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Published online 16 January 2008
Published in Crop Sci 48:85-92 (2008)
© 2008 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Evaluation of Elite Exotic Maize Inbreds for Use in Temperate Breeding

Paul T. Nelson and Major M. Goodman*

Dep. of Crop Science, North Carolina State Univ., Box 7620, Raleigh, NC 27695

* Corresponding author (major_goodman{at}ncsu.edu).


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
While maize (Zea mays L.) is a highly diverse species, this diversity is not well represented in U.S. maize production acreage. Increased genetic diversity can be obtained through breeding with exotic germplasm, especially tropical-exotic sources. However, the pool of available tropical germplasm is large and diverse, making choices of tropical parents difficult. The maize breeding program at North Carolina State University has initiated a large-scale screening effort to evaluate elite exotic maize inbreds, most of which are tropical-exotic in origin. Here we report screening results for 88 inbreds obtained from various international breeding programs. These lines were tested in replicated yield trials in North Carolina as 50% exotic topcrosses by crossing them to a single-cross U.S. tester of stiff-stalk (SS) by non-stiff-stalk (NSS) origin. The more promising lines additionally entered 25% tropical topcrosses with SS and NSS testers and were further evaluated in yield-trials. A handful of tropical inbred lines—CML10, CML108, CML157Q, CML274, CML341, CML343, and CML373—performed well overall. It was further determined that topcrossing to a single SS by NSS tester will suffice for initial screening purposes, allowing for elimination of the poorest performing lines. Topcrossing to additional SS and NSS testers may be of value when determining where, in terms of heterotic patterns, the better-performing lines will fit into a breeding program.

Abbreviations: ARC, Agriculture Research Council • CIMMYT, International Maize and Wheat Improvement Center • IITA, International Institute of Tropical Agriculture • IRA, Institute of Agronomic Research • LAMP, Latin American Maize Project • LSD, least significant difference • NSS, non-stiff stalk • SS, stiff stalk


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication July 27, 2007.

Evaluation of Elite Exotic Maize Inbreds for Use in Temperate Breeding

Paul T. Nelson and Major M. Goodman*

Dep. of Crop Science, North Carolina State Univ., Box 7620, Raleigh, NC 27695

* Corresponding author (major_goodman{at}ncsu.edu).

While maize (Zea mays L.) is a highly diverse species, this diversity is not well represented in U.S. maize production acreage. Increased genetic diversity can be obtained through breeding with exotic germplasm, especially tropical-exotic sources. However, the pool of available tropical germplasm is large and diverse, making choices of tropical parents difficult. The maize breeding program at North Carolina State University has initiated a large-scale screening effort to evaluate elite exotic maize inbreds, most of which are tropical-exotic in origin. Here we report screening results for 88 inbreds obtained from various international breeding programs. These lines were tested in replicated yield trials in North Carolina as 50% exotic topcrosses by crossing them to a single-cross U.S. tester of stiff-stalk (SS) by non-stiff-stalk (NSS) origin. The more promising lines additionally entered 25% tropical topcrosses with SS and NSS testers and were further evaluated in yield-trials. A handful of tropical inbred lines—CML10, CML108, CML157Q, CML274, CML341, CML343, and CML373—performed well overall. It was further determined that topcrossing to a single SS by NSS tester will suffice for initial screening purposes, allowing for elimination of the poorest performing lines. Topcrossing to additional SS and NSS testers may be of value when determining where, in terms of heterotic patterns, the better-performing lines will fit into a breeding program.

Abbreviations: ARC, Agriculture Research Council • CIMMYT, International Maize and Wheat Improvement Center • IITA, International Institute of Tropical Agriculture • IRA, Institute of Agronomic Research • LAMP, Latin American Maize Project • LSD, least significant difference • NSS, non-stiff stalk • SS, stiff stalk


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
THE U.S. MAIZE (Zea mays L.) germplasm base is narrowing. Thus, the ability to adapt to emerging biotic and abiotic stresses is impaired, and there is increased potential for widespread crop failure, as exemplified by the 1970 southern corn leaf blight epidemic (Horsfall et al., 1972). For years, maize breeders have advocated breeding with tropical germplasm (Brown, 1953, 1975; Goodman, 1992, 2004; Lonnquist, 1974; Melhus, 1948; Stuber, 1978; Wellhausen, 1956, 1965), which is the most logical source of added genetic diversity. Currently, however, very little tropical germplasm is represented in U.S. maize breeding programs (Goodman, 1999).

