The anatomy of forage plants impacts forage nutritive value through the physical arrangement, chemical composition, and proportional mass of each tissue type (Grabber et al., 1992). Cool-season grasses possess leaf anatomy that is characteristic of the C3 photosynthetic mechanism, whereas warm-season grasses generally possess leaf anatomy typical of the C4 mechanism (Volenec and Nelson, 2007). Leaves with C3 anatomy have lower cell wall concentrations and a greater proportion of highly digestible cell solubles than leaves of C4 plants (Goering and Van Soest, 1970; Van Soest and Robertson, 1980). Cell wall concentrations in C3 grasses range between 550 and 700 g/kg, whereas they are 600 to 800 g/kg in C4 grasses. The cell walls of C3 plants also are degraded more rapidly in the rumen (Hanna et al., 1973; Akin and Burdick, 1975). For example, leaf tissue of tall fescue after 6, 12, 24, 48, and 72 h of fermentation had DM disappearances of 175, 234, 262, 482, and 449 g/kg, respectively (Akin et al., 1973). This compared with disappearances of 5, 96, 88, 183, and 314 g/kg, respectively, for ‘Coastal' bermudagrass [Cynodon dactylon (L.) Pers.] leaves for the same durations (Akin et al., 1973).
Analyzing forages for their proportions of rapidly vs. slowly degradable tissue types helps explain forage quality differences among species in addition to chemical composition differences. Cool-season C3 grasses have a lower proportion of leaf cross-sectional area composed of slowly rumen-degradable tissue types than C4 grasses and a correspondingly greater proportion of rapidly rumen-degradable tissue types (Akin et al., 1973, 1975; Buxton, 1990; Fisher et al., 1989). The proportional area of rapidly degraded tissue of the C4 grasses bermudagrass, bahiagrass (Paspalum notatum Flüggé), dallisgrass (Paspalum dilatatum Poir.), and pangolagrass (Digitaria eriantha Steud.), averaged 27 to 55% when compared with 56 to 66% for the C3 grasses smooth bromegrass (Bromus inermis Leyss.), orchardgrass (Dactylis glomerata L.), timothy (Phleum pratense L.), Kentucky bluegrass (Poa pratensis L.), and tall fescue (Akin and Burdick, 1975). Slowly degraded tissue area averaged 36 to 56% for the C4 grasses but only 22 to 31% for the C3 grasses. Grouping tissues relative to rate of in vitro degradation in rumen fluid showed that 39% of C4 grass tissues were rapidly degradable, 50% were slowly degradable, and 11% were not degradable. This compares with values of 60, 26, and 15%, respectively, for C3 grass tissues, indicating a physical basis for the greater disappearance of the C3 grasses (Akin et al., 1975).
Within the cool-season grasses, cross-sectional areas of leaf blade tissues were compared among tall fescue, orchardgrass, bromegrass, and Kentucky bluegrass (Akin et al., 1975). Tall fescue leaf blades averaged 11% vascular tissue (slow degradability) compared with 22% for orchardgrass and 20% for bromegrass. Mesophyll tissue (fast degradability) averaged 65% for tall fescue compared with 54% for orchardgrass and 53% for bromegrass. All measurements for Kentucky bluegrass were similar to those for tall fescue (Akin et al., 1975). Furthermore, tissue variation in tall fescue was of sufficient magnitude to permit alteration of certain cell arrangements through breeding methods (Hanna et al., 1973; Soh et al., 1984). These data indicate that tall fescue has the potential to have nutritive value that is comparable to, if not greater than, those of orchardgrass, smooth bromegrass, and Kentucky bluegrass.
Intrinsic differences in plant tissues among forage species are altered differentially by tissue maturity (Buxton and Russell, 1988) and environmental stresses (Kephart and Buxton, 1993; Allinson, 1971). These facets become integrated in the diet of animals, as ruminants selectively ingest green leafy tissue (Hodgson, 1981) one bite at a time in grazing bouts during each 24 h (Ungar, 1998; Burns and Sollenberger, 2002). Because tall fescue possesses characteristic C3 anatomy, its nutritive value and ultimate quality will be discussed relative to those of other C3 perennial grasses.
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