Accounting for marker disassociation when modelling time-dependent ruminal escape of particles based on the faecal excretion of Ytterbium

One of the problems with using rare-earth metals as fibre-bound markers for digestive tract passage investigations in ruminants is that of marker disassociation (leaching) from the fibre and the more rapid movement of disassociated marker through the whole gastrointestinal tract with the liquid phase. Analysing the course of marker concentrations in faecal samples from cattle, sheep and goats fed millet leaves, bush hay or green forage and given a pulse dose of Ytterbium (Yb) bound to fibre particles, it was found that in 28 out of 135 obtained marker excretion curves the Yb-concentrations in the first three faecal samples (excreted 0–16 h after dosing) significantly differed from background values, while this was not the case for the remaining curves. Repeating experiments where marker disassociation has occurred might not always be possible. Therefore, a single compartment model was developed to simulate this disassociation, the ruminal escape of fibre-bound marker assumed to be Gamma-2 dependent, whereas the disassociation and subsequent passage of the marker with the liquid phase were taken to be Gamma-1 (exponentially) dependent. This model, as well as a single compartment model not accounting for marker disassociation, were fitted to the relevant data sets and the results compared. The maximum predicted level of marker disassociation was found to be 45% of the total marker dose. Modelling data sets with marker disassociation with a model not allowing for this resulted in an overestimation of the time taken for half the amount of marker to escape from the rumen (T50) by a maximum of 29.5 h, which could be attributed to a distortion of the curve towards a reduction in the time of first appearance of the fibre-bound marker (TTf). The possible reasons for disassociation of Yb as well as a method for the determination of T50 where this cannot be obtained arithmetically are discussed.