Effect of feed particle size on rumen fluid dynamics in dairy cows

The rumen environment is heterogenous, with lowest pH and highest VFA concentrations in the dorsal rumen.Impairment of fluid flow by feed particles may contribute to this heterogeneity. Eight lactating rumen-cannulatedHolstein cows (110±20 DIM) were used in a cross-over design experiment with the goal to evaluate the effect offeed particle size on intraruminal fluid flow. Treatments were COARSE (48% DM and 30.1% of DM≥8 mm screen)or FINE (34% DM and 15.8% of DM≥8 mm screen) total-mixed rations. Ingredient and nutrient composition (DMbasis) was the same across rations (39% grass silage, 12% corn silage, 6% wheat straw, and 43% concentrates; 38.0%NDF, 15.8% CP, and 15.1% starch). To prepare the different rations, COARSE was mixed for 13 min, and FINE wasmixed for an additional 35 min with added water to aid particle size reduction. Cows were adapted to each ration for20 d before measurement of the rumen fluid dynamics on d 21 of each period. To evaluate intraruminal fluid flows,a pulse dose of Co-EDTA (25 g diluted in 0.5 L) was introduced in the dorsal rumen. Rumen fluid was sampledovertime during the first 4 h after infusion at six sites (3 in dorsal to cranial direction, 3 in dorsal to ventral direction)in the rumen to determine Co concentration. The fractional diffusion rate of Co followed a two-phase model. Duringphase 1 (equilibration phase), which lasted approximately 1 h, Co concentration decreased rapidly in the dorsal rumen(-2.44±0.538 /h), and increased in the cranial rumen at 7× the rate (1.64±0.371/h) of the ventral rumen (0.24±0.856/h). During phase 2, fractional diffusion rate of Co in the dorsal and cranial rumen was (-0.27±0.080 /h) and in theventral rumen (-0.01±0.174 /h). COARSE or FINE did not affect (P>0.12) Co fractional diffusion rates from thedorsal, cranial or ventral rumen sacs. Results indicate that there is a strong fluid flow from the dorsal towards thecranial rumen, and a limited exchange of fluid between the dorsal and ventral rumen. These flows were not affectedby the rations differing in DM content and particle size.