Passage kinetics of concentrate feeds determined from duodenal and faecal sampling

The objective of this study was to determine passage kinetics of concentrate feeds from duodenal and faecal sampling. The study was conducted using 4 × 4 Graeco-Latin square design with four bulls (body weight 400 kg) fitted with a permanent rumen and duodenal cannula. The main treatments were diets based on grass silages ensiled using 0, 2, 4 and 6 l per ton of formic acid (expressed as 100 % formic acid) as an additive. The diet was comprised of grass silage and concentrate (on dry matter basis 0.70:0.30). Four concentrate ingredients (barley, barley fibre, rapeseed meal and soybean meal) were labelled with ytterbium and given as a single dose in the morning meal orthogonally with the diets, i.e., each animal received each one labelled feed, and each one labelled feed was given with each diet. Frequent samples of duodenal digesta and faeces were taken during 120 h after dosing the markers. Digesta kinetic parameters were calculated by arithmetic method, linear regression of natural log transformed marker concentrations in duodenal digesta or faeces, and by non-linear models with either two age-independent compartments (G 1 G 1 ) or gamma time dependency in the first compartment (G n G l , n = 2–4). Increasing the degree of age-dependency improved the fit of the data with G 4 G 1 resulting the best fit. There was an ascending phase of duodenal marker excretion curves demonstrating the mechanism of selective retention of concentrate particles in the rumen. Estimating passage rate only from descending phase of marker excretion curve will therefore underestimate residence time in fermentation compartment. Residence time in first the age-dependent compartment (CMRT 1 ) was longer when estimated from faecal samples compared with duodenal samples, but no differences between the sampling sites were observed for age-independent compartment. The was no significant differences among the labelled feeds in passage kinetic parameters. Calculating the effective ruminal protein degradability using the passage rate estimated from declining phase of marker excretion curve underestimates ruminal degradability and overestimates the supply of rumen undegraded protein compared with models using reciprocal of CMRT determined from duodenal sampling and especially considering selective retention of feed particles in the rumen. It is concluded that the passage kinetics of concentrate particles from the rumen cannot be described as a single-compartment system with first-order passage rate. Ascending phase in duodenal marker excretion curve demonstrated that concentrates are selectively retained in the rumen; therefore, two-compartment models should be used to describe passage kinetics of concentrate feeds. The results indicated that different types of concentrate feeds have similar passage kinetic characteristics.