Plant cell walls are the major source of dietary fiber for animals. Polysaccharides in cell walls cannot be degraded by mammalian enzymes. Instead, animals depend on microbial fermentation, and ruminants are especially well-adapted for using plant fiber for energy. Fiber, measured as neutral detergent fiber (NDF), usually accounts for 30-80% of the organic matter in forage crops. The remaining organic matter, cell solubles, is almost completely digestible. The nutritional availability of fiber to livestock, however, varies greatly, depending on its composition and structure. Lignin has been identified as primarily responsible for limiting digestibility of fiber, but fiber utilization is also limited by physical constraints at the cellular organization level.

AMOUNT OF FIBER IN PLANTS

Differences in digestibility between leaves and stems are normally less in grasses than in legumes. Stems decline in digestibility more rapidly with increasing plant maturation than do leaf blades. Digestibility also declines down stems, and in alfalfa and birdsfoot trefoil (Lotus corniculatus L.) the rate is 20 g·kg¹ per node (Buxton et al. 1995a). In addition to fiber concentration increasing within stems and most leaves with plant maturity, fiber concentration also is increased in total forage because the leaf:stem ratio decreases as plants mature. Albrecht et al. (1987) reported that the leaf:stem ratio of alfalfa decreased from 1.5 at the early vegetative stage to 0.5 by the late flowering stage.

SOURCES OF PLANT LIMITATIONS

Grass leaves develop a lignified midrib to provide mechanical support, which contributes to the high fiber concentration of grass leaf blades. Because the fiber concentration of grass leaf blades is twice that of legume leaves, grass leaf blades are often less digestible than those of legumes. The difference is not as great as suggested by the large difference in NDF concentration, however, because the NDF of grasses is more digestible.

Small particles are digested at faster rates than large particles because they have more surface area exposed per volume of tissue. In ruminants, forage particles are regurgitated and chewed (ruminated) after eating to reduce their size (Wilson and Kennedy 1996). Ruminants spend more time regurgitating and chewing grasses than legumes and more time chewing mature than immature forage. Legume particles in the rumen are often cuboidal, whereas grass particles are elongated and slender (Buxton et al. 1996). Filamentous grass particles are usually slower to pass from the rumen than the cuboidal fragments of legumes.

Not all plant fiber is digestible, even if it remains in the rumen for a long time. In mature forage stems, up to two-thirds of the NDF and more than half of the structural polysaccharides may be completely indigestible (Buxton and Casler 1993). Lignin concentration is closely related to the proportion of indigestible dry matter in forages (Buxton et al. 1996). Legumes generally have a more rapid digestion rate of potentially digestible NDF than grasses, but grasses have a larger portion of NDF that is potentially digestible. Smith et al. (1972) reported that potentially digestible NDF in several legumes digested at an average rate of 0.12 h¹ compared with 0.10 h¹ for grasses. They also found that grasses contained 686 g of potentially digestible NDF per kilogram of NDF compared with 512 for legumes. Additionally, immature forages have a faster rate of digestion than more mature forages. In the study of Smith et al. (1972), the digestion rate of grasses varied from 0.14 h¹ for immature forages to 0.05 h¹ for mature forages. For legumes, the range varied from 0.16 to 0.07 h¹ for immature and mature samples, respectively. Leaves have a shorter rumen retention time than stems due to both faster rates of fiber digestion and higher rates of passage of undigested material. Minson (1982) observed that the mean retention time for leaves was 25 h and that for stems was 33 h.

Depending on maturity, ruminants digest 40-50% of NDF in legumes and 60-70% in C₃ grass (Buxton et al. 1995a). The proportion of digestible energy obtained from NDF varies from 20 to 40% for legumes (60-80% from cell solubles) and from 50 to 80% for grasses (20-50% from cell solubles). Thus, most of the digestible energy in legumes comes from cell solubles, not from fiber.

