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Published in Crop Sci 39:1435-1438 (1999)
© 1999 Crop Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA
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Crop Science 39:1435-1438 (1999)
© 1999 Crop Science Society of America

CROP QUALITY & UTILIZATION

Structural Responses to Selection for Reduced Fiber Concentration in Smooth Bromegrass

M.D. Caslera

a Dep. of Agronomy, Univ. of Wisconsin-Madison, Madison, WI 53706-1597 USA

mdcasler{at}facstaff.wisc.edu


   ABSTRACT
TOP
 NOTES
 ABSTRACT
INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Selection for reduced neutral detergent fiber (NDF) concentration has been practiced as a mechanism of increasing voluntary intake potential of forage crops. Because selection is usually conducted on the basis of whole-plant samples, there exists the potential for structural shifts in plant composition. The objective of this study was to quantify structural changes due to selection for reduced NDF concentration in smooth bromegrass (Bromus inermis Leyss). Three cycles of phenotypic selection led to reduced whole-plant NDF concentration, primarily due to reductions in NDF of stems (-5.7 to -5.8 g kg-1 cycle-1) and leaf sheaths (-3.1 to -4.9 g kg-1 cycle-1). Selection at both the vegetative growth stage (primarily leaf blades) or heading growth stage (all shoot components present) led to similar changes in shoot-component NDF concentrations. Selection on the basis of vegetative samples did not lead to structural changes. Conversely, selection on the basis of headed samples led to an average reduction of -7.5 % cycle-1 in stem component concentration, which was compensated largely by increases in leaf blade and sheath component concentrations. Increases in leaf:stem ratio may partially explain reductions in forage yield associated with reduced NDF concentration. Future selection efforts should attempt to avoid this response by using samples composed of a single shoot component. The NDF concentrations in various shoot components of smooth bromegrass appear to be positively correlated with each other.

Abbreviations: NDF, neutral detergent fiber • NIRS, near-infrared reflectance spectroscopy


   INTRODUCTION
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
PLANT BREEDING EFFORTS to increase voluntary intake potential of forage crops are increasing. The concentration of neutral detergent fiber (NDF) is often used as a laboratory predictor of voluntary intake potential for a forage crop. The concentration of NDF is repeatable, heritable, inexpensive to estimate, and amenable to analysis on thousands of plant samples (Casler, 1999a; Ostrander and Coors, 1997; Surprenant et al., 1988). Progress has been achieved in genetically reducing NDF concentration in smooth bromegrass (Casler, 1999a,b), reed canarygrass, Phalaris arundinacea L. (Surprenant et al., 1988), and maize, Zea mays L. (Ostrander and Coors, 1997). The rate of progress has been estimated to be as high as 12 g kg-1 cycle-1, a rate that can be achieved in 1 yr for annuals or 2 yr for perennials, and has been sustained for at least three cycles (Casler and Vogel, 1999).

Reductions in NDF concentration of forage crops may lead to two potentially serious consequences. In maize, low NDF concentration is associated with increased susceptibility to European corn borer, Ostrinia nubilalis Hübner (Buendgen et al., 1990). Efforts to break this relationship have been unsuccessful (Ostrander and Coors, 1997), indicating that NDF concentration is an important component of the resistance of some maize lines to European corn borer. Second, in reed canarygrass, divergent selection for NDF concentration led to positive and significant correlated responses in forage yield: each 1% change in NDF led to a 3.7% change in yield (Surprenant et al., 1988). Half of this effect was ameloriated by concomitant selection for high yield and divergent NDF, from which each 1% change in NDF led to a 1.9% change in yield. In smooth bromegrass, this effect was less severe; each 1% reduction in NDF concentration led to an average reduction of 1.4% in forage yield (Casler, 1999b).

