RUMINAL AND TOTAL PLANT CELL-WALL DIGESTIBILITY ESTIMATED BY A COMBINED IN-SITU METHOD UTILIZING MATHEMATICAL-MODELS

Three ruminally and duodenally cannulated non-lactating Finnish Ayrshi re cows were used to investigate ruminal and intestinal digestion of c ell-wall carbohydrates by a combined in situ method. Five grasses cut at 10 d intervals were incubated in the rumen for 0, 6, 12, 24, 48, 72 and 96 h, and the undegraded residues were exposed to intestinal dige stion. With advancing maturity of grass both the rate and extent of ce ll-wall digestion decreased. At early stages of growth the decreases w ere faster for the rate of digestion and at late stages of growth for the extent of digestion. Applying a passage rate of 0.02/h in one comp artmental rumen model resulted in digestibility values markedly lower than typically observed in vivo. However, applying a rumen model incor porating a selective retention of particles and time-dependent release of particles from the non-escapable pool resulted in much higher dige stibility values. Recovery of lignin after 96 h ruminal incubation wit h a subsequent mobile-bag incubation was very low (from 244 to 460 mg/ g). Intestinal disappearance of neutral detergent fibre (MDF) and hemi cellulose decreased with advancing maturity of grass and with increasi ng length of preceding ruminal incubation period, i.e. with decreasing potential digestibility of the material. Disappearance of hemicellulo se was much greater than that of cellulose for intact grasses but the difference diminished with increasing length of preceding rumen incuba tion period. On average, 195 mg/g of potentially digestible NDF disapp eared from the mobile bags in the intestines. The post-ruminal digesti on as a proportion of the total NDF digestibility varied between 0.034 and 0.058. Despite methodological problems both in ruminal in situ an d intestinal mobile bag techniques, these methods can be used to inves tigate ruminal and intestinal cell-wall digestion and to partition cel l-wall digestibility between ruminal and post-ruminal digestion provid ing that appropriate rumen models are used.