Forage quality variation among maize inbreds: Relationships of cell-wall composition and in-vitro degradability for stem internodes

Forty-five inbred maize (Zea mays L) lines were evaluated for genetic variation in stem cell-wall concentration, composition and degradability, and for relationships among cell-wall components and polysaccharide degradability. Cell-wall neutral sugars, uronic acids, Klason lignin, and ester- and ether-linked phenolic acids were measured on lower stem internode samples collected at the time of silking in 2 years. Twenty-four and 96 h in-vitro ruminal fermentations were used to determine the rapidly and potentially degradable cell-wall polysaccharide fractions, respectively. Genetic variation (P < 0.05) was found for all measures of cell-wall composition and many estimates of rapidly and potentially degradable cell-wall polysaccharide components. Inbred line means varied by 50–300% for most traits. Three brown midrib mutant inbred lines included in the study were not the lowest in lignin content nor did they exhibit the greatest cell-wall degradabilities in this population of inbred maize. Year of growth (environment) influenced (P < 0.05) cell-wall traits even though reproductive physiological maturity at sampling was similar in both years. Degradability of the cell-wall polysaccharide components were intercorrelated (P < 0.05) within the rapidly and potentially degradable fractions, but rate and extent of degradation of the cell-wall components were not correlated (P > 0.05), except for uronic acids. A multiple regression model of principal components (R² = 0.41, P < 0.05) indicated that cell-wall lignification and substitution of wall polymers with phenolic and uronic acids were negatively associated, and pectic substances were positively related with rapid polysaccharide degradation. Very little of the variation (R² = 0.15, P < 0.05) in potential cell-wall polysaccharide degradation could be explained by this multiple regression analysis. There is a large degree of genetic variation among current inbred maize lines for stem cell-wall quality traits, which should allow improvement of maize as a forage crop. Because of the complex matrix interactions in cell-wall organization, however, no single cell-wall component, or simple combination, can accurately predict degradability of maize cell walls.     

Characterization of p‐coumarate accumulation,p‐coumaroyl transferase,and cell wall changes during the development of corn stems

BACKGROUND: Developmental changes occur in corn (Zea mays L.) stems from cell initiation to fully mature cell types. During cell wall maturation the lignin is acylated with p‐coumarates (pCA). This work describes characterization studies of the p‐coumaroylation process in relation to corn stem development. RESULTS: Corn plants from three locations were harvested and tissues were analyzed from all nodes and even‐numbered internodes above soil line. Changes in carbohydrates reflect a shift to lignification at the expense of structural polysaccharide synthesis. Accumulation of pCA paralleled the incorporation of lignin while ferulate (FA) remained relatively constant as a proportion of the cell wall (5–7 g kg^?1 CW). The p‐coumaroyl transferase (pCAT), which is responsible for attaching pCA to lignin monomers, displayed maximum levels of activity in the middle region of the stem (internodes 10–12, 2–3 nmol L^?1 min^?1 mg^?1). The syringyl content as a proportion of the total lignin did not change significantly with cell wall maturation although there was a trend towards increased amounts of syringyl units in the more mature cell walls. CONCLUSIONS: Incorporation of pCA into corn cell walls not only mirrored lignification but the pCAT activity as well. Levels of pCAT activity may be an indicator of rapid lignification specifically for syringyl type lignin. Copyright © 2008 Society of Chemical Industry     

Biological variability in lignification of maize: Expression of the brown midrib bm3 mutation in three maize cultivars

Cell wall-linked phenolics were investigated in maize internodes located at three positions of the stem (top-middle-bottom). While the lignin content did not change drastically with position, the amount of cell wall-ester linked p-coumaric acid sharply increased from the top to the bottom internodes of the stem. Conversely, the saponified ferulic acid content remained relatively unchanged along the stem. Moreover, the highest syringyl content of the β-O-4-lignin structures was found in the basal (most mature) internode. Therefore, enhanced p-coumaric esterification of the cell wall and preferential deposition of syringyl units in the lignin polymer might indicate an extended maturity stage of the cell wall of maize internodes. The bm3 mutation in the three maize hybrids is expressed by lignin reduction and ester-bound p-coumaric decrease in the mutant lines. Furthermore, all bm3 hybrids synthesised lignin polymers which were characterised by a very low S/G molar ratio (0.16-0.43). This ratio originates from the substantial reduction of the syringyl unit content in β-O-4-lignin structures compared with the normal lignin. The occurrence in the same range of the 5-hydroxyguaiacyl unit in bm3 lignin from each type of hybrids was noteworthy, demonstrating the high heritability of the bm3 mutation at the molecular level. The alkaline solubility of lignin was greater for the three mutant lines compared to the normal cultivars. Furthermore, the alkali-labile fraction of lignin of both normal and mutant lines had a monomeric composition which was consistent with the non-condensed structures of in-situ lignin.     