With the major exception of maize, nearly all selection for increased forage nutritional value is conducted on whole-plant samples. Separations of specific shoot components is too time consuming unless it is considered essential to maximize genetic gains and minimize undesirable consequences. In maize, selection is based either on stalk or sheath samples, or both (Ostrander and Coors, 1997), because of large potential variability in shoot-component composition among plants. Recent results in smooth bromegrass suggest that selection on the basis of whole-plant samples at the heading growth stage may also suffer from this problem (Casler, 1999a). Heritability and realized gains were apparently reduced by excessive sampling variation among plants.

If some of the variation in shoot-component concentration is genetic in nature, selection for whole-plant forage nutritional value may lead to changes in shoot-component composition. This phenomenon has been reported following selection for increased N concentration (Demment et al., 1986) or reduced lignin concentration (Kephart et al., 1989) in alfalfa (Medicago sativa L.). In the former case, forage yield was unaffected, probably because it was included as a selection criterion. In the latter case, forage yield was consistently reduced in low-lignin lines, which also had consistently increased leaf:stem ratios. Thus, structural changes to the plant, resulting from use of whole-plant samples at a reproductive maturity stage, may partially explain some of the observed reductions in forage yield associated with reduced NDF concentration.

The objectives of this study were to quantify structural changes in smooth bromegrass plants resulting from three cycles of selection for reduced whole-plant NDF concentration.


   Materials and methods
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
Results and discussion
 Conclusions
 REFERENCES
 
Phenotypic selection for reduced NDF concentration was practiced in the WB-RP1 smooth bromegrass germplasm pool (Casler, 1992, 1999a). Three cycles of selection were completed between 1985 and 1990 on a Plano silt loam soil (fine-silty, mixed, mesic, Typic Argiudoll) near Arlington, WI. Thirty-five plants were selected from among 350 in each cycle.

Six selection methods were simultaneously, but independently, conducted during that time period. The selection methods differed in three factors: growth stage at which plants were sampled for NDF determination, method of laboratory analysis, and method of intercrossing selected individuals. The two growth stages were: vegetative (20–25 cm tall in early spring when plants consisted exclusively of leaves, and mostly leaf blades) and heading (Stage 59 of Simon and Park, 1983). The two methods of laboratory analysis were wet-chemistry and near-infrared reflectance spectroscopy (NIRS). The two methods of intercrossing selected individuals were open-pollination (plants remained in the original evaluation nursery) and polycrossing (plants were split and transplanted into a randomized and replicated crossing block).

The selection protocol generated 16 populations: the original (C0), six cycle-one (C1) populations, six cycle-two (C2) populations, and three cycle-three (C3) populations. The polycross method was conducted for only two cycles because it required an additional year compared to the open-pollination methods. Additional details of the selection protocols are provided by Casler (1999a).

Twenty seedlings from each of the 16 populations were transplanted to a holding nursery at Arlington, WI, in May 1993. In May 1994, two clonal ramets of each plant, approximately 100 cm2 of crown area, were transplanted to evaluate selection progress at Arlington, WI. The experimental design was a randomized complete block with two replicates. Plots consisted of a row of 20 plants per population, with the two clonal ramets allocated to different replicates and a 0.9-m spacing between all adjacent plants. Plants were watered, fertilized, and hand-weeded to aid establishment.

Plants were fertilized with 112 kg N ha-1 in early April 1995 and 1996. Five tillers were harvested from each plant at the fully headed growth stage in each year and bulked across the 20 plants of each plot. Because plants did not vary in timing of panicle emergence, they were all harvested on one day. Each tiller was separated into four components: stem, leaf sheath, leaf blade, and panicle (detached from the stem at the lowest rachis node). Samples of the four shoot components were dried and ground through a 1-mm screen of a Wiley-type mill and reground through a 1-mm screen of a cyclone mill. The mass of each shoot component sample was used to compute the concentration of each shoot component in the 100-tiller sample. Ground samples were analyzed in duplicate for NDF concentration using the wet-laboratory procedure of Van Soest et al. (1991) with the exceptions that sodium sulfite and {alpha}-amylase were excluded.

