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CROP BREEDING, GENETICS & CYTOLOGY

Inbred Line Evaluation and Breeding for Digestibility-Related Traits in Forage Maize

Unité de Génétique et d'Amélioration des Plantes Fourragères, INRA, 86600 Lusignan, France

barriere{at}lusignan.inra.fr

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Forage maize digestibility is now a major selection criterion in breeding programs, and in hybrid breeding it is useful to select at the inbred level. This study was conducted to evaluate the possibility of doing preliminary selection at the inbred level for digestibility-related traits in forage maize (Zea mays L.). Maize hybrids from a factorial mating design among nine and six maize inbred lines, and the 15 parental inbred lines per se, were evaluated at silage stage at three locations (France) for 2 yr. Whole plant in vitro digestibility of dry matter and of cell walls, and plant biochemical composition traits were investigated by near infra red reflectance spectroscopy (NIRS). Biomass dry matter content and productivity were also collected. In hybrid trials, general combining ability (GCA) of lines was the most important source of variation in all digestibility traits, compared with specific combining ability (SCA), GCA x environment, and SCA x environment interactions. Some plant biochemical components were associated with the biomass yield, whereas digestibility of cell walls, particularly, was found independent from biomass productivity. The efficiency of predicting the hybrid performance from midparent value, as well as the good relationship between inbred line per se and GCA values, for digestibility-related traits, and particularly cell wall digestibility, indicated the possibility for maize breeders to improve digestibility of forage maize by selecting inbred lines.

Abbreviations: ADL, acid detergent lignin • DM, dry matter • GCA, general combining ability • INRA, Institut National de La Recherche Agronomique • IVDDM, in vitro digestibility of dry matter • IVDNSC, in vitro digestibility of non-starch and non-soluble carbohydrate part • IVDNDF, in vitro digestibility of NDF part • NDF, neutral detergent fiber • NIRS, near infra-red reflectance spectroscopy • SCA, specific combining ability

	INTRODUCTION

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FORAGE MAIZE is excellent roughage for ruminants because of its high energy content. Around 3.8 million hectares of silage maize are grown in Europe (Barrière et al., 1997), and in the USA, 2.5 to 3 million hectares of maize are harvested annually for use as whole-plant silage (Coors et al., 1994). For a long time, breeders and farmers relied on the assumption that a good grain maize was also the most suitable for silage. Genetic variation in forage maize digestibility has been clearly proved (Allen et al., 1990; Dhillon et al., 1990; Barrière et al., 1992; Wolf et al., 1993; Coors et al., 1994; Lundvall et al., 1994; Argillier et al., 1995a; Méchin et al., 1998).

Forage maize digestibility is influenced, from a macroscopic point of view, by both grain content and stover digestibility, or, from a microscopic point of view, by both cellular content and cell wall digestibility (cellular contents of forages are nearly completely digestible, whereas cell walls are of lesser and variable digestibility) (Deinum and Struik, 1986; Hunt et al., 1992; Wolf et al., 1993). Forage maize digestibility might be improved by selecting either for high amount of cellular contents, but mostly for high cell-wall digestibility. In a breeding program, maize digestibility has to be taken into account with agronomic criteria such as productivity.

In hybrid breeding, it is useful to select at the inbred level. Selection at the inbred level reduces work during the development of hybrids, and enables one to evaluate a large number of potentially promising genotypes. However, the relationship between digestibility of maize inbreds and hybrids has been reported in only a few studies. Gurrath et al. (1991), in an half-diallel design of 12 inbred lines, found significant correlations between inbred line per se and the mean performance in crosses for in vitro digestibility of organic matter, fiber, and lignin contents. Dolstra et al. (1993) using midparent value to predict the performance of 12 hybrids found stalk cell wall digestibility to be the only stalk quality trait worth evaluating at the inbred line level. In the study of Wolf et al. (1993) after selection for high or low fiber content, 24 families were evaluated in test crosses to two inbreds as well in per se trials of derived S² bulks. They found that hybrid performance was related to performance of partially inbred families for whole plant and stover composition traits and dry matter and cell wall digestibility.

In our study, maize forage quality was evaluated in whole-plant samples, because for breeding purposes this means no additional difficulties and costs at harvest. In addition to the total dry matter digestibility, cell wall digestibility and biochemical constituents were investigated. The objectives of our work included (i) determination of variation for digestibility-related traits in the inbred lines trials; (ii) determination of variation for digestibility-related traits in a large factorial mating design and partitioning into general and specific combining abilities; (iii) study of interrelationships among digestibility-related traits and relation with biomass yield; and (iv) study of the relation between the performance of inbred lines and their hybrids to evaluate the possibility of doing preliminary selection at the inbred level for digestibility traits in forage maize.

