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CROP BREEDING, GENETICS & CYTOLOGY

Identifying Inbred Lines Capable of Improving Ear and Stover Yield and Quality of Superior Silage Maize Hybrids

Dep. of Plant Production, Universidad Nacional de Lomas de Zamora, Camino de Cintura km. 2, (1836) Llavallol, Prov. de Buenos Aires, Argentina

* Corresponding author (bertoia{at}agrarias.net)

	ABSTRACT

TOP NOTES ABSTRACT INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION REFERENCES

 
Maize (Zea mays L.) is an excellent feed, whether fresh, chopped, or ensiled. Silage maize hybrids have been developed for temperate-cold regions of Europe and North America, but limited research has been done to develop silage hybrids for temperate and warm-temperate regions. This study was conducted to identify inbred lines that could be used to enhance the stover and ear fractions of existing hybrids for warm-temperate areas. Forty-five single-cross hybrids were developed from 10 inbred lines and evaluated in six environments. Ten inbred lines were evaluated for potential to improve the best hybrids by Dudley's and Metz's methodologies. Hybrids differed significantly (P < 0.01) for ear (EY), stover (SY), and whole-plant dry matter yield (WY), in vitro digestibility of whole-plant dry matter (iDW), and whole-plant digestible dry matter yield (WDY), differences among hybrids were not significant for in vitro digestibility of ear (iDE) and stover (iDS). Four outstanding hybrids were selected as target hybrids for improvement on the basis of YS_i (Kang's Yield-Stability Statistic) of mean WDY and their stability across environments. Inbred lines that were able to increase SY were not the same ones that would increase EY. Inbred PR4 had outstanding potential to improve the SY of any hybrid. Inbred B84 had greatest potential to enhance EY. Inbred lines showed differential capacity to generate improvement according to the fractions that were evaluated. Finally, inbred lines from the North American Corn Belt did not demonstrate potential for enhancing SY when they were compared with local flint inbred lines.

Abbreviations: EE, Esteban Echeverría • EY, ear dry matter yield • G x E, genotype x environment • FAO, Food and Agriculture Organization • iDE, in vitro digestibility of ear dry matter • iDS, in vitro digestibility of stover dry matter • iDW, in vitro digestibility of whole-plant dry matter • MS, mean square • NIRS, near infrared reflectance spectroscopy • PNG, probability of net gain • SE, Standard Error • SV, San Vicente • SY, stover dry matter yield • VC, Vicente Casares • WDY, whole-plant digestible dry matter yield • WY, whole-plant dry matter yield • YS_i, Kang's yield-stability statistic

	INTRODUCTION

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WHOLE-PLANT MAIZE is an excellent quality feed for ruminants, whether as fresh green-chop or as silage. While silage maize hybrids have been developed for temperate-cold regions of Europe and North America, little work has been done to develop late-maturity silage hybrids.

When maize is used for silage, it can be chopped 40 to 45 d before normal grain harvest. For this reason, tropical and subtropical maize germplasm could be used successfully in temperate environments (Bertoia, 2001). However, use is hindered by a lack of improved forage maize hybrids for these environments. Several plant components, especially the grain, have a high-energy content. Because of the energy contributed by the grain, breeders, until recently, assumed that forage maize quality was determined solely by grain-to-stover ratio, which they used as the basis for hybrid selection. Unfortunately, grain-to-stover ratio will not predict adequately the nutritional value of silage maize; therefore, selection should also be practiced on whole-plant digestibility (Barrière et al., 1997). Stover is as important as the ear because it contributes half of dry matter yield. Roth et al. (1970) suggested that silage maize is primarily a high energy feed, and that its nutritive value can be enhanced by increasing its digestibility. They also proposed that selection for improvement of silage maize should be based on whole-plant digestible dry matter yield. Genetic variation in digestibility and energy value was demonstrated in experimental and commercial hybrids (Vattikonda and Hunter, 1983), suggesting that maize forage quality could be improved via selection. Deinum and Bakker (1981), Vattikonda and Hunter (1983), and Argillier et al. (1997) reported that characteristics related to forage digestibility were consistent over a wide range of environments.

Methods have been developed to select for grain yield and stability of grain yield across environments (Kang and Pham, 1991; Kang et al., 1991; Bachireddy et al., 1992). These methods have not been applied previously to maize forage yield, although both high and stable levels of forage production are desired by growers. In addition to the difficulty in selecting stable, high-yielding hybrids, the maize breeder is faced with the question of how to choose germplasm for a breeding program aimed at improving the best hybrids. The percentage of crosses that lead to advancing parent inbred lines for single-cross hybrids is very low (Hallauer, 1990), so methods for increasing the probability of selecting breeding parents that will give rise to improved inbreds are needed.