Digestibility of silages in relation to their hydroxycinnamic acid content and lignin composition

BACKGROUND: The effectiveness of the analysis of cell wall‐bound hydroxycinnamic acids and the composition of lignin to evaluate the in vivo digestibility of a silage collection with unknown botanical composition was evaluated. RESULTS: Syringyl units content and total etherified phenols showed the highest correlation coefficients with in vivo dry matter digestibility (IVDMD) (r = ? 0.792 and r = ? 0.703, respectively), while guaiacyl units and total phenols showed the highest correlation coefficients with in vivo organic matter digestibility (IVOMD) (r = ? 0.871 and r = ? 0.817, respectively). Using the above‐mentioned chemical parameters, 10 equations were also developed to predict in vivo digestibility. The prediction of IVDMD produced a high adjusted R² value (0.710) using syringyl, total lignin, etherified total phenols, esterified ferulic acid and total phenol content as predictors. The prediction of IVOMD produced a higher adjusted R² value (0.821) using guaiacyl, total phenols, total ferulic acid and etherified p‐coumaric acid content as predictors. CONCLUSION: Cell wall digestibility depends on a multiplicity of factors and it is not possible to attribute a causal effect on in vivo digestibility to any single factor. However, syringyl and guaiacyl content and etherified phenols emerge as good predictors of digestibility. Copyright © 2010 Society of Chemical Industry     

Cell wall composition and degradability of forage stems following chemical and biological delignification

Chemical and biological delignification methods were used to investigate the relationship between the concentration and composition of lignin and degradation of forage cell walls. Stem material from lucerne (Medicago sativa L), smooth bromegrass (Bromus inermis Leyss) and maize (Zea mays L) stalks was treated with alkaline hydrogen peroxide, potassium permanganate, sodium chlorite, sodium hydroxide, nitrobenzene, and the lignolytic fungus Phanerochaete chrysosporium. Klason lignin and esterified and etherified phenolic acids were delermined. Cell wall neutral sugar and uronic acid composition and the extent of in-vitro degradability were measured. Chemical delignification generally removed lignin. but the fungal treatment resulted in the removal of more polysaccharide than lignin. The concentrations of esterfied and etherified p-coumaric and ferulic acids were generally reduced in treated cell walls; chlorite treatment preferentially removing p-coumaric acid whereas nitrobenzene treatment removed more ferulic acid. Syringyl moieties were completely removed from the core lignin polymer by nitrobenzene treatment of forage stems. Alkaline hydrogen peroxide and nitrobenzene were generally the most effective delignification treatments for improving polysaccharide degradability, with the grass species responding similarly to delignification whereas lucerne was somewhat less responsive. Fungal delignification, under these experimental conditions, did not improve cell wall degradability of these forages. Multiple regression and covariate analyses indicated that the lignin components measured were not powerful predictors of cell wall degradability. Neither the concentration nor the composition of the lignin fractions was consistently correlated with degradation. This lack of effect was attributed to the more generalised disruption of the cell wall matrix structure by delignification treatments.     

