3 - 4% of grain weight at maturity. This period occurs during the month of August. A killing frost during this period will result in grain of extremely poor quality, in addition to a substantial reduction in yield.
Physiological maturity (or black layer) usually occurs in Ontario during the second half of September or the first half of October; grain moisture percentage at physiological maturity is in the low 30s. Another measure of grain development is half milk line. Half milk line occurs when the grain moisture percentage is about 40% and grain has been filled for more than 90% of its potential. In Ontario, half milk line occurs usually in the early part of September when about 90% the seasonal heat units have been accumulated. This is the stage at which corn is harvested for silage. Consequently, yield differences due to a killing frost after half milk line among hybrids of similar relative maturity could be as high as 3 - 5%, but in most cases differences will be neglible. A frost occurring after physiological maturity will have no effect on either grain yield or grain quality.
Harvest maturity is a function of grain moisture percentage. Total grain yield and bushel weight have been fixed at physiological maturity. However, grain drying adds to the cost of corn production and, therefore, grain moisture at harvest is the dominant feature in assessing hybrid maturity.
Heat Units
Temperature is the most important among all environmental factors that influence rate of plant development. The temperature response of most metabolic processes in corn are similar: the general shape of the response curve for the extension of growth of shoots or roots, the appearance of new leaves in the whorl, and leaf photosynthesis, shows a minimum temperature of between 0 and 10°C, an optimum at approximately 30°C, and a maximum at about 45°C. For instance, the response of rate of leaf appearance in corn to temperature has an optimum temperature of 31°C and rate of leaf appearance is zero below 6°C and above 45°C (Fig. 1). Air temperature is used in quantifying the response of crop development to temperature. Growing-point temperature should be used although this is much more difficult to monitor. Differences between growing-point temperature and air temperature can be large when (i) growing point is below or close to the soil surface and the soil is wet and cold and (ii) a high rate evaporation and/or transpiration occurs at the growing point, resulting in a low plant-tissue temperature.
The importance of temperature in crop development has been recognized for a long time and several simple formulas have been developed to account for the effect of temperature in crop development (i.e, heat-unit systems). The two heat-unit system that have been used most frequently are the Growing Degree Day system (GDD) and Crop Heat Units (CHU). Recently, a new heat-unit system has been developed based on different responses of corn from planting to silking and from silking to maturity, the General Thermal Index (GTI). Finally, a methodology used in crop modeling that is called the Thermal Leaf Unit method (TLU) is based on the response of rate of leaf appearence to temperature. These heat unit systems were compared with calendar days for the corn hybrid Pioneer 3902 (2700 CHU relative maturity) grown at the Elora Research Station from planting to silking during 10 years in the period from 1987 to 1997 (table below). Results show that silking of Pioneer 3902 occurred between 69 and 88 days after planting, a 20-day range between the earliest and latest silking date during the 10-year period. Coefficient of variability (CV; standard error of the mean divided by the mean) is a measure of the reliability of the method to predict silking date (i.e., low CV means high reliability). The accuracy of predicting silking date using GDD or CHU was almost double compared to that of calendar days, but the accuracy of those two methodologies was somewhat lower than that of either GTI or TLU in this comparison.
| Method* | Mean | Range | Coefficient of variation (%) |
* Click on any of the heat-unit methods for a detailed description. Approximate conversions among various relative maturity systems are presented in a table at the bottom of this article.
Growing Degree Days (GDD)
GDDs are calculated for each day using the maximum daily temperature (Tmax), the minimum daily temperature (Tmin), and a base temperature (Tbase) as:
GDU = (Tmax + Tmin) / 2 - Tbase
The base temperature (Tbase), i.e., the temperature below which development is zero, varies among crop species. Tbase is between 6 and 10°C for corn. Sometimes the GDD equation is modified to include an upper limit (e.g., 30°C), beyond which development either remains constant or declines linearly with temperatures above the maximum temperature. The simple relationship between average daily temperature and GDD accumulation is depicted in Fig. 2.
Crop Heat Units (CHU)
CHUs are used in Ontario for field and horticultural crops (more information [137 KB]). Crop heat units are calculated separately for the day (CHUday) and for the night (CHUnight):
CHUday = 3.33 x (Tmax - 10) - 0.084 x (Tmax - 10)2
CHUnight = 1.8 x (Tmin - 4.4)
CHU = [CHUday + CHUnight] / 2
The relationship between daily temperature and CHU accumulation is depicted in Fig. 3. The CHU system suggests that the reponse of development to temperature differs between the day and the night, but there is no physiological basis for this assumption. Despite this flaw, the CHU system works well and is recognized around the world as one of the best methods to quantify the effect of temperature on crop development.
General Thermal Index (GTI)
GTIs are calculated from mean daily temperature (T) in a different manner for the period from planting to silking (FT(veg)) than for the period from silking to maturity (FT(fill)).
FT(veg) = 0.0432 T2 - 0.000894 T3
FT(fill) = 5.358 + 0.011178 T2
GTI = FT(veg) + FT(fill)
Hence, the GTI takes account for the apparent difference in response of corn development to temperature before and after silking. The two responses are depicted in Fig. 4
Other Environmental Factors
Other environmental factors can have a "temperature-like" response. For instance, rate of leaf appearance in maize will be lower when plants are exposed to low soil N, low soil-moisture, or low incident solar radiation. The effects of those environmental factors on rate of leaf appearance, however, are small. For instance, rate of leaf appearence declines by 50% when temperature is reduced from 20 to 15°C, a 2% decline on the absolute temperature scale (°K), but rate of leaf appearences declines only by 30% when N availability is reduced from superoptimal N levels to conditions of N starvation, a 97% decline in available N.
Relative Maturity Conversion Guidelines
General guidelines for conversions among various relative-maturity rating systems have been reported by Dwyer et al. (Agron. J. 91: 946-949). Conversions among CHU, GDD, and the Corn Relative Maturity rating system (CRM), also called the Minnesota Relative Maturity Rating, are listed in the table below. The CRM rating system is widely used in the US to characterize hybrid relative maturity. The CRM rating is not based on temperature, but rather the rating of a hybrid is based on the duration in days from planting to maturity (presumably in an average year somewhere in Minnesota) relative to a set of standard hybrids. The approximate conversion from one rating system to another can be estimated from simple linear regression equations of the form Y =a + b x X. For instance, the relative maturity of a 80-day CRM hybrid is approximately (776 + 23 x 80 =) 2620 CHU, and the relative maturity of a 3300-CHU hybrid is approximately (-33.7 + 0.043 x 3300 =) 108 days CRM.
Caution! These conversions are reasonably close only for season totals and when using GDDs calculated from temperatures expressed in degree Celcius. Many US data sets calculate GDDs from degree Fahrenheit, resulting in a number that is 1.8 x larger than that when using degree Celcius in the estimation of CHU or CRM from GDD (or 1.8 x smaller when estimating GGD from CHU or CRM).
| Y | X = CHU | X = GDD | X = CRM |
| CHU | -- | 769 + 1.77 x GDD | 776 + 23.0 x CRM |
| GDD | -436 + 0.567 x CHU | -- | -12.3 + 13.2 x CRM |
| CRM | -33.7 + 0.043 x CHU | 0.934 + 0.076 x GDD | -- |
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