ANIMAL FEED SCIENCE AND TECHNOLOGY ELSEVIER Animal Feed Science and Technology 48 (1994) 211-227 The use of internal markers to predict total digestibility and duodenal flow of nutrients in cattle given six different diets P. Huhtanen*, K. Kaustell, S. Jaakkola Department of AnimaI Science, University of Helsinki, P.O. Box 28, FIN-O0014 Helsinki, Finland (Received 28 July 1993; accepted 9 February 1994) Abstract Six ruminally and duodenally cannulated cattle were used in a 6 × 6 Latin square exper- iment 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-i 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 (IDM4~), neutral detergent fibre (INDF41) and acid detergent fibre (IADF, I ) determined by a 288 h ruminal incubation in nylon bags with a pore size of 41/tm, and corresponding indigestible fractions deter- mined by using bags with a pore size of 6/zm were IDM6, INDF6 and IADFr. 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 differ- ences between the diets, none of the markers met this criterion. Of the markers evaluated, the most suitable was AIA, followed by INDF6, INDF4~ and IADF4~. 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, INDF4~ and IADF4~ 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. Dou- ble-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 ob- tained 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 *Corresponding author. Present address: Agricultural Research Center of Finland, Institute of Animal Production, FIN-31600 Jokioinen, Finland. 0377-8401/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSD10377-8401 ( 94 ) 00622-G