Effect of down-regulation of cinnamyl alcohol dehydrogenase on cell wall composition and on degradability of tobacco stems

The effect of down-regulation of tobacco cinnamyl alcohol dehydrogenase (CAD) on cell wall composition and degradability has been assessed. CAD activity was only 20, 16, 14 and 7%, relative to the controls, in four populations of plants (designated 40-1, 40-2, 48 and 50, respectively) transformed with CAD antisense mRNA. Cell wall residues of stem samples were analysed for polysaccharide composition, gravimetric and acetyl bromide lignins and lignin nitrobenzene oxidation products. In situ disappearance and cellulase solubility of both initial dry matter and CWR were determined. The populations of plants with depressed CAD activity showed no change in lignin content but some consistent changes in cell wall composition and digestibility were identified. The syringyl content of lignins decreased and the syringaldehyde to vanillin ratio (S/V) was consequently reduced. Dry matter degradability, as measured by both methods, was significantly improved in all CAD-depressed samples except for population 40-1, which was the least CAD-depressed. Increased in situ disappearance of cell wall (ISCWD) was found in all plants exhibiting more than 80% CAD down-regulation and was maximal (7 percentage units) in population 50 which had the greatest CAD depression. The rates of ISCWD increased slightly in some populations (40-2 and 50). The relationship between S/V and ISCWD was significant (R = -0·68) only in the samples from a selected population of mature, most depleted plants. Other modifications may therefore also contribute to the improvement in degradability. However the changes in lignin composition that were observed in CAD-depressed tobacco are largely similar to those seen in some maize and sorghum mutants with altered lignification and improved digestibility. These data therefore suggest that depressing CAD activity may be an effective method for improving digestibility in forage crops. © 1998 SCI.     

Biological variability in lignification of maize: Expression of the brown midrib bm2 mutation

The cell wall phenolic components in the internodes of three maize genotypes, namely normal, bm2 and bm3 maize, were determined. The bm2 and bm3 brown midrib mutations lowered the lignin content of the bottom, middle and top internodes to a similar extent. However, unlike bm3, the bm2 trait did not induce a sharp reduction of the level of ester-bound p-coumaric acid in maize internodes. The other main alkali labile phenolic acid, ferulic acid, reached similar levels in the three genotypes. The main difference between bm2 and bm3 mutations occurred in the alkyl aryl ether linked structures of the lignin component. In contrast to bm3 lignins, which are characterised by a low syringyl content, the bm2 lignin had a lower content of guaiacyl units than lignin of normal maize internode. Consequently, the syringyl/guaiacyl molar ratio of bm2 lignin gave higher values (2.7–3.2) than those from either normal (0.9–1.5) or bm3 lignins (0.3). The alkali solubility of lignin was also compared between the three genotypes. Incorporation of the bm3 trait in maize led to a high recovery of alkali soluble lignin whereas the bm2 lignin had a similar solubility to the normal one in 2 M NaOH. The monomeric composition of the alkali soluble lignins was consistent with the non-condensed structures of the in-situ polymer. Although the bm3 and bm2 mutations had different effects on lignification, the modification of the cell wall phenolic level was also found in the bm2 maize stem as previously studied.     

Characterisation of Lignin from Parenchyma and Sclerenchyma Cell Walls of the Maize Internode

Internodes of maize (Zea mays L, Co125), harvested 5 days after anthesis, were sectioned into five equal parts and samples of sclerenchyma and parenchyma cells mechanically isolated from each section. Phenolic acids and syringyl and guaiacyl degradation products of lignin were released from the walls of the two cell types by microwave digestion with 4 M NaOH. Aryl ether bonded units were selectively released by thioacidolysis. Total phenolic content of cell walls from the youngest (basal) sections were approximately two-thirds of those of the oldest, topmost sections (parenchyma 70·8–99·0 and sclerenchyma 72·5–114·1 mg g^-1) indicating that the process of lignification was already well advanced amongst most of the cell walls of the youngest section. The total phenolic content was marginally, but significantly, greater (P<0·05) in sclerenchyma walls than in parenchyma walls at all stages of maturity. There was no significant difference in phenolic acid concentrations between cell types from the same section but p-coumaric acid concentration increased with maturity (P<0·001) in walls from both cell types. The increase in p-coumarate with age was matched by an increased recovery of syringyl units resulting in a constant coumaroyl: syringyl molar ratio. Recovery of acetosyringone was significantly greater (P<0·001) from sclerenchyma than parenchyma walls and, in sclerenchyma, acetosyringone as a proportion of total syringyl recovery, increased significantly with age (P=0·015). Digestion with NaOH and thioacidolysis released comparable amounts of guaiacyl residues but NaOH digestion released approximately twice the amount of syringyl residues. This difference may be explained by the retention of the ester-bond between p-coumaric acid and syringyl units during thioacidolysis but not during digestion with 4 M alkali. The similarity in phenolic composition suggested that both cell types, despite their considerable anatomical differences, were exposed to a common flux of lignin precursors during the later stages of lignification as illustrated by the internode sections. Differences between cell walls arose because of differences in the regiochemistry of precursor incorporation. © 1997 SCI.     

