THE DIGESTIBILITY OF STRUCTURAL CARBOHYDRATES OF GRASSES BY RUMEN MICRO‐ORGANISMS IN VITRO

Abstract

In vitro digestion of grass samples has shown that the amount of digestible cellulose, hexosan or pentosan is not greatly affected by species or by stage of maturity. A survey of published data on crude fibre and cellulose digestibility has confirmed these findings. Marked differences between grasses and legumes were observed. The rate of digestion has also been shown to be independent of the total amount of structural material present. The relevance of these observations to the selection of herbage varieties with greater intake by ruminants is discussed. INTRODUCTIONRuminant animals are capable of digesting structural carbohydrates, the main constituents of the fibrous cell walls of forage plants. However, the amount of a forage which a ruminant is able to consume appears to be limited in part by the fibre content of the feed, the amount of this fibre which can be digested, and the rate of its digestion (e.g. 3).This paper describes experiments conducted in vitro which were designed to throw light on the relationship between these three factors. EXPERIMENTALThe in vitro digestion procedures have been described already (16,17). Hexosan and pentosan contents were measured on forages, and on the residues from in vitro digestion, by the colorimetric procedures of Deriaz (6); when hexosan only was measured on the residues from digestion, the preliminary stages of extraction with an alcohol/benzene mixture and the first treatment with A' sulphuric acid were omitted, and the 72% sulphuric acid reagent was applied direct to the indigestible residues which had been recovered and oven-dried (1(K)°C) in 50 ml beakers.Cellulose was estimated by the method of Crampton and Maynard (5), on residues in 50 ml beakers; 25 ml portions of the acetic acid/nitric acid reagent were added, the beakers 238 were covered with a glass basin containing about 15 ml of cold water and were then refiuxed gently for 20 min. After cooling, the residues were filtered through asbestos in Gooch crucibles, dried at 100°C, weighed, ashed at 600°C and reweighed. Cellulose content was measured as the loss of weight on ignition. For grasses, the cellulose, hexosan and crude-fibre contents appear to be similar. RESULTSFigure 1 (i) shows data obtained with a wide range of cocksfoot samples (Dactylis ghmerata, cv. S37). As the grass matured, hexosan content rose from 21% to 37% of the herbage DM and the in vitro hexosan digestibility fell from 80% to 45%. However, the content of digestible hexosan (16% to 22%) showed no clear trends, similar values being found for the least and the most mature samples. Corresponding data for perennial ryegrass samples (Lolium perenne, cv. S24) are shown in Fig. 1 (ii). For the range of samples examined, hexosan content was rather less variable at 23 to 29%, hexosan digestibility fell from 85 to 54%, and the content of digestible hexosan varied between 16 and 20%, the lowest values tending to occur in the more mature samples.The rate of digestion of two samples of S37 cocksfoot and two of S24 perennial ryegrass were measured...

Effect of ensiling and silage additives on fatty acid composition of ryegrass and corn experimental silages1

“…A saline solution of silages was used to measure the pH and ammonia-N was measured by the micro-diffusion method (Portuguese Standard NP4038, 1990, Lisbon, Portugal). Gross energy was determined by calorimetric method (ISO 9831, 1998, Geneva, Switzerland) and digestibility was measured in vitro by the method of Tilley et al (1969). A water extract of silages were prepared by adding deionized water (50 mL) to 10 g of silage that was used to measure fermentation acids (lactic acid, VFA) and ethanol.…”

The interrelationship of some components of forage quality

“…', or both. Tilley et al (1969) have stated that selection for increased 'digestible fibre etc' will result mainly in an improvement in digestibility, but that selection for a faster rate of digestion, i.e. for higher PS, may have more effect on the intake characteristics of the herbage.…”

Pasture quality and ruminant nutrition

“…sampling time (e.g., Tilley et al 1969). Hydrolysis of cell-wall preparations with mixtures of the appropriate carbohydrases, cellulases, and hemicellulases offers a simpler method, as aliquots for measurement of liberated reducing sugars may be removed from the digests at intervals.…”