Relationships of fibre, lignin, and phenolics to in vitro fibre digestibility in three perennial grasses
Introduction
Livestock performance can be improved by increasing the digestibility of feeds, one of the key elements of feed quality. Digestibility of feeds can be improved by breeding forage crops with modified cell-wall structure, increasing the relative availability of energy in the cell wall to rumen microbes and livestock (Casler, 2001, Fahey and Hussein, 1999). Because effective plant breeding requires evaluation of thousands of forage samples within a relatively short time period and under reasonably uniform conditions, in vivo digestibility cannot be used as a direct selection criterion in plant breeding programs. Indirect measures of digestibility are required for the selection and breeding process, followed by validation using in vivo measurements of digestion, lactation, weight gain or other indicators of animal performance (Argillier et al., 1998, Casler, 2001).
In vitro dry matter digestibility (IVDMD) has received considerable attention from forage breeders, with the demonstration of heritable variation in numerous species (Buxton and Casler, 1993). Genetic improvements in IVDMD are generally a result of decreases in cell-wall concentration, decreases in lignin concentration, or increases in water-soluble carbohydrate concentration (Casler, 2001). In vitro neutral detergent fibre digestibility (IVNDFD) has received relatively little attention from forage breeders, despite its potentially greater relevance to cell-wall utilization by ruminants. Because the cell wall represents the most indigestible portion of a forage plant, the digestibility and composition of the cell wall may comprise the factors most limiting to livestock production on high-forage diets (Van Soest, 1994).
Within grass cell walls, the relative abundance of lignin and the frequency of ferulate cross-links with arabinoxylans appear to be the most important factors limiting energy utilization by rumen microbes (Casler, 2001, Jung and Deetz, 1993). In vitro neutral detergent fibre digestibility can be increased by reducing lignin concentration or cross-linking between lignin and cell-wall carbohydrates (Casler, 2001). Lignin concentration has been identified as a key factor limiting IVNDFD in lucerne (Medicago sativa L.) (Jung and Lamb, 2003) and smooth bromegrass (Bromus inermis Leyss) (Casler and Jung, 1999, Jung and Casler, 1991). The concentration of etherified ferulate, a measure of cross-linking between lignin and arabinoxylans, has a strong negative influence on IVNDFD of smooth bromegrass, an effect that seems to be independent of the concentration of lignin in the fibre fraction (Casler and Jung, 1999). The objectives of this research were to identify interrelationships among lignin and phenolic components of the fibre fraction of three perennial grasses and to determine their influence on IVNDFD.
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Materials and methods
Selection nurseries of orchard grass (Dactylis glomerata L.) and reed canarygrass (Phalaris arundinacea L.) were established from 70-day-old plants in May 1997 and a similar nursery for smooth bromegrass was established in 1998. Each plant was raised in a glasshouse from an individual seed. For cocksfoot, the selection nursery consisted of a total of 1160 plants, 40 plants from each of 29 cultivars and experimental populations, all classified as medium-late to late in reproductive maturity. For
Results
Variability among clones was repeatable across harvests for all variables measured on all three species (P<0.001 in all cases, except for 24-h IVNDFD of reed canarygrass). Intraclass correlation coefficients were lowest for 24-h IVNDFD for all three species (Table 2), indicating the difficulty in precisely measuring clonal differences for digestibility following relatively short digestion periods. Repeatability tended to be greatest for cocksfoot, followed by smooth bromegrass and reed
Discussion
The moderate to high intraclass correlation coefficients indicated fairly consistent differences among clones and rankings across harvests. The moderate repeatability of clone performance across harvests was similar to observations made by Casler and Jung (1999). The moderate to high repeatability across harvests can be partially attributed to lack of variation in leaf:stem ratio, because these samples consisted entirely of leaf tissue (Casler, 1999a, Casler, 1999b). Because leaves and stems
Conclusions
Concentrations of NDF, Klason lignin, and ferulate ethers were all negatively correlated with IVNDFD in all three perennial grasses examined. The impact of ferulate cross-linking was more pronounced for potentially digestible NDF (96-h incubations) than the rapidly digestible NDF fraction (24-h incubations). However, the results indicate some serious potential problems associated with etherified ferulic acid as a selection criterion to improve digestibility of perennial grasses. For all three
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