Lignin composition is more important than content for maize stem cell wall degradation

BACKGROUND

The relationship between the chemical and molecular properties – in particular the (acid detergent) lignin (ADL) content and composition expressed as the ratio between syringyl and guaiacyl compounds (S:G ratio) – of maize stems and in vitro gas production was studied in order to determine which is more important in the degradability of maize stem cell walls in the rumen of ruminants. Different internodes from two contrasting maize cultivars (Ambrosini and Aastar) were harvested during the growing season.

RESULTS

The ADL content decreased with greater internode number within the stem, whereas the ADL content fluctuated during the season for both cultivars. The S:G ratio was lower in younger tissue (greater internode number or earlier harvest date) in both cultivars. For the gas produced between 3 and 20 h, representing the fermentation of cell walls in rumen fluid, a stronger correlation (R² = 0.80) was found with the S:G ratio than with the ADL content (R² = 0.68). The relationship between ADL content or S:G ratio and 72‐h gas production, representing total organic matter degradation, was weaker than that with gas produced between 3 and 20 h.

CONCLUSION

The S:G ratio plays a more dominant role than ADL content in maize stem cell wall degradation. © 2017 The Authors. Journal of The Science of Food and Agriculture published by John Wiley & Sons Ltd on behalf of Society of Chemical Industry.     

The newly extended maize internode: A model for the study of secondary cell wall formation and consequences for digestibility

The upper five internodes were collected from maize (Zea mays L) inbred cell lines Co 125 and W401 harvested at the same developmental stage, 5 days after silking. Each internode was dissected into ten equal lengths labelled A (top) to J (base). The youngest cells were found in section J, which contained the intercalary meristem, and the oldest in section A. Internodes 1, 3 and 5 provided material for chemical analysis and internodes 2 and 4 for degradability measurements. Cell wall material accounted for one-third of dry matter in section J, doubling to two-thirds in the upper half of each internode. Only section J exhibited a polysaccharide profile typical of primary cell walls. In all other sections, 1,4-linked glucose (± 46% of cell wall) and xylan largely free from side chains (± 25% of cell wall) predominated. Net accretion of cell wall polysaccharide reached a maximum by segment G and thereafter little additional carbohydrate was deposited. Lignification appeared to be separated from the biogenesis of structural carbohydrate and continued over much of each internode reaching a maximum in section C. Degradability measurements, made using a modified neutral-detergent cellulase digestibility method, showed substantial differences between sections. In line Co 125, cell wall degradability fell from over 95% in the youngest section (J) to approximately 24% in section B. Internode 4 of line W401 failed to show the same pattern of degradabilities, probably because of a sequential rather than simultaneous pattern of internode elongation. Saponifiable p-coumaric acid appeared to provide a more sensitive marker than lignin of the extent of secondary wall development. The inverse relationship between extent of lignification in each section and its degradability confirmed the value of the internode model for the study of secondary wall formation and its biological consequences.     

Relationship of cell wall composition to in vitro cell wall digestibility of maize inbred line stems

The phenolic equipment of maize stem tissues was investigated in relation to the feeding value of the detergent fibre components. Sixteen maize inbred lines, including three brown‐midrib 3 mutants and their normal counterparts, were selected for highly divergent in vitro cell wall digestibility. These lines were grown during two years. Maize stems were analysed for detergent fibre concentration, esterified and etherified p‐hydroxycinnamic acids, lignin content and structure and in vitro digestibility. A large genotypic variation was found for neutral detergent fibre, cell wall phenolic composition and cell wall digestibility. Within the normal maize lines the in vitro neutral detergent fibre digestibility (IVNDFD) of stem fractions was negatively correlated with their Klason lignin content. A multiple regression model based on esterified p‐coumaric acid and lignin composition as two explanatory variates accounted for 58% of the IVNDFD variation. In this study, three normal maize inbred lines displaying a lignin content and a cell wall digestibility level close to those observed in the three bm3 lines could be detected, which opens up new breeding avenues. © 2000 Society of Chemical Industry     