Many elite tropical lines are publicly available from maize breeding programs in the tropics (Nelson et al., 2006). One reason for the underuse of this germplasm resource is the lack of available data on relative line performance. Breeders have little information on which to base parental choices. Breeding with exotic germplasm in a temperate environment is difficult, and various temperate breeding programs have largely failed in their efforts to use tropical germplasm because their choice of an exotic collection has often been more or less random (Lonnquist, 1974; Stuber, 1978). Thus, there is a need for performance data on tropical inbred lines if they are to be successfully used in temperate maize breeding.

Various studies have already been conducted that provide useful information about tropical germplasm. Melhus (1948) crossed multiple native Guatemalan collections with U.S. inbreds and evaluated them in Guatemala. Wellhausen (1956) made exotic intra- and interracial crosses and evaluated them in Mexico. Stuber (1978) screened exotic maize races from Latin America in North Carolina. The Latin American Maize Project (LAMP) evaluated more than 12,000 tropical exotic accessions in a 5-yr cooperative public–private effort (Salhuana et al., 1991). These screening trials were performed in the tropics and led to the identification of many potentially useful tropical accessions (Salhuana et al., 1998). Castillo-Gonzalez and Goodman (1989), Holland and Goodman (1995), and Mickelson et al. (2001) performed screening trials of tropical accessions and improved populations in temperate environments. Whitehead et al. (2006) incorporated elite tropical maize lines into elite temperate germplasm and evaluated the resulting populations in Iowa. These studies provide information on tropical accessions, landraces, improved populations, and tropical x temperate populations; however, they shed little light on elite tropical inbreds. Han et al. (1991), Vasal et al. (1992a,b), Hede et al. (1999), and Menkir et al. (2004) evaluated elite tropical inbred maize lines for combining ability and agronomic performance in tropical environments. While these studies do provide direct comparative data on elite inbred line performance in the tropics, results from such studies usually do not directly translate to temperate U.S. environments. Aside from work done by Nelson et al. (2006), there have not been any published studies that have reported results from comparative yield trial screenings of elite tropical maize inbreds in a temperate environment.

Here we report the results from a screening study, similar to that by done by Nelson et al. (2006). Eighty-eight elite exotic maize inbred lines were topcrossed to U.S. testers and evaluated in replicate yield trials in North Carolina. Our objective was to identify elite exotic maize inbreds that can be used in temperate maize breeding. The use of such material will aid in broadening the U.S. maize germplasm base by introducing novel alleles for yield potential, disease resistance, and other agronomic characteristics. The yield data generated in this study will provide temperate maize breeders with a comprehensive resource for selecting exotic germplasm. Breeding with exotic germplasm is a costly and time-consuming endeavor, the success of which is largely determined by the choice of germplasm (Goodman, 1992).


   MATERIALS AND METHODS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Germplasm Selection
We chose 88 inbred lines, most of tropical origin, for screening. Many of these lines were identified through unpublished sources, including conference talks, poster sessions, and word of mouth. Table 1 lists each line and its country and breeding program of origin. Sixty-five of the lines were developed by the International Maize and Wheat Improvement Center (CIMMYT) in Mexico (Srinivasan, 2001). Eight lines are of temperate exotic origin, developed at the University of Novi Sad in Serbia. Seven were developed in Cameroon through joint cooperation of the International Institute of Tropical Agriculture (IITA) and the Cameroon Institute of Agronomic Research (IRA) (Everett et al., 1994a, 1994b). Four were developed by Hans Gevers at the Agricultural Research Council's (ARC) Grain Crop Institute in South Africa. Three were developed by IITA in Nigeria. One line included in the study, an NC296A derivative, is an all-tropical line that was developed at North Carolina State University but that underwent a gametophyte factor conversion attempt at Novi Sad University, Serbia. All of the lines in this study except the seven from Serbia are tropical in origin; however, for this study all lines will be referred to as "exotic."