All response variables were analyzed by mixed-models analysis of variance, assuming blocks, years, and their interactions to be random effects and populations (cycles and methods) to be fixed effects. The main effect of years was treated as a repeated measures factor. The linear and quadratic effects of selection cycles were tested by contrasts, and selection responses were computed by linear regression. Differences among linear selection responses for the six selection methods were tested by contrasts formulated to test the methods x cycles interaction.


   Results and discussion
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
Conclusions
 REFERENCES
 
The two selection growth stages (vegetative vs. heading) resulted in differential selection responses for nearly all measured variables. The other factors (open-pollination vs. polycrossing and wet-laboratory vs. NIRS) had no effect on selection responses of measured variables (data not shown). Therefore, selection responses for NDF and shoot component concentrations are reported separately for vegetative and heading selection growth stages, but pooled for all other selection methods. Selection responses were homogeneous for the 2 yr, so they are reported as means over years.

NDF Concentration of Shoot Components
Selection for reduced NDF concentration at the vegetative growth stage numerically reduced NDF concentration for all shoot components by an average of -2.0 to -5.8 g kg-1 cycle-1 (Fig. 1) . Of these responses, the only significant response was for leaf sheaths (-4.9 g kg-1 cycle-1, P < 0.01). Whole-plant NDF at the heading growth stage declined by an average of -4.8 g kg-1 cycle-1 (P < 0.05) due to selection at the vegetative stage (Casler, 1999a). Leaf sheaths made up an average of only 172 g kg-1 of total dry matter (17.2%). Thus, while the leaf sheath NDF response was the only response to show significance, it could not explain the entire NDF response for whole plants. The numerical reduction in NDF of all shoot components suggested a positive ontogenic correlation for NDF of the various shoot components. Limited replication may have reduced statistical precision more for the non-sheath components than for the sheath component.



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  		Fig. 1 Selection cycle means and linear responses to selection for reduced whole-plant neutral detergent fiber (NDF) concentration at a vegetative growth stage, measured as NDF concentration of individual shoot components at the heading growth stage. Units of linear regressions coefficients are grams per kilogram per cycle. Data points represent means over two replicates and 2 yr

 
Selection for reduced NDF concentration at the heading growth stage numerically reduced NDF concentration for all shoot components by an average of -1.1 to -5.7 g kg-1 cycle-1 (Fig. 2) . Both leaf sheath and stem NDF responses were significant (P < 0.05). These two components combined to make up an average of 520 g kg-1 (52.0%) of whole-plant dry matter, but their high NDF concentration meant that they contained 59.6% of the total NDF in the plant. Their inherently high NDF concentration was likely the reason that stem and sheath NDF showed the greatest selection responses.



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  		Fig. 2 Selection cycle means and linear responses to selection for reduced whole-plant neutral detergent fiber (NDF) concentration at the heading growth stage, measured as NDF concentration of individual shoot components at the heading growth stage. Units of linear regressions coefficients are grams per kilogram per cycle. Data points represent means over two replicates and 2 yr

 
The consistent negative responses for all shoot-component NDF concentrations suggested that selection acted to reduce NDF concentration on all shoot components. This appeared to be true for selection based on leaf sheaths and blades or based on whole plants. These responses suggest a strong ontogenic autocorrelation for NDF concentration. Plants that are inherently lower in NDF concentration tend to have lower NDF concentration in all their shoot components, regardless of growth stage. This result is consistent with the observation of high genetic correlations between NDF concentration measured at different growth stages of smooth bromegrass (Ehlke et al., 1986; Reich and Casler, 1985). Stems and sheaths were most important because of their higher NDF concentration, while leaf blades and panicles appeared to require greater replication to detect their lower selection responses.

