Published online 25 April 2006
Published in Crop Sci 46:1346-1353 (2006)
© 2006 Crop Science Society of America
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CROP BREEDING & GENETICS
Biplot Analysis of Forage Combining Ability in Maize Landraces
Luis Bertoia*,
César López and
Ruggero Burak
Dep. of Agronomy, Universidad Nacional de Lomas de Zamora, Camino de Cintura km. 2, (1832) Lomas de Zamora, Prov. de Buenos Aires, Argentina
* Corresponding author (bertoia{at}agrarias.unlz.edu.ar)
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ABSTRACT
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In Argentina, maize (Zea mays L.) hybrids selected for silage production and local adaptation are not available for most temperate regions where silage maize can be produced. This study sought maize landraces that could be used as germplasm sources to enhance forage yield and quality in warm temperate areas and emphasized eight landraces not previously selected for grain yield. These were crossed in a diallel mating design and the 28 F1 population hybrids, the eight parental populations, and four commercial hybrids were evaluated in four environments in Argentina for combining abilities and to determine heterotic patterns among germplasm sources, using a biplot diallel analysis. Significant midparent heterosis (MPH) was observed for ear yield (EY), stover yield (SY), and whole plant yield (WY). General combining ability (GCA) effects were significant for EY, SY, and WY. Specific combining ability (SCA) effects were not significant for any trait. The graphical representation offered by biplot analysis allowed a rapid and effective overview of GCA and SCA effects of the populations, their performance in crosses, as well as grouping patterns of similar genotypes. Significant variation among checks was observed for EY and SY. On average, commercial hybrids had greater EY, but lower SY, than landraces and population hybrids. Some landrace crosses showed superior or similar WY than commercial checks, indicating the breeding potential of the evaluated germplasm.
Abbreviations: ATC, average tester coordinate • ED, in vitro digestibility of ear dry matter • EY, ear dry matter yield • GCA, general combining ability • GGE, genotype main effect plus genotype x environment interaction • INTA, Instituto Nacional de Tecnología Agropecuaria • LAMP, Latin American Maize Project • MPH, midparent heterosis • NIRS, near infrared reflectance spectroscopy • PC, principal component • PC1, first principal component • PC2, second principal component • SCA, specific combining ability • SD, in vitro digestibility of stover dry matter • SY, stover dry matter yield • WD, in vitro digestibility of whole-plant dry matter • WY, whole-plant dry matter yield
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INTRODUCTION
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ENSILING MAIZE stabilizes forage availability throughout the year, since it permits a supply of good quality forage when seasonal production is low. Although the whole maize plant (ears plus stover) is used for silage, most hybrids utilized for silage were selected for grain production alone. Several authors have emphasized the importance of improving the vegetative component in silage (Argillier et al., 1995; Barrière and Traineau, 1986; Dhillon et al., 1990), and if this is important, two constraints must be considered. First, most temperate breeding programs are based on the use of the Stiff Stalk x non–Stiff Stalk heterotic pattern within the Corn Belt Dent race, which has already undergone several cycles of selection primarily for improved grain yield. Second, breeding efforts devoted to improving maize for forage production were made mainly with early maturing genotypes not suitable for warm temperate or subtropical regions. Because of these issues, we believe that exotic germplasm should be considered in breeding programs for which the objective is selection of genotypes for forage production.
An appropriate starting point in such a program could be the evaluation of accessions in germplasm banks that have undergone little or no selection for grain production. Many authors have emphasized the importance of broadening the genetic variability on which most maize breeding programs are based (Goodman et al., 1988; Wilkes, 1993). A result of this concern is the Latin American Maize Project (LAMP), which has cooperatively evaluated nearly 15 000 Latin American landraces (LAMP, 1991; Taba, 1994; Salhuana and Sevilla, 1995). Even where landraces have been used as a source of inbred lines in the past, their exploitation has generally been limited (Smith, 1988).
Little attention has been paid to the introduction of exotic germplasm for the production of forage maize, despite some promising initial results. Thompson (1968) found that a group of exotic and semi-exotic populations yielded on average 28% more digestible dry matter than adapted hybrids, and Stuber (1986) suggested that some semi-exotic materials might be suitable for silage, given their good grain production and great vegetative development. More recently, Bertoia (2001) noted that landraces with no history of breeding for grain production generated crosses with good forage potential, whereas inbred lines from the North American Corn Belt did not demonstrate potential for enhanced stover yield and quality when compared with inbred lines from Argentine germplasm (Bertoia et al., 2002).