	Materials and methods

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The genetic material consisted of 53 maize hybrids, established by crossing nine maize inbred lines with six maize inbred lines in a factorial mating design (one crossing was missing), and of the 15 parental inbred lines (Table 1) . These maize lines, with different genetic backgrounds, represent variation for digestibility traits, according to previous experiments. `Anjou 285', registered in France in 1994, was added as an elite check hybrid.

Experimental units consisted of single or double-row plots, 4 to 6 m long (double-row plots at Le Pin, and single-row plots at all other locations) with 0.80 m between rows. Plant density was about 95 000 plants/ha. Hybrids were grown in a lattice design (eight sub-blocks) with three replicates. Inbred lines were arranged in a randomized block design with three replicates. Hybrids and inbreds trials were planted adjacent to each other in the same field. Fertilizers were given according to usual requirements for high yielding maize, taking into account the soils characteristics and the previous cropping in each locations.

At the silage stage (average dry matter content 32%), whole plants in each plot (single or double-rows) were machine-harvested (all plots were harvested on the same day, in each of the environments). Biomass yield and dry matter content of each plot were measured in each environment. A representative sample of 1 to 1.5 kg chopped material per plot was taken. Samples were then dried at 70°C for 48 h, and ground in a hammer mill to pass a 1-mm screen.

Samples were analyzed at INRA Lusignan by near infra red reflectance spectroscopy (NIRS). The following traits were predicted: starch (Ewers method), soluble carbohydrates (Lila, 1977), neutral detergent fiber (NDF), acid detergent lignin (ADL) (Goering and Van Soest, 1970), and in vitro digestibility of dry matter (IVDDM) (enzymatic digestion according to Aufrère and Michalet-Doreau, 1983). All compositional data were calculated on a dry matter basis, except ADL which was reported as a proportion of NDF. NIRS values were collected with a NIRS system 6500 with wavelengths 1100 to 2500 nm and 4-nm steps. Calibration equations were obtained at INRA Lusignan (France) or were provided by SHB Libramont (Belgium). Standard errors of cross-validation and determination coefficients of cross-validation, were respectively equal to 1.7 and 0.87 for starch, 1.2 and 0.93 for soluble carbohydrates, 1.9 and 0.93 for NDF, 0.4 and 0.82 for ADL, and 1.8 and 0.91 for IVDDM.

On the basis of these traits, the in vitro digestibility of the non-starch and non-soluble carbohydrate part of the plant (IVDNSC) was computed as

assuming that starch and soluble carbohydrate are completely digestible, and the in vitro digestibility of the NDF part of the plant (IVDNDF) was computed as

assuming that the non-NDF part was completely digestible.

Analyses of variance were conducted considering all effects as fixed (both genotypes and environments were chosen, and could not be considered as representative of the whole existing variation).

For inbred lines experiment, the model used was:

where Y_iek is the value of Inbred Line i in the Environment e in the Block k, µ the grand mean, E_e the main effect of Environment e (each year-location combination was considered as a separate environment), B_ke the effect of Block k nested in Environment e, G_i the main effect of nbred Line i, GE_ie the interaction effect between Inbred Line i and Environment e, and R_iek the residual term.

For the hybrid experiment, the model used was:

where Y_lteku is the value of the cross between Line l and Tester t, in the Environment e, in the Block k, and in the Subblock u, µ the grand mean, E_e the main effect of Environment e, B_ke the effect of Block k nested in Environment e, U_uke the effect Subblock u nested in Block k and Environment e, g_l the general combining ability (GCA) of line l, g_t the general combining ability of Tester t, s_lt the specific combining ability (SCA) between Line l and Tester t gE_le the interaction between GCA of Line l and Environment e, gE_tê the interaction between GCA of Tester t and Environment e,sE_lte the interaction between SCA of the cross (between Line l and Tester t) and Environment e, and R_lteku is the residual term.

Data were combined over environments and replications before determination of phenotypic correlation coefficients between (i) traits, (ii) the performance of inbred lines per se and the mean performance of the crosses involving that line (GCA), (iii) the performance of hybrids and midparent value.

The analyses were performed with the S-plus package (Venables and Ripley, 1994).

	Results

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Variation among Inbred Lines Per Se
The mean squares due to environment and inbred line per se were highly significant for all traits (Table 2) . Inbred line per se x environment interactions were significant for all traits; however, the ratio of mean squares due to inbred line per se to mean squares due to inbred line per se x environment were at least equal to 10, except for starch content (ratio=7.7), and NDF content (ratio=4.8). Ranges among inbred lines were 67.6 to 77.4% for IVDDM, 57.0 to 68.0% for IVDNSC, 31.6 to 46.3% for IVDNDF, 5.6 to 23.2% for starch, 6.7 to 16.5% for soluble carbohydrates, 40.3 to 49.8% for NDF, and 4.5 to 7.3% for ADL in NDF.

