Comparison of methods, markers, sampling sites and models for estimating digesta passage kinetics in cattle fed at two levels of intake

Six ruminally and duodenally cannulated Friesian bulls were used in a switch-back experiment to compare passage models, markers and sampling sites, and duodenal or faecal marker excretion curves and rumen evacuation technique for estimating kinetics of digesta passage. Cr-mordanted and Yb-labelled hay were used as particulate markers and CoEDTA as a liquid-phase marker. The cattle were fed on a dry matter (DM) basis (g kg−1) a diet of hay (600), barley (396) and urea (4) at either low or high DM intake (40 and 80 g DM (kg0.75 live weight)−1). Post-duodenal digesta kinetics were estimated by administering the markers (CoEDTA, Cr-mordanted and Yb-labelled duodenal digesta particles) into the duodenum and collecting faecal samples. Digesta kinetic parameters were calculated by linear regression of natural log transformed marker concentrations in ruminal fluid, duodenal digesta or faeces, and by non-linear models with either two age-independent compartments (G1G1) or gamma time dependency in the first compartment (GnG1, n = 2–4). the models with gamma age dependency improved the curve fitting which suggested an age-dependent flow. In most cases the error mean squares were smallest with the G3G1 model. Total compartmental retention time (CMRT), transit time (TT) and total mean retention time (TMRT) of the markers decreased as DM intake increased. Regardless of marker, sampling site or model, the relative differences in digesta kinetics between the dietsʹ kinetics were similar. Shorter post-duodenal retention time of markers at high compared to low DM intake resulted from both shorter CMRT and TT. Similar post-duodenal kinetics of Co, Cr and Yb indicate that there was no differential passage of liquid and solid digesta in the caecum and proximal colon. e CMRT was shorter for Yb than for Cr when derived either from duodenal (57.4 vs. 66.8 h) or faecal sampling (61.1 vs. 73.5 h). Faecal sampling resulted in 3.7 h (Yb) and 6.6 h (Cr) longer CMRT than duodenal sampling. Post-duodenal CMRT was only 0.223 (Yb) and 0.146 (Cr) of the retention time in the fast age-dependent compartment derived from faecal sampling. This suggests that preduodenal sites accounted for the major proportion of the retention time in this compartment. ssage rate estimates derived from rumen evacuation were much slower than those calculated from the exponential decline in duodenal and faecal Cr and Yb concentrations, mainly because rumen evacuation derived passage rate includes the retention time in both ruminal compartments. However, when a simple first-order passage rate was calculated from the passage rate from the two ruminal compartments and rate of neutral detergent fibre (NDF) digestion, the values for indigestible NDF, NDF and digestible NDF obtained from the two methods were fairly similar; rumen evacuation: 0.0165, 0.0126 and 0.0095 h−1; Cr: 0.0159, 0.0111 and 0.0085 h−1; Yb: 0.0179, 0.0123 and 0.0099 h−1.