Several chemical and structural features have been identified that may limit fiber digestibility. Of these, lignin is most prominently mentioned (Jung and Deetz 1993). Lignin is thought to interfere with microbial degradation of fiber polysaccharides by acting as a physical barrier. Recently the importance of cross-linking of lignin to polysaccharides by ferulate bridges has been implicated as an additional factor inhibiting digestion of grass fiber (Jung and Allen 1995). Similar cross-linking of lignin to fiber polysaccharides has not yet been identified in legumes.

Lignin is necessary to provide mechanical support for stems and leaf blades and to impart strength and rigidity to plant walls. Also, much evidence shows that lignin and lignin-like compounds along with other cell wall constituents provide resistance to diseases, insects, cold temperatures, and other biotic and abiotic stresses. Thus, practical limits exist as to how much lignin and other cell-wall constituents can be reduced in forages through breeding to improve digestibility without adversely affecting the ability of forages to grow and survive in field environments. Alfalfa selected for low lignin concentration grew less vigorously and had poorer field survival than alfalfa selected for high lignin concentration (Buxton and Casler 1993). Likewise, switchgrass selected for three cycles of high digestibility had lower field survival than the base population (Buxton et al. 1995b).

Physical and structural barriers may limit fiber digestibility beyond the effect of lignin. Waxes and the cuticle of the epidermis covering plants restrict microbe and enzyme access to forage tissues (Wilson and Kennedy 1996). Ruminal microbes normally enter interior cells in leaf blades and stems through stomata, fractures in the cuticle, or through cut or sheared surfaces. The epidermis of C₃ grasses is linked to thin-walled parenchyma cells, whereas in C₄ grasses the linkage is to thick-walled bundle sheath cells. The epidermis is usually shed within 24 h during digestion of legumes and C₃ grasses but may remain attached for longer than 48 h in C₄ grasses (Akin 1989, Wilson 1993). Both grass and legume stems have a ring of thick-walled, lignified cells resistant to digestion (Akin 1989).

Several grass cell types become lignified during maturation, whereas in legumes, xylem and tracheary cells are the only major tissues lignified (Wilson 1993). Lignified secondary walls of grass cells are digestible when microbes have access to these walls. Lignified primary walls and middle lamella, however, are not digestible. In contrast, lignified secondary walls of xylem and tracheary cells in legumes are not digestible (Wilson and Mertens 1995). Lignification in these legume cells is much higher than those in grasses, which might be the reason secondary walls in legumes cannot be digested.

For the past two decades, a growing awareness has developed that plant anatomy at the cellular level also influences fiber digestibility and that cell types differ in digestibility (Akin and Burdick 1975) (Table 1). Sclerenchyma, vascular tissue, and sometimes stem parenchyma and stem epidermis are digested very slowly and contain a substantial indigestible component. These tissues are the most highly lignified.

Table 1. Summary of plant tissues and their relative digestibility1 [View Table]

Leaf blades of C₃ grasses are usually more digestible than those of C₄ grasses because they have more mesophyll cells (Table 2). In C₃ species, mesophyll, phloem, epidermal and parenchyma bundle sheath cells are all totally degraded. In contrast, the epidermal and parenchyma bundle sheath cells in C₄ grasses may be slowly or only partially degraded (Akin 1989). The greater fiber concentration in C₄ compared with C₃ grasses is primarily due to larger amounts of vascular tissue and parenchyma bundle sheath cells (Akin and Burdick 1975).

Table 2. Relative tissue type proportions for cross-sections of warm- and cool-season perennial forage grasses [View Table]

Wilson and Mertens (1995) presented evidence that anatomical features of thick-walled grass cells may be as important as lignin in limiting fiber digestion. They noted that when the physical organization of cells is disrupted, fiber digestibility is improved. They identified five basic features of structural limitations to fiber digestion in grasses: 1) microbial degradation can proceed only from the interior of lignified, thick-walled cells because the lignified middle lamella and primary wall are indigestible; 2) particles pass from the rumen quickly in comparison with the digestion rate so that 20% of thick-walled cells can be digested; 3) access to cell interiors is not instantaneous because many cells composing fiber particles are not exposed or disrupted by mastication; 4) digestion of thick-walled parenchyma and sclerenchyma cells is limited by their low surface area:volume ratio; and 5) phenolic-carbohydrate compounds may be toxic to fiber-degrading bacteria on a microclimate level within cells, though average levels in the rumen are not high enough to be toxic.