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  		Table 1. Experimental lines and origin.{dagger}

 

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  		Table 2. Yields for 88 experimental entries and 13 commercial checks. Standard errors (SE) of least squares yield estimates are included since data are from varying numbers of environments.

 
The exotic inbreds in this study were included for the following reasons. First, the lines are exotic from a U.S. perspective, containing many alleles not found in the U.S. maize germplasm base. Second, unlike accessions and unimproved landraces that have been the focus of many studies with exotic germplasm (Castillo-Gonzalez and Goodman, 1989; Holland and Goodman, 1995; Melhus, 1948; Salhuana et al., 1998; Stuber, 1978; Wellhausen, 1965), these lines have already been subjected to multiple cycles of inbreeding and selection. Third, these lines are considered elite in their countries of origin, showing good yield potential, disease resistance, and overall favorable agronomic performance. Fourth, they are all publicly available and the seed for most can be obtained either from the breeding program of origin (Table 1) or from the North Central Regional Plant Introduction Station in Ames, IA.

Yield Trial Evaluation
Yield trial evaluations were performed from 2001 to 2005. Each exotic line was initially crossed to a temperate U.S. tester, LH132 x LH51 (Holden's Foundation Seed, Williamsburg, IA), an improved B73 x Mo17 hybrid of stiff-stalk (SS) x non-stiff-stalk (NSS) origin. The resulting 50% exotic topcrosses were then evaluated in replicated yield trials. Thirteen commercial hybrid checks, representing a range of maturities grown in North Carolina at the time the study was conducted, were also included in the study. Following each year of testing, the poorest performing lines were eliminated from the study. In general, experimental entries had to be within one least- significant difference (LSD) of the mean check yield to remain in the study. Lines that remained after their first 2 yr of screening then entered two modified three-way topcrosses, each resulting in a 25% exotic topcross. In the first of these topcrosses, the exotic line was crossed to a line of SS origin and then crossed to a NSS x NSS sister-line tester. The SS line used was NC374, and the NSS x NSS sister-line tester was FR615 x FR697 (Illinois Foundation Seeds, Champaign, IL). In the second of these modified three-way topcrosses, each exotic line was crossed to a line of NSS origin and then crossed to a SS x SS sister-line tester. The NSS line used was either NC382, NC414, or NC418. NC382 and NC418 are sister lines and also share a common pedigree with NC414. The SS x SS sister-line tester used was FR992 x FR1064 (Illinois Foundation Seed). Therefore, by its third year of testing, an exotic line was represented in yield trials by three topcrosses, one 50% exotic topcross and two 25% exotic topcrosses. These three topcrosses were then tested together for the remainder of the study. Because inbreds 326172w, 326633A, and 327609A are of known SS origin, these lines were crossed to FR615 x FR697 for initial screening.

The number of entries included in any single year of testing varied because newly acquired exotic lines were continually being added to the study and poorly performing lines were continually being eliminated. Thus, the experimental design was unbalanced across years. The experimental design within any given year was balanced across environments.

Yield trial evaluations were conducted over 5 yr at five North Carolina locations: the Central Crops Research Station in Clayton, the Peanut Belt Research Station in Lewiston, the Tidewater Research Station in Plymouth, the Sandhills Research Station in Jackson Springs, and the Caswell Research Station in Kinston. These locations were used because they represent the primary maize production regions in North Carolina. Yield trial data were collected at Clayton and Lewiston all 5 yr. Plots were grown at Plymouth all 5 yr, but yield trial data from this location was not used in 2003 or 2004 because of hurricane damage. Plots were grown at Sandhills for 4 of the 5 yr, 2003 being the exception. Yield trial data were collected at Kinston in 2004 only. Thus, 18 North Carolina environments were represented in the study.

