A review of the use of intrinsically 14C and rare earth-labeled neutral detergent fiber to estimate particle digestion and passage

Methodology that allows simultaneous measurement of dynamic events affecting NDF digestion in passage from the rumen should improve our understanding of factors influencing intake and digestion. Ideally, particle flow is measured with a marker indelibly attached to or intrinsically part of the feed. If flow measurements are to reflect physiological conditions, marked and unmarked feed must be digested and passed with identical fractional rates. Application of 14C-labeled plant fiber to the study of ruminal dynamics has been slow because of expense and difficulty in producing 14C-labeled plant material. Recently, alfalfa was intrinsically labeled with 14C under field conditions to produce plant material similar in composition to unlabeled material. Carbon-14 specific activity was similar in all particle sizes, and in indigestible and digestible NDF. Greater concentrations of ytterbium (Yb) were associated with smaller vs larger particles. Larger differences in turnover rates among animals than differences attributable to treatments force comparisons of markers to be made within animal or with in vitro systems. The uncertainty about how extrinsic markers respond under various environments resulting from interaction of feed properties and gut function, and the high error inherent in measuring dynamic systems, raise serious questions on the interpretability of results. Advantages of 14C-labeled NDF over other markers include simultaneous measurements of particle breakdown, digestion and passage rates as well as the potential to study microbial attachment, VFA, CO2 and CH4 production and rate of incorporation of labeled metabolites into tissues.