Shoot-Component Concentrations
Selection for reduced NDF concentration at the vegetative growth stage had no effect on the relative amounts of shoot components (stems, leaf sheaths, leaf blades, and panicles). Vegetative plant samples were highly uniform, composed mostly of leaf blades and a small amount of sheath tissue. Thus, there was no opportunity for selection at this growth stage to act on structural composition of these plants.

Selection for reduced NDF concentration at the heading growth stage had a large effect on structural composition of these plants (Fig. 3) . Stem component concentration decreased by -18.0 g kg-1 cycle-1 (-7.5% cycle-1). Increases in panicle, leaf blade, and leaf sheath component concentrations compensated for the decrease in stem concentration, with leaf blades and sheaths making up 89% of this response. The overwhelming importance of stem component concentration was likely due to the combination of higher NDF concentration and higher concentration per se of stems, compared with the other three shoot components.



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  		Fig. 3 Selection cycle means and linear responses to selection for reduced whole-plant neutral detergent fiber (NDF) concentration at the heading growth stage, measured as concentration of individual shoot components at the heading growth stage. Units of linear regressions coefficients are grams per kilogram per cycle. Data points represent means over two replicates and 2 yr

 
Smooth bromegrass plants at the heading growth stage are highly variable in structural composition and tiller size (Casler, 1999a). This variation is sufficiently large to reduce appreciably realized heritability and genetic progress for reduced NDF concentration. Much of this variation appears to be genetic in nature, as indicated by the large and significant selection responses for shoot component concentrations. An inability to control plant structural composition led to unconscious selection pressure for this trait. These results are similar to observations made on alfalfa, in which increased forage quality was associated with structural shifts toward more leaf and less stem tissue (Demment et al., 1986; Kephart et al., 1989).

These results may partly explain the consistent reductions in forage yield associated with selection for reduced NDF concentration in smooth bromegrass (Casler, 1999b) or selection for reduced lignin concentration in alfalfa (Kephart et al., 1989). The latter authors suggested that reductions in the amount of lower-stem tissue of alfalfa, associated with higher whole-plant forage quality, would result in reduced forage yield. Due to their structural and supportive function and their high concentration of highly-lignified schlerenchyma cells, stems appear to be more important contributors to forage yield than leaves. While the photosynthetic contribution of leaves cannot be discounted, it appears to be less limiting to forage yield than the amount of structural support tissue (stems).


   Conclusions
TOP
 NOTES
 ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
REFERENCES
 
Results from smooth bromegrass and alfalfa support a hypothesis of a minimum amount of stem tissue required to maintain forage yield potential. Any reduction in stem component concentration appears to limit forage yield potential of a plant, despite an increase in the amount of photosynthetic leaf tissue. The great majority of selections for increased forage nutritional value have shown no association between forage yield and nutritional value (Casler and Vogel, 1999). This has been due either to a lack of true genetic correlation between forage yield and nutritional value (Casler and Vogel, 1999) or to concomitant selection pressure for both traits (Demment et al., 1986; Hopkins et al., 1993).

Until further reports are made, the structural changes associated with reduced NDF concentration in smooth bromegrass and lignin concentration in alfalfa appear to be unique to these two traits of these two species. Kephart et al. (1989) suggested that such a response can be avoided by practicing selection for increased forage nutritional value of individual shoot component tissues, such as stem bases of alfalfa. While this should be effective at minimizing structural changes to the plant, it demands a significant increase in effort associated with selection. This will have the effect of reducing population size, selection pressure, and realized gains. In smooth bromegrass, selection on the basis of leaf tissue of vegetative plants offers the advantages of a reasonably high heritability, relatively uniform plant samples (no selection pressure for structural changes), and minimal effort to collect plant samples (Casler, 1999a).


   NOTES
TOP
 NOTES
ABSTRACT
 INTRODUCTION
 Materials and methods
 Results and discussion
 Conclusions
 REFERENCES
 
Research supported by Hatch formula funds.

Received for publication November 23, 1998.


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




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