The effects of diet on water flux and volatile fatty acid concentrations in the rumen of growing beef steers fed once daily.

In vivo temporal changes in ruminal liquid flow, liquid volume and VFA concentrations were determined in growing steers following once-daily feeding. Crossbred beef steers (n = 8; 278 +/- 8 kg), used in a crossover design, were trained to consume 180% of their maintenance energy intake within 2 h of either a high-forage (54% hay: 46% concentrate; HF) or high-concentrate (28% hay: 72% concentrate; HC) diet. For each animal on each diet, ruminal VFA concentrations, liquid volumes and liquid dilution rates were determined during a 2-h period before feeding and during three consecutive 2-h periods following feeding. Water was withheld during these periods to observe physiologically rather than behaviorally induced changes. At equal energy intakes, diet alone had no effect on VFA concentrations (P greater than .10), although changes in total VFA, acetic and isobutyric acids differed during the time periods following feeding due to diet (diet x time period interaction; P less than or equal to .10). Ruminal volume and dilution rate were altered in a reciprocal manner due to diet, with greater volumes and lower dilution rates observed in HC than in HF steers. Differences among time periods were observed for dilution rate and propionic acid concentrations. In general, these differences can be explained by comparison of the prefeeding time period with the postfeeding time periods. A numerical, but not statistically significant, increase in ruminal volume was observed following feeding. The effects of diet on volume and liquid dilution rate of the rumen must be considered when assessing total nutrients available for animal absorption.