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Published in Agron J 91:946-949 (1999)
© 1999 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
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Agronomy Journal 91:946-949 (1999)
© 1999 American Society of Agronomy

GRAIN AND OIL CROPS

Guidelines for Comparisons among Different Maize Maturity Rating Systems

Lianne M. Dwyera, Douglas W. Stewarta, L. Carriganb, B.L. Maa, P. Neavea and D. Balchina

a Agric. & Agri-Food Canada, Eastern Cereal & Oilseed Res. Ctr., Ottawa, Ontario, K1A 0C6, Canada
b Pioneer Hi-Bred Int., Plant Breeding Div., Willmar, MN, 56201 USA

dwyerl{at}em.agr.ca


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
Several systems are extensively used to rate maize (Zea mays L.) maturity in North America, including growing degree days, crop heat units, and Minnesota relative maturity rating days. Correspondence between the different systems varies with the temperature range of the environment, because of different bases of calculation and unit size. However, general guidelines for conversion from one system to another would aid communication among researchers, producers, distributors, and extension personnel, particularly where regional preference for one system exists, but information developed in another region is shared. Simple regression equations were developed from a data set with 4 years, 28 hybrids, and 19 locations (between 39° and 48° N lat). Coefficients of determination were >=0.91, indicating that these equations could be used as a general guideline to compare maturity ratings developed using the different systems.

Abbreviations: CHU, crop heat units • GDD, growing degree days • GTI, general thermal index • MRMR, Minnesota relative maturity rating


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 REFERENCES
 
RATING

of hybrid maize maturity and zonation of production areas in North America employ several systems, including growing degree days (GDD; Wang, 1960) calculated in either °C or °F, crop heat units (CHU; Brown and Bootsma, 1993) calculated in °C, and the Minnesota relative maturity rating (MRMR; Peterson and Hicks, 1973) expressed in days. The MRMR provides a relative ranking of the number of days a hybrid requires to reach maturity in relation to the time required by previously ranked hybrids. In practice, MRMR is often based on the grain moisture content of hybrids at harvest relative to previously ranked hybrids. Growing degree day and CHU requirements of hybrids have also been assigned relative to those of previously ranked hybrids, but these systems are based on an assumed quantitative response of maize development rate to temperature.

Growing degree days (GDD) required to reach maturity are calculated from

(1)
where TA is the average of daily maximum (Tmax) and minimum (Tmin) air temperatures, TB is a base temperature below which development is assumed to cease, m is date of planting, n is date of maturity, and t is a time step in days. In maize, TB for the entire period from planting to maturity is usually set at 10°C. In addition, temperatures below 10°C and above 30°C are assumed to be ineffective for development and Tmax values > 30°C are set to 30°C and Tmin values < 10°C are set to 10°C (e.g., Cross and Zuber, 1972; Gilmore and Rogers, 1958). Crop heat units required to reach maturity are calculated from the daily average of Ymin, representing nighttime temperature relationships

(2)
and Ymax, representing daytime temperature relationships

(3)

Recently, a new thermal index, termed the general thermal index (GTI), has been developed from fitted maize development temperature response functions for the vegetative and grain-filling periods with the same unit size as GDD (Stewart et al., 1998). Dwyer et al. (1999) determined that

(4)
with transition from the temperature function for the vegetative period (FT(veg))

(5)
to that for the grain-filling period (FT(fill))

(6)
set at Day 213 (1 August).

With the exception of GDD and GTI, which have equivalent units (1 GDD is equivalent to 1 GTI), each system uses different units based on different temperature response functions, making direct comparison difficult. Correspondence between the different systems will vary with the temperature range of the environment considered; however, guidelines to the general correspondence among systems would facilitate comparison of hybrid rating and thermal zonation developed in different regions. Schmidlin and Dethier (1987) correlated GDD and CHU calculated for a small number of stations in the northeastern USA, and DeGaetano et al. (1996) developed maps of GDD and CHU accumulations to specific growth stages in the northeastern USA and southeastern Canada. Our objective was to compare calculated GDD, CHU, GTI, and MRMR for a large group of maize hybrids grown at 19 North American locations from 39° to 48° N lat and to determine if there are quantitative relationships that can be the basis of guidelines for conversion from one system to another.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 REFERENCES
 
Twenty-eight Pioneer brand maize hybrids were grown at 19 locations in the north-central and northeastern USA and southern Ontario from 1992 to 1995 (Table 1) . Hybrids in this study were rated from 75 to 110 d in maturity based on the MRMR, but distribution of location-years is more heavily weighted to long-season hybrids (Fig. 1) . These hybrids, at locations from 39° to 48° N lat, represent maize production systems that depend on a thermal index for management decisions. Later-maturing hybrids grown further south are less limited by adequate thermal units to reach maturity. At each location, a randomized complete block design with two replications was used. Hybrids were planted at the recommended times and population densities. Since not all hybrids were adapted to all locations, not all 28 hybrids were grown at each location.