Dudley (1984a) proposed a method to screen donor germplasm systematically to find sources of favorable alleles that were not present in the parents of the elite hybrids. He defined eight classes of loci (A–H) that existed for any three homozygous parent inbreds on the basis of the occurrence of favorable (+) and unfavorable (-) alleles in the inbreds (Table 1). The genotypic values of the three possible genotypes (++, +-, --) at a single locus are µ, µ, and -µ, respectively. A general model based on Comstock and Robinson (1948) is z + 2µ, z + µ + aµ, and z, where z is the value of recessive genotype, µ is half the difference between homozygotes, and a is a degree of dominance (a = 1 for complete dominance, a < 1 for partial dominance and a > 1 for overdominance). Using the above notation, Dudley developed theory useful to identify those donor inbred lines (I_w) having a higher relative number of loci homozygous for favorable dominant alleles (µG) for a quantitative trait, that are not present in the parents (I₁ and I₂) of the target elite single-cross. The theory was based on four assumptions: (i) a = 1, (ii) the number of loci for which I₁, I₂, and I_w were all ++ (class A loci) was equal to the number for which they were all -- (class H loci), (iii) negligible epistasis, and (iv) µ constant for all loci. Using this assumption, Dudley (1984a) could evaluate I₁, I₂, I_w, and the single crosses among them as function of the mean effect and the frequency of the six remaining loci classes (B, C, D, E, F, and G) (Table 1). The six simultaneous functions were solved for the relative frequency of classes B, C, D, E, F, and G. Under the assumptions of complete dominance, µ constant for all loci, µA = µH, and z = -µ, he obtained estimators for µB, µC, µD, µE, µF, and µG. Later on, Dudley (1987) presented a modified model for, a less biased, estimator of favorable alleles (µG') in which the assumptions that µA = µH and z = -µ were removed. Other methods have been developed as alternatives to Dudley's modified method (Gerloff and Smith, 1988a, b; Bernardo, 1990). Although ranking of donors sometimes varies with different statistics, the correlations among donor statistics are usually positive and significant (Cheres et al., 1999). Finally, Metz (1994) demonstrated how to estimate the probability of net gain (PNG) from an inbred. Probability of net gain estimates the relative number of favorable alleles that can be gained as a proportion of the relative number of loci where favorable alleles can be either gained or lost during selection when I₁ is crossed to I_w (PNG₁) or I₂ is crossed to I_w (PNG₂).

	MATERIALS AND METHODS

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Field Trials
Ten inbred lines were selected for evaluation to represent different racial origins, maturity, and grain type (Table 2). The inbreds were crossed in a diallel mating design without reciprocals to form 45 F₁ hybrids. Two trials were grown simultaneously for each environment. One included 45 F₁ hybrids and three commercial checks (DEKALB 4F37, MORGAN 369, and CARGILL Semiden 5); the second trial included the 10 lines per se. Locations were within the Buenos Aires Province dairy region: San Vicente (SV - 35°24' S, 58°30' W), Vicente Casares (VC - 35°18' S, 58°56' W), and Esteban Echeverría (EE - 34°38' S, 58°48' W). Experiments were grown during the 1994-1995 (SV and VC), 1995-1996 (SV and EE), and 1996-1997 (VC and EE) growing seasons. The soils were typical Argiudolls (SV and VC) and Aquic Argiudoll with silty clay loam and B2t horizon (EE). The design of the experiment for each environment-planting date combination was a randomized complete block, with three replications for the F₁ + checks trials, and two replications for inbred lines trials. Experimental units consisted of two 5.20-m rows, spaced 0.70 m apart. Plots were planted at a rate of 52 seeds per row. When the plants reached the four-leaf stage, plots were thinned in order to achieve an approximate plant density of 71 500 ha^-1. Each experimental unit was harvested by hand when the kernel milkline reached 2/3 of the way down the kernels at the center of the ear (Hunt et al., 1989). Ears and stover were separated and fresh weights of each fraction were measured in each plot. Then, a representative sample of each plant fraction was dried to calculate the dry matter percentage and to perform the laboratory analyses. Dried samples were milled to a 1-mm particle size. On all samples, near infrared spectra were collected (NIRS; NIRS 6500, NIRSystem Inc., Silver Spring, MD) between 1100 to 2500 nm at every 2 nm. Ear (iDE) and stover (iDS) in vitro dry matter digestibility were predicted by NIRS equations, calibrated by the enzymatic method (Gabrielsen, 1986).