Cell wall fermentation kinetics are impacted more by lignin content and ferulate cross‐linking than by lignin composition

BACKGROUND: We used a biomimetic model system to ascertain how reductions in ferulate–lignin cross‐linking and shifts in lignin composition influence ruminal cell wall fermentation. Primary walls from maize cell suspensions with normal or reduced feruloylation were artificially lignified with various monolignols previously identified in normal, mutant, and transgenic plants. Cell wall fermentability was determined from gas production during in vitro incubation with rumen microflora and by analysis of non‐fermented polysaccharides. RESULTS: Hemicellulose fermentation lag time increased by 37%, rate decreased by 37%, and the extent declined by 18% as cell wall lignin content increased from 0.5 to 124 mg g^?1. Lignification increased lag time for cellulose fermentation by 12‐fold. Ferulate–lignin cross‐linking accounted for at least one‐half of the inhibitory effect of lignin on cell wall fermentation. Incorporating sinapyl p‐coumarate, a precursor of p‐coumaroylated grass lignin, increased the extent of hemicellulose fermentation by 5%. Polymerizing varying ratios of coniferyl and sinapyl alcohols or incorporating 5‐hydroxyconiferyl alcohol, coniferaldehyde, sinapyl acetate, or dihydroconiferyl alcohol into lignin did not alter the kinetics of cell wall fermentation. CONCLUSION: The results indicate that selection or engineering of plants for reduced lignification or ferulate–lignin cross‐linking will improve fiber fermentability more than current approaches for shifting lignin composition. Copyright © 2008 Society of Chemical Industry     

Changes during growth in anatomical and chemical composition and in-vitro degradability of lucerne

Lucerne (alfalfa) was harvested at different stages of maturity and the plants classified according to a phenological scale. The main class was separated into anatomical parts and analysed for content of extractives and crude protein and in-vitro degradability. Whole crop samples were also analysed for polysaccharide residues, Klason lignin and in-vitro degradability of crude protein and polysaccharide residues. The composition of the plant changed during maturation with an increase of the stem fraction from 18.5% to 50.7% of dry matter and a corresponding decrease in the leaf fraction from 72.9% to 18.4% of dry matter. The leaf fraction changed least in content of extractives and crude protein and in-vitro degradability while in the stems all these paramaters decreased radically. Chemical analysis of residues after incubation with rumen liquor in vitro of whole crop samples revealed that the degradability of crude protein declined from about 90% to about 80% and of non-starch polysaccharides from about 90% to about 60% during maturation. Xylose was the least degradable polysaccharide residue at all harvests. Xylose residues also showed the greatest decrease in degradability during growth. A comparison showed that stage of development and harvest date were of practically equal value as predictor of crude protein content in lucerne.     

Impact of accessibility and chemical composition on cell wall polysaccharide degradability of maize and lucerne stems

Although lignification of forages is generally accepted as limiting cell wall degradability, prediction of degradation from cell wall composition is often difficult when forages are of similar maturity. It has been proposed that rumen microbe accessibility to potentially degradable cell walls is limited by the presence of non‐disrupted cells in forage particles with lignified middle lamella/primary walls acting as barriers to microbial access. We tested this accessibility hypothesis by evaluating the impact of reducing particle size of maize and lucerne stems to the level of individual cells by ball‐milling, in order to eliminate accessibility as a limiting factor. While cell wall concentration and composition were not influenced by ball‐milling compared with grinding to pass a 1 mm screen in a cyclone‐type mill, degradability of total cell wall polysaccharides was dramatically increased. However, only those polysaccharides (cellulose and xylan) which are most abundant in cell types with lignified middle lamella/primary and secondary walls increased in degradability owing to particle size reduction. Degradability of pectins, which are abundant in non‐lignified tissues in lucerne, did not respond to ball‐milling. Contrary to our expectations, ball‐milled forages showed fewer correlations for cell wall composition with degradability than observed for the larger‐particle‐size grinding treatment. Many components of the cell wall were correlated with polysaccharide degradation for the cyclone‐ground samples; however, the results were inconsistent as to which cell wall components were correlated with degradation among and within forages. This observation does not clarify the role of cell wall chemical structure as a limiting agent to wall degradation in the absence of accessibility barriers, but this study does provide support for the hypothesis that lignified middle lamella/primary walls act as barriers to microbial access for degradation. © 2000 Society of Chemical Industry     