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  		Table 1 Sites in the 19-location, 4-year (1992–1995) data set

 


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  		Fig. 1 Distribution of location-year data as a function of the Minnesota relative maturity rating (MRMR) of each maize hybrid

 
Dates for planting, silking (50% of plants with silk), and maturity (50% of plants at 0% milk line) were recorded. Phenology dates were based on observations made a minimum of three times weekly on the center two rows of four-row plots. Daily maximum and minimum air temperatures were recorded from planting to maturity. Based on these measurements, approximately 400 estimates of maturity dates of all 28 hybrids were made using GDD, CHU, and GTI, and these individual location-year estimates were used to calculate the average number of heat units required by each hybrid over the four years and 19 locations. Minnesota relative maturity ratings for each hybrid were obtained from the Pioneer data base.

Simple linear regressions of mean calculated GDD (°C), GTI (°C), and CHU (°C) against MRMR (d) for each hybrid were run using the SAS regression procedure. Confidence intervals (95% level) around the regression line were calculated using the CLI option of the SAS regression procedure (SAS Inst., 1997). The GDD, GTI, and CHU were also regressed against each other to determine conversion factors for each pairing. As it was desirable that each rating system could be treated as either a dependent or independent variable, fitted lines were determined using the method of maximum likelihood where equal errors are assigned to each axis (in-house software; Bard, 1974). The SAS mixed procedure (SAS version 6.12; SAS Inst., 1997) calculated comparable coefficients.


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
REFERENCES
 
Coefficients of determination between GDD or GTI and MRMR were lowest (0.91 and 0.92), followed by correlation between CHU and MRMR (0.94); all other correlations were >=0.99 (Table 2) . Because MRMR is an estimate of relative maturity based on ranking of field observations, it cannot be calculated from temperature data and it has no associated variance. Table 3 indicates that hybrid ranking based on MRMR differed from that based on GDD, GTI or CHU. Pioneer hybrids 3563, 3861, 3845, 3893, 3905, 3921, 3963, and 3947 would be placed higher (i.e., later-maturing) in rankings based on GDD, GTI, or CHU than in the ranking based on MRMR. In contrast, there was only one minor discrepancy in ranking based on GDD or GTI compared with CHU. A limitation of MRMR is that, although it provides a simple estimate of relative maturity, it cannot account for variation in development rates resulting from different conditions in different location-years. In addition, MRMR cannot be used in comparing rate of development with significant intermediate growth stages (e.g., six-ligule stage, or anthesis), as the proportion of the total development period required for intermediate stages is not constant among hybrids and cannot be inferred from the total days to maturity (Stewart et al., 1998).


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  		Table 2 Fitted coefficients and their standard errors from simple linear regression (y = a + bx) and coefficients of determination (r2) for each pairing of systems. First three rows: the independent variable (MRMR) is set for each hybrid and has zero variance; remaining rows, the independent variable has associated variance (n = 28 for all comparisons).{dagger}

 

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  		Table 3 Maize hybrids in the 19-location, 4-year (1992–1995) data set

 
All systems compared here are widely used to characterize hybrid maturity and zonation requirements in North America. The fact that coefficients of determination (r2) of all linear regressions are >=0.91 and confidence intervals around the fitted lines are narrow (Fig. 2) suggests that the regressions developed will be useful in simplifying communication on relative maturity of maize hybrids and zonation of production areas. Because of the large number of data points and few coefficients, degrees of freedom remained high. Although all data were used in developing coefficients, Stewart et al. (1998) determined that under these conditions (large data set, few fitted coefficients), there is no difference in fit to dependent or independent data. However, testing of the regression equations on data sets representing additional hybrids and locations would demonstrate the generality of the relationships. The goodness of fit of the regressions suggests that these relationships can be reliably used as guidelines in comparing heat unit and maturity ratings in the northern USA and Canada.SAS Institute 1997



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  		Fig. 2 Fitted linear regression lines (solid line) with 95% confidence intervals (dotted lines) for mean calculated (a) growing degree days (°C), (b) crop heat units (°C), and (c) general thermal index units (°C) against the Minnesota relative maturity rating for maize. Fitted coefficients and coefficients of determination are listed in Table 2 (first three rows)

 

   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 
ECORC contribution No. 991412.

Received for publication October 16, 1998.
   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 REFERENCES
 




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