Statistical Analysis
The ANOVA used to evaluate inbred lines, crosses, and hybrids was performed according to a mixed model (McIntosh, 1983), where the environments and theirs interactions were considered random effects (Table 3). When the mean square (MS) for interaction (principal sources x environment) was not significant, it was replaced by the MS for error to calculate F value of principal sources of variation. To identify experimental hybrids that were better than the best check, the simultaneous selection method of Kang (1993) was applied to WDY because it combines both the yield and quality of two plant components (ear and stover). Kang's (1993) yield-stability statistic (YS_i) was calculated for each hybrid using the statistical software package STABLE (Kang and Magari, 1995).

_j

_k

Means for each trait were calculated for inbred lines (Table 4) and hybrids + checks (Table 5). Positive estimates of µD' and µF' indicate that I₁ and I₂, respectively, contain favorable alleles at loci where the other two lines have unfavorable alleles. The µC' + µF' estimate the relative number of loci when I₁ and I_w have the same (either + or -) alleles while µD' + µE' is the total relative number of loci for which I₂ and I_w have the same alleles. To maintain the heterotic pattern in I₁ x I₂, those lines with µC' + µF' > µD' + µE' (most closely related to I₁) would be crossed with I₁ and those with µD' + µE' > µC' + µF' (most closely related to I₂) would be crossed with I₂ (Dudley, 1984b). The difference µG' - µD' indicates whether selfing will start directly in F₁ generation or whether backcrossing to I₁ is advisable. If µG' - µD' is not significantly different from zero, direct selfing of the F₁ generation in the process of selection is suggested. If µG' - µD' > 0, then backcrossing to I_w is suggested. If µG' - µD' < 0, then backcrossing to I₁ is suggested. If I₂ is to be improved, then class F loci are important. Therefore, the characteristics of an "ideal" donor line to improve I₁ are (i) µG' large, (ii) µC' + µF' > µD' + µE', and (iii) µG' - µD' = 0. To improve I₂ the donor should have (i) µG' large, (ii) µD' + µE' > µC' + µF', and (iii) µG' - µF' = 0.

PNG₁ = [µG'/(µG' + µD')] when a new inbred line is to be developed from I₁ x I_w and PNG₂ = [(µG'/(µG' + µF')] when a new inbred line is to be developed from I₂ x I_w. Probability of net gain estimates the relative number of favorable alleles that can be gained as a proportion of the relative number of loci where favorable alleles can be either gained or lost during selection when I₁ is crossed to I_w (PNG₁) or I₂ is crossed to I_w (PNG₂). These statistics (PNG₁ and PNG₂) also allow one to select which parent inbred line should be hybridized to the donor to maximize the probability of recovering a new inbred that will improve the hybrid. If PNG₁ > PNG₂, then the greatest gain can be made by hybridizing the donor to I₁, whereas the reverse holds if PNG₁ < PNG₂.

Contrasting µG' with PNG, µG' would tend to estimate the maximum potential of I_w to improve I₁ x I₂, without regard for the potential loss of favorable alleles in I₁ x I₂. Probability of net gain, in contrast, estimates the probability of a net gain of favorable alleles from I_w, without regard to the absolute magnitude of that potential gain. µG' used in conjunction with PNG to select I_w to improve I₁ x I₂ is useful. µG' estimates how much improvement in I₁ x I₂ might be possible, while PNG estimates the probability of achieving that improvement (Metz, 1994).

The standard error (SE) of the estimators was calculated as the square root of the variance of the linear function associated with each estimator ignoring covariance terms. All estimators were considered different from zero at P < 0.05 if they exceeded twice their SE. All statistical analyses, except YS_i, were performed with the SAS/STAT procedure of the SAS software package (SAS Institute, 1999).

	RESULTS

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Environmental effects for hybrids for all variables were significant (Table 3). There were also significant differences among hybrids for EY, SY, WY, iDW, and WDY (Table 3). Crosses did not differ significantly for traits related to the quality of ear and stover fractions (iDE and iDS, respectively), but showed significant differences in WDY because of differential contribution of the ear to the total yield in each hybrid. Hybrids x Environment interaction was significant for all traits, except iDE and iDS (Table 3). Four experimental hybrids were selected because they had Kang's YS_i greater than the best commercial check (DK4F37) for WDY (Table 5). One experimental hybrid (ZN6 x B84) was significantly greater than DK4F37 for WDY. ZN6 x B84 had the highest YS_i followed by PR4 x B84, P465 x B84, and PR4 x ZN6. In contrast to previous studies, where the hybrid to improve was chosen a priori, we selected the best hybrids on the basis of their performance and considered them to be the hybrids to improve.