The races from which most current commercial maize lines were developed represent a very restricted sample of the genetic variability available within the species (Hallauer, 1990), and landraces from tropical and subtropical areas may harbor new favorable alleles that are lacking in more elite temperate germplasm. For hybrid breeding programs, evaluation requires determination of both combining ability and per se performance. Abel and Pollak (1991) suggested that the effective value of a germplasm depends on its heterotic response with other genotypes. Thoughtful utilization of local germplasm accessions and the search for alternative heterotic patterns are therefore important research objectives. Exploitation of germplasm from non–Corn Belt Dent racial backgrounds in hybrid breeding programs will be facilitated by the identification of heterotic patterns between germplasm sources.
Diallel crosses have been widely used to determine heterotic responses and heterotic patterns in maize populations (Hallauer and Miranda Filho, 1981), with conventional diallel analysis limited to partitioning total variation into GCA and SCA. Recently, Yan and Hunt (2002) suggested the application of principal component biplot techniques to diallels. In the original application, the first two principal components of the data matrix obtained from a multienvironment trial were used to display genotype effects and genotype x environment interactions in a single two-dimensional figure. By analogy to multi-environment trials, the first two principal components of a diallel data matrix can be extracted and used to display GCA and SCA effects.
Based on results of previous evaluations of germplasm accessions in Argentina, and in some instances in cooperation with LAMP (Ferrer and Solari, personal communication, 1997), eight landraces from Argentina were selected and crossed following a diallel mating scheme. The objectives of this study were: (i) to evaluate the performance of the eight selected landraces, (ii) to determine their genetic potential as sources of germplasm for a forage maize–breeding program, and (iii) to identify heterotic groups among populations with different geographic origins by principal component biplot analysis.
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MATERIALS AND METHODS
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Eight maize landraces were selected for their agronomic response, differences in geographic origin, maturity, and grain type (Table 1). Seeds were supplied by the Maize Germplasm Bank at INTA Pergamino, Argentina. Landraces were crossed following a diallel mating design without reciprocals. Crosses were performed in eight isolation blocks. In each isolation block, one population was used as the male and the other seven populations were detasseled and used as females. At least 150 ears per cross were obtained. Landraces per se, the 28 F1 crosses, and four commercial check hybrids (Cargill Semiden 5, Dekalb 4F37, Morgan 369, and Syngenta Pucará, selected for grain production but widely used for forage production in Argentina) were evaluated during two growing seasons (1997–1998 and 1998–1999) at Esteban Echeverría (34°38' S, 58°48' W) and Vicente Casares (35°18' S, 58°56' W) in the Buenos Aires Province dairy region. Soils are typical Argiudoll (Vicente Casares) and Aquic Argiudoll with silty clay loam and B2t horizon (Esteban Echeverría), respectively.
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Table 1. Population name, germplasm bank code, geographic origin, race, grain type, and plant height of maize populations evaluated in diallel crosses in two environments during 1997–1998 and 1998–1999 growing seasons.
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The experimental design was a randomized complete block with three replications within each environment. Experimental units consisted of two 5.20-m rows, spaced 0.70 m apart. Plots were over-planted at 52 seeds per row, then thinned to a density equivalent to 71 500 plants ha–1 at the three-leaf stage. Each experimental unit was harvested by hand when the kernel milk line in approximately 50% of the plants reached two-thirds of the way down the kernels at the central part of the ear (Hunt et al., 1989). Ear and stover were separated and weighed fresh. A representative sample of each plant component was taken, weighed fresh, and dried with dry forced air, then weighed dry to permit estimation of dry matter percentage. Dried samples were milled to 1-mm particle size and analyzed with near-infrared reflectance spectroscopy. Near infrared spectra between 1100 and 2500 nm at every 2 nm were collected on all milled samples using an NIRS 6500 spectrophotometer (NIRSystem Inc., Silver Spring, MD). In vitro dry matter digestibility of ear (ED) and stover (SD) were predicted by NIRS equations, which were calibrated by the enzymatic method (Gabrielsen, 1986). Whole-plant dry matter digestibility (WD) was computed as the sum of ED and SD.
Statistical Analyses and Mathematical Model for GGE Biplot
Analyses of variance were performed for each variable, using a mixed model (McIntosh, 1983), where replications, environments, and genotype x environment interactions were considered random effects.