Phenolic components and degradability of the cell walls of the brown midrib mutant,bm3, of Zea mays

Cell walls separated from the leaf blade, leaf sheath and stem of the brown midrib mutant, bm₃, of Zea mays were more degradable by a commercial cellulase than the corresponding part of the isogenic normal inbred cultivar (Tr). The walls of each part of the mutant when compared with the corresponding part of the normal cultivar contained less lignin and bound phenolic components released by treatment with NaOH. The major phenolic components detected were trans-p-coumaric and trans-ferulic acids together with small amounts of their cis isomers and diferulic acid. Cell walls of stem of the mutant contained a total of 17.3 mg g^?1 of these bound acids compared with 9.8 mg g^?1 for leaf sheath and 3.5 mg g^?1 for leaf blade: there was more than twice as much p-coumaric acid in cell walls of stem as in those of leaf sheath and more than seven times as much as in those of leaf blade. When cell walls of the stem from the mutant or the normal cultivar were treated with NaOH their degradability by cellulase was highly correlated with the amounts of phenolic components released by the alkali.     

Composition of cell walls isolated from cell types of grain sorghum stems

Cell types were isolated from sorghum stems at two stages of development, anthesis and grain maturity, to study cell wall characteristics. Cell walls were isolated from epidermis (EPID), sclerenchyma (SCL), vascular bundle zone (VBZ), inner vascular bundles (IVB) and pith parenchyma cells (PITH) and analysed for total carbohydrate, acid insoluble lignin, total uronosyls, neutral sugars and hydroxycinnamic acids. In addition, walls from SCL, VBZ, IVB and PITH were subjected to chemical fractionation to separate wall carbohydrate into polysaccharide groups. Although wall characteristics were similar at both plant maturities, there were differences in lignin concentration, hydroxycinnamic acids, and carbohydrate composition among the cell wall types. Lignin was lowest in the PITH walls (169 g kg⁻¹) and highest in SCL and EPID (c 211 g kg⁻¹). Cellulose was most abundant in VBZ and SCL walls with greater secondary wall formation. Pectic materials were most abundant in PITH walls. Xylans were similar among wall types except for EPID that contained higher amounts of xylose. Releasable hydroxycinnamates were not as consistent among the cell wall types. Total ferulates, including ester linked and releasable ether linked, tended to increase from PITH to SCL (8 to 15 g kg⁻¹ CW) with an increase in the proportion etherified within the wall matrices (PITH 51%; SCL 66%). Total p‐coumarates showed opposite trends with PITH walls having significantly more (35 g kg⁻¹ CW) than VBZ or SCL (19 and 13 g kg⁻¹ CW). EPID walls contained the least pCA (6.5 g kg⁻¹ CW). Except for the hydroxycinnamates, compositional trends for the different wall types would reflect changes from primary walls to increased amounts of secondary wall. Neutral sugar analysis of indigestible residues indicated similar carbohydrate compositions among the cell wall types, with xylose being less degradable than all other wall sugars. © 1999 Society of Chemical Industry     

Effect of lucerne preservation method on the feed value of forage

BACKGROUND: Natural climatic wilt (NCW) and induced industrial wilt (IIW) are widely used as preservation methods for lucerne. Both of these methods reduce the quality of green forage due to respiration under NCW and heat damage under IIW. We compared the influence of these two preservation methods on nutritive value across a wide range of harvest conditions. RESULTS: Cell wall content and cell wall‐linked nitrogen values were higher (P < 0.05) in IIW than NCW. The preservation methods differed significantly (P < 0.05) in terms of soluble fraction, insoluble potentially degradable fraction of dry matter and effective degradability of dry matter. Nitrogen disappearance kinetics showed that the interactions of preservation by cut and preservation by phenological state were significant for the effective degradability of nitrogen. Organic matter digestibility was higher in lucerne preserved by NCW than IIW whereas cell wall digestibility was higher in lucerne preserved by IIW than NCW. Digestible organic matter intake did not differ between preservation methods (P > 0.05). CONCLUSION: Natural climatic wilt forage hay presents similar feed value to the induced industrial wilt alfalfa. Copyright © 2009 Society of Chemical Industry     