Donor Inbreds for Enhancing ZN6 x B84
The best donor inbred line for the improvement of EY (Table 6) was inbred P465. Its µG' was the largest (0.75*), suggesting that inbred P465 had favorable alleles for EY that can be transferred to the hybrid. The inequalities µC' + µF' > µD' + µE' and significant µG' - µD' < 0 indicated that the most promising strategy for developing superior inbred lines should be to cross the selected donor (P465) to the female parent (ZN6) and backcross to inbred ZN6 (since positive and high values of µD' revealed that inbred ZN6 had favorable alleles in that class of loci). The probability of net gain, however, was relatively low, PNG₁ = 0.34.

Donor Inbreds for Enhancing PR4 x B84
Inbred ZN6 was the best potential donor for EY having the highest significant values of µG' (Table 7). According to µC' + µF' > µD' + µE' and significant µG' > µD', and PNG₁ = 0.82, the process of improvement would be successful and initiated by making a cross between inbreds ZN6 and PR4, and backcrossing to inbred ZN6. The low values of µG' and PNG for six donor lines suggest that improvement for SY may be difficult. Only inbred ZN6 was capable of improving this trait, but the probability of gain was low (PNG₁ = 0.49). When taking account of µC' + µF' > µD' + µE', inbred ZN6 should be crossed to inbred PR4. Due to µG' = µD', then selfing of the F₁ (PR4 x ZN6) is recommended. Inbred A632 would arise as donor to inbred B84 because of its significant µG' for SY, µC' + µF' < µD' + µE' and PNG₂ = 0.48 > PNG₁ = 0.18. Similarly, inbreds ZN6 and P465 are possible donors for WDY. These lines should be crossed to inbred PR4 since in both cases their µC' + µF' > µD' + µE' and µG' > µD' were significant. Therefore, PR4 x B84 should be backcrossed to inbred ZN6. PNG₁ for this inbred was the highest (0.73) and > PNG₂. The probability to achieve this improvement is high.

	DISCUSSION

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The application of the methodologies of Kang (1993), Dudley (1987), and Metz (1994) in a combined form allowed us to identify genotypes with superior forage yield and digestibility, and inbred lines with potential to improve forage characters in other inbred lines. However, we found that some lines had higher µG', but lower PNG. Therefore, to transfer this maximum potential of favorable alleles to parent lines will be complicated because of a low probability for success. To apply these methodologies most efficiently, plant components (ear and stover) should be evaluated separately. Inbred lines in this study showed differential capacity to generate improvement according to the component that were evaluated. The lines are in a sense specializations, since those that were able to introduce improvement in SY were not the same ones that would improve EY. Significant differences among crosses for iDW but not for EY and SY support the hypothesis that iDW was affected by the percentage of ear in whole-plant yield (forage harvest index). Since variability was not detected among the evaluated hybrids when stover digestibility was considered, selecting hybrids on the basis of their iDW is equal to selection by harvest index and not by digestibility of their components. It is risky and prevents finding potential variability in stover digestibility.

Silage maize usefulness depends upon the contributions of both vegetative and reproductive plant fractions. Therefore, limiting the genetic base to inbred lines belonging to classic heterotic groups defined on the basis of grain yield in temperate environments would result in overlooking some outstanding lines derived from unique pedigrees, such as flint lines PR4, ZN6, P465, and P21.

	ACKNOWLEDGMENTS

 
Many helpful suggestions for improving this paper were given by Drs. Andrea Cardinal and James Holland, North Carolina State University. Also, we thank Dr. Manjit Kang, Dep. of Agronomy, Louisiana Agric. Exp. Stn., Louisiana State Univ. Agric. Ctr. for supplying the STABLE software package, and Dr. R.T. Boca, Dep. of Plant Production, Universidad Nacional de Lomas de Zamora for generating the SAS analysis used in this study.

	NOTES

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This research was supported by the Dep. of Animal Production, Facultad de Ciencias Agrarias, Universidad Nacional de Lomas de Zamora.

Received for publication May 8, 2001.

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This Article Abstract Full Text (PDF) Free Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in ISI Web of Science Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via ISI Web of Science (5) Citing Articles via Google Scholar Google Scholar Articles by Bertoia, L. M. Articles by Torrecillas, M. Search for Related Content PubMed Articles by Bertoia, L. M. Articles by Torrecillas, M. Agricola Articles by Bertoia, L. M. Articles by Torrecillas, M. Related Collections Crop Genetics Maize Maize