Applying GGE biplot methods to diallel data, the terms "average yield" and "stability" of the genotypes correspond to GCA and SCA, respectively. The mean values for hybrids and parental populations across environments are used to form a symmetrical diallel data matrix from which the first two principal components are extracted. In this matrix, each population corresponds to one row and one column of data, where the row is considered an "entry" and the column a "tester" (Yan and Hunt, 2002). Thus, each population can be considered both an entry and a tester. Means of each column are calculated and a new, adjusted (nonsymmetrical) data matrix is obtained by subtracting the column (tester) mean from each cell. After obtaining the first two principal components of the adjusted data matrix, the biplot model can be written as:
where 
ij is the genotypic value of the cross between entry i and tester j for the trait of the interest; ßj is the mean of all crosses involving tester j; 
1 and 2 are the singular values for the first and second principal components (PC1 and PC2 respectively); 
i1 and i2 are the PC1 and PC2 eigenvectors, respectively, for entry i; 
j1 and j2 are the PC1 and PC2 eigenvectors, respectively, for tester j; and 
ij is the residual of the model associated with the combination of entry i and tester j. When i 
j, the genotype is a population hybrid. When i = j, the genotype is a landrace.
For each trait, we obtained the eigenvectors of the first two PCs for each genotype, and the eigenvalues for PC1 and PC2. The singular value for a PC was obtained as the square root of the sum of squares explained by the PC, which is the product of the eigenvalue multiplied by the number of landraces. Principal components scores for entries and testers were scaled symmetrically.
Following Yan and Hunt (2002), we displayed the results of the principal components analysis in two ways: as an average tester coordinate view and as a polygon view. The average tester coordinate (ATC) view is displayed by defining the average tester position in the biplot as that position with average PC1 and PC2 scores of all testers. The ATC is established with its abscissa passing through the origin and the average population, and its ordinate passing through the origin and perpendicular to the abscissa. The GCA effects of the populations are then approximated by their projections as entries onto the ATC abscissa. Grid lines perpendicular to the ATC abscissa are displayed to help to rank the populations in terms of GCA. Projections of the populations as testers onto the ATC ordinate approximate their SCA effects, which represent the trend of the populations to produce superior hybrids with specific genotypes.
The polygon view of a biplot provides an alternative graphical presentation that permits identification of interactions between populations. Polygon view biplots were drawn, resulting in the partitioning of the biplot into sectors, with entries farthest from the center of the biplot representing the vertices of the polygon. Testers within a sector form the best hybrids with the entry at the vertex of the sector. Entries located near the biplot origin are less responsive to the change of testers.
All statistical analyses were performed with the Proc Princomp and Proc Mixed procedures of the SAS software package (SAS Institute, 1999).
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RESULTS
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Genotypes varied significantly (P < 0.01) for all yield traits (SY, EY, and WY), but not for digestibility traits (SD, ED, and WD) (Table 2). Significant variation among parents (P < 0.01) and among F1s (P < 0.01) was also observed for all yield traits. The difference between check hybrids and experimental genotypes was significant for SY (P < 0.01) and EY (P < 0.01). Stover dry matter yield of some landraces and crosses was significantly greater than the best checks, but no landraces or crosses had EY as high as the two best checks (Tables 3 and 4). On average, checks had greater EY but lower SY than the unimproved genotypes (Tables 3, 4, and 5). As WY is the sum of SY and EY, the differences between these two traits counteracted each other when WY is considered, resulting in no significant difference between checks and unimproved genotypes (Tables 2 and 5). Variation among checks was observed for SY and EY (P < 0.01), but not for WY. Crosses had greater values than parental landraces (P < 0.01) for all yield traits, indicating that heterosis was significant. The GCA effects were significant (P < 0.01) for SY, EY, and WY, but SCA was not significant for any trait.
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Table 2. Analysis of variance for stover (SY), ear (EY), and whole-plant (WY), and in vitro digestibility of stover (SD), and whole-plant (WD), of eight maize landraces, 28 single-crosses and four commercial checks evaluated in two environments during the 1997–1998, 1998–1999 growing seasons.
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Table 3. Mean ear dry matter yield (EY) across four environments for landraces per se (diagonal), F1 hybrids among landraces (above diagonal), and commercial checks. Percentage of midparent heterosis is shown below the diagonal.
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Table 4. Mean stover dry matter yield (SY) across four environments of landraces per se (diagonal), F1 hybrids among landraces (above diagonal), and commercial checks. Percentage of midparent heterosis is shown below the diagonal.
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Table 5. Mean whole plant dry matter yield (WY) across four environments of landraces per se (diagonal), F1 hybrids among landraces (above diagonal), and commercial checks. Percentage of midparent heterosis is shown below the diagonal.