Lignification of switchgrass (Panicum virgatum) and big bluestem (Andropogon gerardii) plant parts during maturation and its effect on fibre degradability

Five cultivars of switchgrass (Panicum virgatum L) and four cultivars of big bluestem (Andropogon gerardii Vitman) were harvested at vegetative, boot and heading stages of maturity. Leaf and stem fractions were analysed for detergent fibre composition and 48-h ruminal in-vitro degradability, ester- and ether-linked non-core lignin phenolic acids, and core lignin composition. Big bluestem leaves contained more neutral detergent fibre than switchgrass, but general composition of the fibre did not differ. Stem fibre of switchgrass had relatively lower levels of cellulose and lignin at the vegetative stage than observed in big bluestem. Esterified and etherified p-coumaric and ferulic acid concentrations were generally higher in switchgrass plant parts. Yield of nitrobenzene oxidation products from core lignin was greater for switchgrass leaves, but very little difference in composition was noted. Leaf tissue contained lower concentrations of all lignin components than stems. Maturation resulted in increased total lignification, but all components did not respond in the same manner. Variation for all measures of lignification seemed to be as great within species as between the grass species. Degradability of fibre declined with maturation. The only species difference was that switchgrass fibre was more degradable at the vegetative stage. Relationships between lignification and fibre degradability were in agreement with some, but not all, previously reported relationships. Concentration of core lignin was only a significant predictor of fibre degradability when the relationship was examined across maturity stages. Within a stage of maturity, lignin composition was more closely related to fibre degradability than was concentration. However, the best predictors of fibre digestibility differed among species, plant part and maturity.     

Transformations of (14C-lignin) cell walls of wheat by rumen microorganisms

The lower halves of apical internodes of wheat harvested at the flowering stage were labelled with [U-¹⁴C] phenylalanine (phe) or with [O¹⁴CH₃] sinapic acid (sin). Cell wall residues (CWR) and saponified residues (SR) were incubated in a fermenter simulating the rumen for 7 days with rumen fluid or without microorganisms (controls). PheCWR was labelled in all lignin units (measured as aldehydes from nitrobenzene oxidation), in phenolic acids and slightly in proteins. Labelling of pheSR was more lignin-specific. SinCWR and sinSR were specifically labelled in syringyl units of lignin. The fermentation of CWR resulted in phenylpropane-derived unit losses in the following decreasing order: ferulic acid>p-coumaric acid>syringaldehyde>vanillin>p-hydroxybenzaldehyde. If allowance is made for slight losses in controls, 61, 52, 61 and 63% of the phenylpropanes of pheCWR, sinCWR, pheSR and sinSR, respectively, were transformed into an acid-precipitable fraction, an acid-soluble fraction and ¹⁴CO_2. The comparison of pheCWR and sinCWR degradation showed that syringyl units were solubilised into acid-precipitable molecules to a greater extent than the other lignin units; demethylation of the syringyl units of lignins was also evident from the different productions of ¹⁴CO₂. Alkali-resistant lignins of SR were mainly transformed into acid-precipitable molecules and were weakly degraded. Lignin solubilisation and degradation seem to be governed by different mechanisms which depend on both cell wall structure and rumen microflora.     

Changes in the biodegradability of ryegrass and legume fibres by chemical and biological pretreatments

Two moderately lignified lignocellulosic substrates, ryegrass (Lolium perenne L) and lucerne (Medicago sativa L) stems, were modified by treatment with different chemical and biological reagents. The reagents were chosen to have effects on one or more of the components of the cell wall. The residues were analysed for yield of DM, composition of carbohydrate and non-carbohydrate constituents and degradability as determined by both rumen liquor and a pepsin/cellulase method. The results were compared with the previous results for a highly lignified substrate, barley straw. Both degradability methods again gave results with similar ranking, and the rumen liquor method was more efficient than the pepsin/cellulase method. In these series, only cellulase treatment gave residues with decreased degradability. Treatments attacking the core lignin gave residues with the greatest increase in degradability, but the changes were not as pronounced as for the straw-owing to the lower inherent lignin contents. Some differences between the grass and the legume may be due to the higher protein and lower esterified phenolic acid content of the lucerne.