Interrelationships among Digestibility-Related Traits Within Hybrids Experiments and Relation with Agronomic Traits
IVDDM was positively correlated with IVDNSC and IVDNDF and negatively correlated with NDF (Table 4) . IVDNSC was positively correlated with IVDNDF, and both were negatively correlated with ADL (%NDF) content, and in a lesser extent with NDF content. Starch content was correlated with soluble carbohydrates and NDF contents. The correlation coefficient between the sum starch+soluble carbohydrates and NDF was high .

Relationship between the Performance of Inbred Lines Per Se and Hybrids
As expected, there were greater ranges in digestibility traits among inbred lines per se than among line and tester mean performances in crosses (Table 5) .

	Discussion

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A range of more than 10 points for dry matter and cell wall digestibility was observed among hybrids and among inbred lines. In this study, low and high digestibility values were not closely associated with the genetic origin nor with the relative maturity.

The high negative correlation between starch and soluble carbohydrates is in agreement with findings of Russell et al. (1992) and could be attributed to remobilization and translocation of available carbohydrates from the vegetative part to the ear, which is strongly influenced by harvest stage (as shown by the significant correlations between dry matter content at harvest and starch and soluble carbohydrates contents, respectively). A negative linear relationship between the sum starch + soluble carbohydrates and NDF content was observed. Between the three traits (NDF, starch and soluble carbohydrates), approximately 92% of whole-plant dry matter was accounted for.

Whole-plant dry matter digestibility was, as expected, negatively related to NDF content and positively to IVDNDF and IVDNSC. The two cell wall digestibility estimates (IVDNSC and IVDNDF) were, as expected, tightly interrelated. In our study, lignin content in the cell walls determined about 80% of the genotypic variation of cell wall digestibility. Lignification is considered a major factor associated with reduced cell wall digestibility among forage maize genotypes (Wolf et al., 1993; Lundvall et al., 1994; Argillier et al., 1995a; Méchin et al., 1998). Lignin may influence the extent of cell wall digestion by means of physical shielding of cellulose or hemicellulose, or the effect of lignin may involve more specific molecular interactions (Buxton and Casler, 1993).

Biomass productivity of hybrids was significantly correlated with IVDDM, starch, soluble carbohydrates, and NDF contents of the whole plant and most likely attributable to relative maturity differences among hybrids. This is in agreement with studies of Allen et al. (1990), Geiger et al. (1992), Argillier et al. (1995b) using different genetic material. Cell wall digestibility appears to be independent of biomass yield. This was also observed by Allen et al. (1990), with U.S. commercial hybrids bred for grain yield, and Argillier et al. (1995b) within a factorial mating design of genetically diverse material. Therefore, it seems possible that maize breeders can select hybrids with good cell wall digestibility and simultaneously high biomass yield.

The low inbred-hybrid correlation coefficient for either starch or soluble carbohydrate content was partly related to the process of remobilization of soluble carbohydrates to the ear, which may have developed or not at the time of sampling, according to genotype, and partly to different sink source relationships in inbred lines and hybrids. For other traits, the correlation between inbred lines and hybrids was generally significant, which reinforced results of Gurrath et al. (1991) and Wolf et al. (1993). Inbred line per se IVDNSC, IVDNDF, and ADL values were strongly correlated with GCA values for these traits. This confirmed results of Dolstra et al. (1993) who found stalk cell wall digestibility to be the only stalk quality trait worth evaluating at the inbred line per se level.

Selection for whole-plant IVDDM of forage maize might produce hybrids with increased grain or starch content but unimproved cell wall digestibility. Hence, separate evaluation of starch content (or inversely NDF content) and cell wall digestibility would allow selection for both, and maximize genetic progress for digestibility. More progress is to be expected from selection for cell wall digestibility, because in addition to be less affected by plant maturity than other quality traits (Dolstra and Medema, 1990; Argillier and Barrière, 1996), it is a relevant discriminant criterion which can be evaluated at the inbred line level, and it is independent from biomass yield.

	ACKNOWLEDGMENTS

 
The authors thank all the persons at INRA Lusignan for their assistance in managing the experiments and conducting field studies, NIRS prediction and biochemical analyses. We also acknowledge J.P. Lundvall and A.R. Hallauer for having kindly sent to us the LAN496 inbred line.

Received for publication January 1, 1999.

	REFERENCES

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