Digestible NDF is selectively retained in the rumen of dairy cows compared to indigestible NDF

Abstract

Data was compiled from four experiments (60 observations) with fistulated dairy cows. Different forages were fed ad libitum as the only feed or supplemented with a fixed amount of concentrate. Three rumen evacuations were performed on different days, and fibre kinetics was determined based on average rumen pools and average daily flows. Mean retention time in the rumen (MRT) of indigestible NDF (INDF) varied from 35 to 92 h. MRT determined using the rumen evacuation technique has traditionally been based on first-order kinetics and a one-compartment model, and overestimates MRT of DNDF, but not of INDF. A new method is presented for correcting MRT of DNDF for the simultaneous digestion and the selective retention in a more biologically reasonable model with two successive rumen compartments. MRT of DNDF using the traditional rumen evacuation approach is merely a function of fractional rate of passage, whereas MRT in the new equation also depends upon fractional rate of digestion and distribution of MRT between rumen compartments. The distribution of MRT between rumen compartments was obtained from interpretation of ytterbium marker excretion curves. Use of uncorrected rumen evacuation data for calculation of MRT for DNDF overestimated MRT with on average 52 h, equal to a 64% overestimation compared to corrected MRT of DNDF. The degree of overestimation increases with increasing fractional rate of degradation, decreasing fractional rate of passage, and as the distribution of MRT between compartments approaches 0.5:0.5. The overestimation was higher for highly digestible grass silages, grass hay, lucerne hay, and pea silage, compared to low digestible grass silage, maize silage, and whole crop barley silage. This was probably due to differences in INDF:NDF ratio between different parts in the plant. Corrected MRT of DNDF varied from 52 h for supplemented maize silage to 116 h for unsupplemented grass hay, and was on average 19 h higher than MRT for INDF, proving selective rumen retention, probably because INDF and DNDF are heterogeneously distributed between different particle size fractions and between different parts of the plant, differing in chemical composition and digestion and passage kinetics (leaves, steams or seeds).