THE USE OF INTERNAL MARKERS TO PREDICT TOTAL DIGESTIBILITY AND DUODENAL FLOW OF NUTRIENTS IN CATTLE GIVEN 6 DIFFERENT DIETS

Six ruminally and duodenally cannulated cattle were used in a 6 x 6 Latin square experiment with a 2 x 3 factorial arrangement of treatments. The cattle were fed diets based on silage or dried grass cut simultaneously from the same sward, each fed with 250, 500 or 750 g concentrate dry matter (DM) kg-1 total DM. The feed components investigated as potential markers to determine total DM digestibility (DMD) were acid insoluble ash (AIA) and acid detergent lignin (ADL), indigestible DM (IDM41), neutral detergent fibre (INDF41) and acid detergent fibre (IADF41) determined by a 288 h ruminal incubation in nylon bags with a pore size of 41 mum, and corresponding indigestible fractions determined by using bags with a pore size of 6 mum were IDM6, INDF6 and IADF6. INDF and IADF were also determined by a 96 h in vitro incubation in rumen fluid (IVINDF, IVIADF). If the criterion of an ideal marker is to predict the digestibility and the differences between the diets, none of the markers met this criterion. Of the markers evaluated, the most suitable was AIA, followed by INDF6, INDF41 and IADF41. ADL was the least acceptable marker. Three double-marker systems were used to calculate duodenal flows of organic matter (OM), non-ammonia N (NAN), microbial N and NDF. Cr-mordanted straw, INDF41 and IADF41 were used as particulate markers in conjunction with Co-EDTA as a liquid phase marker. Duodenal OM flows were also estimated using a single-marker system. Double-marker systems reduced variability in duodenal OM flow estimates compared with single-marker systems. The choice of the particulate marker affected (P<0.001) duodenal flow estimates but there were no indications of marker x diet interactions. Neither was the statistical significance of dietary effects changed. When compared with the values obtained with Cr as a particulate marker, INDF estimated a greater relative increase in NDF flow (0.143) than in OM (0.062) or NAN flow (0.018). This suggests that there may be differences in the distribution of solid phase markers in various particle size fractions. In such cases, if the digesta sample contains different proportions of large and small particles compared with the true digesta, then a double-marker system may not be sufficient to correct unrepresentativeness of digesta sampled from a simple T-shaped cannula.