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Biplot Analysis
Ear Yield
The first two principal components explained 67% (42.1 and 24.9% by PC1 and PC2, respectively) of the variation for EY. The average tester coordinate biplot indicates that landraces B, G, D, and F had positive GCA effects (order also indicates ranking order), whereas landraces E, A, H, and C had negative GCA effects (Fig. 1
). Although landraces E and H combined well with landraces A and C, their SCA effects were not significant (Table 2).
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Fig. 1. Biplot based on diallel data of eight landraces for ear yield. Codes for genotypes are: A, ARZM 17–034; B, ARZM 03–056; C, ARZM 01–150; D, ARZM 03–054; E, ARZM 16–062; F, ARZM 16–042; G, ARZM 19–006; H, ARZM 01–088. Genotypes are labeled with uppercase letters when viewed as entries and with lowercase letters when viewed as testers. ATC, average tester coordinate.
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The biplot in Fig. 1 is divided into five sectors with landraces A, E, H, D, G, and B as the vertex. Two well differentiated and opposite groups can be observed, E to H and A to C. Landraces A, E, H, and C produced the worst combinations with themselves, since testers e, h, a, and c fell into opposite sectors. Testers d, e, f, g, and h fell into sector B to G, indicating that their crosses with B, G, and F generated good hybrid combinations. Sector D contained three adequate hybrid combinations: b x D, a x D, and c x D. High values of MPH for these crosses were observed (Table 3). Both b x G and d x G represent crosses with interesting potential as breeding materials, since they did not have significant differences with the second highest yielding genotype (Cargill Semiden 5), and showed very high MPH (29 and 24%, respectively). Another promising cross is a x D with an EY similar to Syngenta Pucará and Morgan 369 and a MPH of 26%.
Stover Yield
Principal components 1 and 2 together explained 87.5% of the observed variation for SY (76.2% and 11.3%, respectively, for PC1 and PC2). The corresponding biplot (Fig. 2
) shows that landraces A, B, C, and D had positive GCA effects for SY. We suggest that maturity has a strong influence on SY, since only later flowering landraces (
T10°C > 800°C) showed positive GCA effects. Consequently, it is possible to observe a clear separation between landraces with positive and negative GCA effects for SY. Landraces A, B, C, and D exhibited good combining ability with all testers including themselves. The polygon view biplot demonstrates that landraces A and B produced good hybrid combinations with c, d, e, g, and h for SY (sector A–B, Fig. 2). Similarly, it can be observed in sector D that landraces C and D possess good combining ability with testers a and f. The distance between the x axis and a genotype in the biplot is an estimation of its SCA effect (analogous to stability in GGE biplots). All landraces clustered near the x axis, indicating that SCA effects were not important for SY. The entries most distant from the x axis were populations C and D, but their distances from the x axis were nonsignificant. This observation agrees with the ANOVA, where variation due to SCA effects was not significant (Table 2). According to the biplot view, as a is in sector C–D, and c and d are in sector A–B, crosses a x C and a x D are predicted to be the best combinations.
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Fig. 2. Biplot based on diallel data of eight landraces for stover yield. Codes for genotypes are: A, ARZM 17–034; B, ARZM 03–056; C, ARZM 01–150; D, ARZM 03–054; E, ARZM 16–062; F, ARZM 16–042; G, ARZM 19–006; H, ARZM 01–088. Genotypes are labeled with uppercase letters when viewed as entries and with lowercase letters when viewed as testers. ATC, average tester coordinate.
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Crosses a x (C, E, G), b x (C, D, E, G), c x E, and d x (E, F, G) did not show significant differences with the best commercial hybrid (Morgan 369), while a x B and a x D yielded greater than this check (Table 4). These two crosses also showed significant MPH values (10 and 17%, respectively).