Mesophyll, parenchyma and sclerenchyma cells in grasses exhibit large variations in size, shape and wall thickness, which affect the amount of surface area. Wilson and Mertens (1995) noted that mesophyll cells are not lignified and are often spherical with a diameter of ~50 µm. Wall thickness is only ~0.15 µm, and these cells can be digested from the outside inward. Stem parenchyma cells are cubical and ~100 µm on a side, whereas sclerenchyma cells are cylindrical in shape with dimensions of ~7 × 1000 µm. Wall thicknesses of parenchyma and sclerenchyma cells are much greater than for mesophyll, typically 1.0 µm and 2.4 µm, respectively. Because of the barrier from the lignified middle lamella and primary wall, these cells likely can be digested only from the interior outward.

Because the primary fiber-degrading bacteria are immobile, movement through sclerenchyma cells may be by diffusion. Wilson and Mertens (1995) estimated that 4.3 d would be required for bacteria to diffuse 500 µm into sclerenchyma and 17.0 d to diffuse the entire length of a sclerenchyma fiber 1000 µm in length. These observations strongly suggest that physical barriers to fiber digestion have a significant influence on fiber degradability.

IMPROVEMENTS THROUGH BREEDING

One of the first forage cultivars developed with improved dry matter digestibility was Coastcross-1 bermudagrass. Coastcross-1 was 12% more digestible than `Coastal' (Burton 1989). There are only small differences in leaf cell types (Table 2), but Coastcross-1 had bundle sheath cells that were more digestible than those in Coastal (Akin and Burdick 1975). In other work, smooth bromegrass genotypes selected divergently for forage digestibility differed in digestibility by 48 g·kg¹ (Ehlke and Casler 1985). The high digestibility genotypes had less bundle sheath and sclerenchyma in leaf blades (Table 2). Further analyses of these genotypes showed that high digestibility genotypes had larger leaf blades but fewer leaves per tiller than low digestibility genotypes (Casler and Carpenter 1989). In a third study, orchardgrass clones, selected for divergent leaf blade width (mean of 5.9 vs. 8.3 mm), differed in digestibility (Lentz and Buxton 1991). Digestibility averaged 19 g·kg¹ higher in leaf blades and 48 g·kg¹ higher in stems of wide-blade clones compared with narrow-blade clones. Wide-blade clones had fewer tissues that stained for lignin as a percentage of cross-sectional area than did narrow-blade clones.

Single-gene mutations (brown-midrib phenotypes, bmr), which affect fiber concentration, digestibility and composition, have been identified in corn (Zea mays L.), sorghum [Sorghum bicolor (L.) Moench.] and pearl millet [Pennisetum typhoides (Burm.) Staph and C. E. Hubbard]. These mutations are simply inherited recessives and can be readily identified by the brown color they induce in the stem and the leaf midrib (Cherney et al. 1991). The bmr mutants have a more condensed, highly cross-linked lignin that is more soluble than normal lignin. Digestibility is generally higher in bmr mutants. Studies comparing digestion of normal and bmr tissues show that slowly degraded sclerenchyma and bundle sheath cells in leaf blades and parenchyma cells in the midvein are more readily degraded in bmr than in normal plants (Akin and Chesson 1989). In contrast, highly lignified tissues were not degraded in either plant type.

PROSPECTIVE FOR THE FUTURE

A small portion of the total cells in forage seems to exercise large control over fiber digestibility. In many of these cells, the middle lamella and primary wall are probably responsible for lower fiber digestibility. Altering the digestibility of these cell types could improve the digestibility of fiber by providing access to many already potentially digestible cells. This could also enhance cell separation and particle disintegration and increase surface area for microbial action. This might be done by modifying lignin to reduce its inhibitory nature in these cell regions. Selecting for anatomical components in a breeding program, however, will be difficult because of the labor intensive nature of histological techniques and the generally small change in cell type associated with improvements in dry matter digestibility.