EFFECTS OF MATURITY AND ENVIRONMENT ON THE AMOUNT, COMPOSITION, AND IN VITRO DISAPPEARANCE OF HEMICELLULOSE IN TALL FESCUE
Abstract
The purpose of this study was to determine the extent to which environmental conditions during growth influence the amount, chemical composition and in vitro disappearance of tall fescue hemicellulose and its constituents. First growth was sampled and cut on May 16, June 14, and July 19. Regrowth was sampled at weekly intervals following each cutting. Samples were extracted with neutral detergent and the neutral detergent fiber (NDF) was treated with 1.0 N H(,2)SO(,4) to hydrolyze hemicellulose (HC). A high performance liquid chromatography (HPLC) technique, using cation exchange resins and water as the sole eluent, was adapted to measure concentrations of xylose and arabinose in the HC hydrolyzates. The remaining portion of HC was assumed to consist primarily of glucuronic acid. Selected samples were incubated in rumen fluid to determine the in vitro digestibility of the various cell wall constituents. Neutral detergent fiber in first growth leaf tissue increased 10.2 percentage units of dry matter between May 16 and June 14, and increased 5.3 additional units between June 14 and July 19. Acid fiber (AF) percentage increased 7.3 units and hemicellulose (HC) increased 3.9 units between May 16 and June 14. Between June 14 and July 19, AF and HC increased by 2.2 and 3.2 percentage units respectively. Xylose percent of dry matter increased 5.1 units between May 16 and June 14, and increased 2.0 units between June 14 and July 19, while arabinose percentage remained about the same during the entire period. Between May 16 and July 19, the in vitro disappearance of NDF decreased from 37.9 percent to 22.4 percent, AF disappearance decreased from 27.1 percent to 9.2 percent, and HC disappearance decreased from 50.1 percent to 39.4 percent. Xylose in vitro disappearance decreased from 40.8 percent to 36.5 percent, arabinose disappearance decreased from 78.1 percent to 53.8 percent, and glucuronic acid disappearance decreased from 50.0 percent to 39.1 percent. Second growth NDF increased over time from May 16 (early summer growth) and from June 14 (mid-summer growth), but it decreased in regrowth from July 19 (late summer growth). The trends if AF paralled the NDF trends, but HC trends were opposite. Xylose percentage in HC followed the NDF trends more closely than the HC trends, while arabinose showed little variation. In vitro NDF disappearance in second growth decreased over time during all three periods, and was related more to AF disappearance than HC disappearance, suggesting that the effect of time was greater on cellulose than on hemicellulose. Decreases in HC in vitro disappearance were related to decreases in both xylose and glucuronic acid disappearance. The effect of environment on fiber digestibility was indicated because NDF disappearance in three week-old regrowth was higher in mid-summer growth than in the same age material from the other two periods. When calculations were made to estimate changes that occurred in the composition of newly synthesized cell wall, the magnitude of differences was much greater than indicated by the original data, although the trends were similar. Differences were associated more with time of season than with tissue age (weeks). No consistent trends were found linking amounts of cell wall constituents to temperature. However, concentrations of NDF, AF, and xylose increased during the period of increasing daylength and decreased during the period of decreasing daylength. These results suggest that changes in the amount and composition of newly synthesized fiber may have been influenced by photoperiod.
Degree
Ph.D.
Subject Area
Agronomy
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