Two replications were planted in a double lattice design at all 2001 and 2002 environments and at Clayton, 2004. All other environments had three replications planted in a triple lattice design. All plots were two rows, 4.88 m in length measured from the center of the alley, with 1-m alleys, and row spacing of 96.5 cm at all locations except Lewiston, where row spacing was 91.4 cm. Plots were planted with 44 seeds plot–1 with a target plant density of 43,000 plants ha–1 at all locations except Lewiston, where target plant density was 45,000 plants ha–1. Data reported here are limited to yield, grain moisture at harvest, ear height, plant height, percentage erect plants at harvest, and days to anthesis. Days to anthesis was recorded at Clayton only; all other data were collected at all locations.

Data Analysis
Statistical analysis was done using SAS version 9.1.3 (SAS Institute, 2003). Stand was fitted as a covariate (p < 0.001) in the analysis of yield data from Lewiston, 2003, and Sandhills, 2005. Stands at these environments were poor due to nongenotypic effects: wet cool weather after planting at Lewiston and bird damage at Sandhills. Where informative, various methods of spatial analysis, as described by Brownie et al. (1993), were also used in the data analysis within environments.

Entry means across environments were obtained in two ways. First, entry means from all 18 environments were included in a mixed model analysis using PROC MIXED in SAS despite the unbalanced nature of the experiment across years. Through this analysis, least squares entry means for all 88 exotic lines on the SS x NSS tester and for the 13 checks were obtained. However, because of the unequal representation of entries across environments (not every entry was grown in every environment; few were grown in all 18 environments), entry means using this method are calculated with varying levels of precision. For this reason, standard errors were calculated for values obtained through this analysis (Table 2). Second, entry means from individual environments were used to perform across environment analysis for a balanced subset of entries that were grown together in 10 environments from 2003 to 2005. By maintaining a balanced experimental design, entry means were estimated with equal precision, thus allowing more appropriate comparisons between entries. For this analysis, a protected LSD was generated for pairwise comparisons between experimental entries and the mean of the commercial checks (Table 3 ).


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  		Table 3. 25% and 50% exotic entry means from 10 environments. Data are given for yield, grain moisture, ear height, plant height, percent erect plants at harvest, and days to anthesis.

 
Line x tester interaction for exotic line yield on each of the three testers was evaluated using the SLICE option with the LSMEANS statement with PROC GLM in SAS. The SLICE option partitions interaction LSMEANS effects and produces tests of simple effects (SAS Institute, 2003). For this procedure, yields were expressed as a deviation from the mean yield on the respective tester (Table 4 ).


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  		Table 4. F values and significance levels of line x tester interactions expressed as deviations from the mean. Tests are based on data from the number of environments given.

 

   RESULTS AND DISCUSSION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Entry Performance
In all analyses, F values for the null hypothesis of no differences between entries for the given trait were highly significant (p < 0.01). Mean yields for each of the 88 lines on the SS x NSS tester, LH132 x LH51, are given in Table 2. The CML343 and 89291 topcrosses were the highest yielding at 7.7 Mg ha–1 each. Other superior topcrosses were the NC296A, CML274, CML341, CML108, CML157Q, and CML373 topcrosses.

Many of the entries in Table 2 were grown in only a few environments because they either (i) performed poorly in the initial years of testing and were thus dropped from the experiment or (ii) were new additions to the experiment in the latter years of testing. The CML48 topcross, for example, was only tested in four environments because its performance did not merit further testing; however, the 89291 topcross, although it showed superior performance, was also tested in only four environments simply because it was a new addition to the experiment in 2005, the final year of testing. Several lines that fit either of these two categories may merit additional attention, including inbred lines 89291, 89199, C70, CML142, CML184, and CML273.

