Rumen liquid dilution rate in dairy cows fed once daily: effects of diet and sodium bicarbonate supplementation.

Three ruminally cannulated Holstein cows fed total mixed diets of hay crop silage and concentrate (30:70, 50:50, 70:30, 100:0) were used to determine liquid volume ruminal and dilution rate with polyethylene glycol as a marker. Dosing 5 h prefeeding produced nonlinear marker dilution curves that were divided into prefeeding, eating, and resting phases, whereas dosing 2 h postfeeding produced linear dilution curves. Ruminal liquid volume calculated from either dosing method overestimated liquid volume compared to manual estimates. Mean liquid dilution rates did not differ between methods of dosing, and differences between diets were closely related to dry matter intake. Supplementation with 0, 68, or 114 g/day sodium bicarbonate did not affect ruminal liquid volume or dilution rates. Across diet and buffer treatments, liquid dilution rate during eating (23 to 32%/h) was greater than mean, prefeeding, or resting dilution rates (10 to 21%/h). Correct interpretation of ruminal volume and dilution data requires full details of dosing method, dosing time in relation to feeding, and sampling times.