Lignin Constituents and Cell‐Wall Digestibility of Grass and Legume Stems

Abstract

Concentration and chemical constituents of lignin are factors that may influence cell‐wall (CW) digestibility of forages. This study was conducted to determine the apparent effect of lignin concentration on extent of CW digestion in immature and mature stems of grasses and legumes and to estimate and relate monomeric products of saponification and nitrobenzene oxidation of lignin to CW digestion. The basal 150 mm of stem from cultivars of field‐grown alfalfa (Medicago sativa L.), birdsfoot trefoil (Lotus corniculatus L.), smooth bromegrass (Bromus inermis Leyss.), orchardgrass (Dactylis glomerata L.), and red clover (Trifolium pratense L.) were harvested during spring growth. Laboratory evaluations included CW digestibility after 72 h of fermentation, neutral‐detergent fiber (to estimate CW concentration), permanganate lignin, alkali‐labile phenolic acids in CW, and nitrobenzene oxidation products (NBO). The apparent inhibition of CW digestion by lignin was 62% greater in grasses than in legumes. In grass stems, ferulic acid was positively correlated with arabinose [r = 0.58 (arabinose determined for another study)], whereas p‐coumaric acid was positively associated with lignin (r = 0.52). The syringaldehyde proportion in NBO increased 61% in grasses and 31% in legumes with stem maturity. The NBO as a proportion of lignin was closely related to CW digestion (r = −0.86) when both grasses and legumes were correlated. The syringaldehyde proportion in NBO was closely related to CW digestion in grass stems (r = −0.93), but not in the legumes (r = −0.23). Thus, there was close agreement between lignin concentration and CW digestibility within grass and legume stems, and the results indicate lignin chemical constituents can be important in accounting for differences in inhibition to digestion between grass and legume lignins.

Downregulation of Caffeic Acid 3-O-Methyltransferase and Caffeoyl CoA 3-O-Methyltransferase in Transgenic Alfalfa: Impacts on Lignin Structure and Implications for the Biosynthesis of G and S Lignin

“…Lignin levels increase with progressive maturity in stems of forage crops, including legumes such as alfalfa (Jung et al, 1997b) and in grasses such as tall fescue (Buxton and Redfearn, 1997). In addition, the lignin composition changes with advanced maturity toward a progressively higher S/G ratio (Buxton and Russell, 1988). Both lignin concentration (Albrecht et al, 1987;Casler, 1987;Jung et al, 1997a) and lignin methoxyl content (S/G ratio; Sewalt et al, 1996) have been negatively correlated with forage digestibility for ruminant animals.…”

“…Both lignin concentration (Albrecht et al, 1987;Casler, 1987;Jung et al, 1997a) and lignin methoxyl content (S/G ratio; Sewalt et al, 1996) have been negatively correlated with forage digestibility for ruminant animals. Although several studies have linked decreased forage digestibility to increased S/G ratio as a function of increased maturity (Buxton and Russell, 1988;Grabber et al, 1992), other studies have questioned the effect of lignin composition on digestibility (Grabber et al, 1997). Softwood gymnosperm lignins essentially lack S residues and therefore are highly condensed.…”

Targeted down-regulation of cytochrome P450 enzymes for forage quality improvement in alfalfa ( Medicago sativa L.)

“…Gymnosperm wood is therefore less amenable than angiosperm wood to the harsh chemical and physical treatments (pulping) necessary to remove lignin from cellulose during paper production (6), and pulping properties are improved by genetic modification of trees to increase the S͞G ratio (5). In many forage crops, lignin content and S͞G ratio increase with stem maturity (7,8), and both parameters correlate negatively with forage digestibility in ruminant animals (8)(9)(10)(11)(12). The amount of G lignin also has been linked with reduced cell wall degradability in forages (13), although studies with synthetic lignins (14) have questioned effects of lignin composition on forage digestibility.…”

Lignin and fiber digestion

“…The negative relationship of lignin concentration and cell-wall digestibility is true regardless of the method of lignin analysis employed and has been observed with in vivo and in vitro measures of digestibility (Jung et al 1997). Generally, the slope of this negative relationship is less for legumes than grasses, suggesting that lignin is more inhibitory of digestion in grasses (Van Soest 1964, Buxton andRussell 1988). This conclusion has been drawn primarily from research where lignin was measured as ADL and it should be considered suspect because the ADL method under-estimates lignin concentration more severely in grasses than legumes.…”