Entry means for a subset of the better-performing entries are given in Table 3. This subset includes data on the six traits measured from 10 environments from 2003 to 2005. Several experimental entries did not differ significantly from the mean of the checks. Among the 50% exotic entries in Table 3, the CML343 topcross was the highest yielding (7.7 Mg ha–1), followed by the CML274 (7.4 Mg ha–1), CML157Q (7.3 Mg ha–1), CML373 (7.2 Mg ha–1), and CML108 (7.2 Mg ha–1) topcrosses. The CML103, CML108, and CML91 topcrosses were the driest, with grain moistures around 170 g kg–1 at harvest. The CML154Q, CML108, and CML69 topcrosses were the earliest-maturing entries, all flowering at about the same time as the checks. The CML108 topcross was within one LSD of the check mean for all traits except yield and moisture, although its performance for these two traits was also quite good.

Among the 25% exotic entries, the two CML341 and the two CML10 topcrosses were the four highest yielding. Each was within one LSD of the mean of the checks, the highest yielding being the CML341.NC374 topcross (7.7 Mg ha–1). More than half of the 25% exotic entries had moistures within one LSD of the check mean (165 g kg–1), and all but one were as dry or drier than the wettest check, Garst 8288 (17.2%). The latest-maturing experimental entry, the CML10.NC374 topcross, still flowered within 2 d of the check mean. The CML10.NC414 topcross was within one LSD of the check mean for all traits except moisture.

Data on additional subsets of entries including individual year means can be found in Nelson (2006).

Line x Tester Interaction
Some lines exhibited significant line x tester interaction. For example, in 2005 the CML10 topcrosses with the SS and NSS testers ranked 9th and 10th respectively for yield, yet the CML10 topcross on the SS x NSS tester ranked 64th overall (data not shown). Table 4 contains F values and significance levels of line x tester interactions for yield for 19 of the exotic lines. For investigating line x tester interaction, yields were expressed as deviations from the mean yield of all lines on the respective tester. Only three lines showed significant line x tester interactions: CML10, CML269, and CML274. CML10 and CML269 showed superior performance on the SS tester; CML274 showed superior performance on the SS x NSS tester.

Line x tester interactions were not significant for most of the lines in Table 4. These results support conclusions by Nelson et al. (2006) that for initial screening purposes, tropical maize inbreds can be screened on a SS x NSS tester only, followed by screening of the better-performing lines on SS and NSS testers independently. This approach provides an indication of relative line performance while minimizing the resources required for screening large numbers of tropical lines.


   CONCLUSIONS
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Most of today's elite U.S. hybrids derive from a small pool of inbreds that were developed almost half a century ago (Goodman, 1992; Smith, 1988; Troyer, 1999; Mikel and Dudley, 2006). While maize yields in the United States continue to improve (USDA-NASS, 2006), and the long-foretold yield plateau (Wellhausen, 1956) has not yet been reached, there is little evidence that the United States has a monopoly on yield genes (Goodman, 1992). The results presented here certainly indicate that there is yield potential outside the U.S. Corn Belt. A handful of 50% exotic topcrosses presented here rivaled the check mean in yield performance. Seven lines stand out across analyses for yield performance: CML10, CML108, CML157Q, CML274, CML341, CML343, and CML373. The CML341 topcrosses were the most consistently high yielding entries across testers. The CML108 topcross consistently exhibited superior performance across all of the traits measured.

Breeders who are working with tropical-exotic germplasm face numerous challenges, namely photoperiod sensitivity, disease susceptibility, and weak roots and stocks (Holland and Goodman, 1995). The magnitude of these effects can be minimized by selecting tropical-exotic parents that are more easily adapted to temperate U.S. environments. The maize breeding program at North Carolina State University has already begun breeding with many of the lines presented in this study. These lines are being used in both exotic x temperate and exotic x exotic breeding crosses and populations. The lines presented here, in conjunction with lines presented by Nelson et al. (2006), provide temperate breeders with a sizable pool of potentially useful exotic maize inbred lines. These lines certainly deserve further attention in temperate breeding efforts.

All rights reserved. No part of this periodical may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or any information storage and retrieval system, without permission in writing from the publisher. Permission for printing and for reprinting the material contained herein has been obtained by the publisher.

Received for publication July 27, 2007.


   REFERENCES
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 





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