The extent of cellulose digestion by rumen micro‐organisms in vitro is dependent on the pH of the medium. Digestion is greatly reduced at pH values similar to those found within the rumen of sheep fed with diets rich in readily digestible carbohydrates. It is suggested that this may be one of the reasons for reduced digestion of the fibre component of forages when carbohydrate supplements are fed.
Four feeds, two duodenal digesta samples, two ileal digesta samples and two faecal samples were analysed in quintuplicate for their long-chain fatty acid content by a new method (direct methylation) whereby the dried samples were heated with benzene and 5 % w/v methanolic HCl in screw-capped bottles at 70°C for 2 h. The methyl esters, were extracted with hexane, purified by thin-layer chromatography, and measured by gas-liquid chromatography. The values obtained were compared with those obtained using an established method (saponification) in which the dried samples were refluxed with ethanolic KOH, the fatty acids liberated by acidification, extracted and methylated by refluxing with 5 % wlv methanolic HCl and the methyl esters extracted, purified and measured as before. Statistical analysis indicated no particular bias for either method though in general the direct methylation appeared to have a lower variance than saponification. A regression analysis of the direct methylation means (2) against the saponification means (i) gave the equation: -d=0.0073 + 0.9883 r2 = 0.994 (n=96) (i 0.0255) (i 0.0079)
Three mature wether sheep, equipped with re-entrant cannulae at the proximal duodenum and terminal ileum were fed either frozen, or dried and chopped or dried and ground red clover herbage, and samples of duodenal and ileal digesta and faeces were collected. Analysis of the feed and digesta samples for their individual long chain fatty acid content enabled the quantitative digestion of herbage lipids to be investigated.The flow of total fatty acids into the small intestine was greater than the amount daily consumed when the dried diets were fed, whilst on the frozen diet there was a net loss between mouth and duodenum (P i 0.01). There was a marked increase in the proportion of saturated fatty acids in the fatty acid fraction collected at the duodenum relative to the feed on all diets, but some dietary differences were noted. The extent of hydrogenation was reduced on the ground diet and, to a lesser degree, on the chopped diet. Consequently these diets had significantly higher proportions of C18:l in the duodenal fatty acid fraction than the frozen diet (ground,22.5 %; choppedl7.0,frozen 13.5; P
Three wether sheep equipped with re-entrant cannulae at the proximal duodenum and terminal ileum were given dried cocksfoot in the chopped or ground form and the samples of duodenal and ileal digesta and faeces obtained for both diets were analysed for individual long-chain fatty acids. The total quantities of fatty acids entering the proximal duodenum (15.2 and 15.4 g/24 h on chopped and ground feed, respectively) were markedly higher than the amounts consumed (8.7 and 7.8 g/24 h, respectively), whilst losses within the small intestine were 13.9 and 14.2 g/24 h, respectively. None of these effects was found to be significantly influenced by the processing of the diet (P 0.05). The quantity of CIS unsaturated fatty acids in the two feeds comprised 78 % of the total fatty acids but the level of these acids was considerably reduced (19%) in duodenal digesta and was not significantly influenced by diet (P > 0.05), whilst the levels of stearic acid (CIS: , , ) entering the small intestine were markedly increased on both diets.
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...
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