Whole-Plant Yield
The first two principal components together explained 81.1% (66.3 and 14.8%, respectively) of the variation for WY. The resulting biplot revealed a response similar to that observed for SY (Fig. 3
), where the later flowering populations (A, B, C, and D) had positive GCA effects. Based on the distance between each entry and the ATC abscissa, landrace B had the greatest GCA effect, followed by landraces D, A, and C, respectively. Negative GCA effects (in decreasing order) were observed in landraces H, E, F, and G. The polygon biplot view reveals four well-defined sectors, named A, B, D, and H. SCA values for this trait were small and nonsignificant in the ANOVA (Table 2). Consequently, it was not possible to observe clear heterotic groups. Good hybrid combinations were d x A, b x A, and c x A in sector A; h x B, g x B, and e x B in sector B; and f x D and f x C in sector D. Sector H included landraces E, F, and H, but no testers fell into this sector, indicating that these landraces exhibited poor performance for WY both per se and in hybrid combinations. Landrace G did not fit in a well-defined sector, since it could be assigned to sector A or H. In sector A, d was predicted to be the best mating partner for A, while in sector D, a is the best partner for D. Both A and D were, therefore, identified to be the best partners for each other. This combination significantly out yielded all commercial hybrids (Table 5) and showed an MPH of 20%. Crosses b x G and d x G did not show WY differences with the two best checks, Morgan 369 and Cargill Semiden 5.
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Fig. 3. Biplot based on diallel data of eight landraces for whole plant yield. Codes for genotypes are: A, ARZM 17–034; B, ARZM 03–056; C, ARZM 01–150; D, ARZM 03–054; E, ARZM 16–062; F, ARZM 16–042; G, ARZM 19–006; H, ARZM 01–088. Genotypes are labeled with uppercase letters when viewed as entries and with lowercase letters when viewed as testers. ATC, average tester coordinate.
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DISCUSSION
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No significant breeding effort to improve corn forage yield or quality attributes has been undertaken by corn breeders (Lauer et al., 2001), and silage maize currently lacks both well-defined heterotic groups and an appropriate ideotype that can be used to guide forage maize breeders. Breeding of highly digestible forage maize may depend on the re-evaluation and use of old genetic resources (e.g., landraces) that are not currently used or that were never used in maize breeding (Barrière et al., 2005). Selection for modern grain type hybrids changed plant design (Evans and Fischer, 1999), with an improvement in whole plant digestibility as a result of the increased contribution of EY to WY in commercial checks. However, such improvement was not observed in this study, since WD of commercial hybrids and landraces were not significantly different (Table 2). According to Roth et al. (1970), Gunn (1975), and Twumasi-Afriyie and Hunter (1982), this result may be explained by the hypothesis that selection not only resulted in an increase in harvest index and stalk lodging resistance in modern grain type hybrids, but also in a decrease in SD. Again, however, this effect was not observed in this study, given that there were no differences between landraces and checks for SD. Argillier et al. (1995) reported similar results. According to Lauer et al. (2001), little change has occurred in the quality of the stover portion of corn forage cultivars available in northern corn belt from 1930 to the present.
Biplot analysis was preferred over conventional diallel analysis because its graphical representation jointly gave information about GCA and SCA effects of the populations, their performance in crosses, as well as grouping patterns of similar genotypes. The heterosis values observed and the performance of crosses compared to the hybrid checks, suggest that some populations have potential as breeding material to select genotypes with improved forage production. More than one breeding strategy could be adopted in breeding for improved corn silage, but in all cases, a recurrent selection scheme is recommended to increase the probability of obtaining competitive inbred lines. Two heterotic patterns can be defined: (i) one white dent x white dent grain type, with landraces B and D as a group and landrace A as the other group; and (ii) one white dent x orange flint with B and D as a group and G as the other. Landraces B and D can be intercrossed to form a composite and reciprocal recurrent selection can be implemented with it and landraces A or G. Although we believe that an interpopulation scheme is more appropriate, intrapopulation recurrent selection also can be adopted in the composite and landraces A and G. Another potential strategy would be to cross the selected landraces with inbred lines from well-known heterotic patterns like Reid x Lancaster, to study their performance in hybrid populations. Thus, a group of outstanding lines can be crossed with composite B x D, and the best combination identified. The selected inbred line can then be used as the tester in a recurrent selection program and/or in the selection of new inbred lines from the composite, taking advantage of (i) the normally observed interracial heterosis, and (ii) the use of a line with good per se yield and stalk quality as the female parent of the potential hybrids. At the present time, landraces B and D are being intercrossed to form composite B–D.
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ACKNOWLEDGMENTS
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Sincere thanks are extended to Drs. Jim Holland, North Carolina State University, and Peter Graham, University of Minnesota, for valuable critiques and suggestions of previous versions of this paper. We also thank Dr. Marcelo Ferrer, Germplasm Bank at INTA Pergamino, Argentina, for supplying seeds of the evaluated Landraces.
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NOTES
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This research was supported by the Dep. of Agronomy, Facultad de Ciencias Agrarias, Universidad Nacional de Lomas de Zamora.
Received for publication September 27, 2005.
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TOP
ABSTRACT
INTRODUCTION
