Ruminal production and interchange rates of volatile fatty acids (VFA) were measured for steers fed corn grain in the whole or ground form. Liquid turnover rate was 29% greater (P less than .10) with steers fed the corn in the whole form. On a molar basis (mol/100 mol), ruminal concentrations of propionate were lower while those of butyrate, 3-methyl butyrate, valerate and caproate were greater with whole than with ground corn diets. Acid production differences followed similar trends with de novo production of butyrate being seven times greater and irreversible butyrate loss three times greater with whole than with ground corn diets. Conversion of acetate to butyrate was about 70% greater with whole than with ground corn diets. This conversion far exceeded other VFA exchange rates. Outflow of VFA to the omasum was calculated to equal 20, 25 and 12% of the irreversible loss of acetate, propionate and butyrate, respectively. Ruminal output of energy as VFA from the rumen equaled 60 and 43% of consumed energy with the whole and ground grain diets, respectively. Production rates of VFA were poorly correlated with ruminal concentrations except for butyrate. Less than 10% of the labeled carbon from acetate and butyrate was detected in plasma glucose, but the transfer coefficient from propionate uncorrected for crossover of label was 59%. Gluconeogenesis from propionate alone exceeded literature estimates for basal glucose requirements of nonlactating ruminants.
Volatile fatty acids concentrations are shown as μM C in Fig. 4 . Acetate occurred in the highest concentrations in all samples, followed by propionate, lactate, and isobutyrate (data not shown). Lactate concentrations remained around 1–2 μM throughout the incubation. In the uninhibited 0–2 cm slurry the acetate concentrations increased from 45 to 78 μM after 2 days and showed a second transient increase with maximum concentrations after 12 days ( Fig. 4 ). The concentrations decreased to 3–8 μM towards the end of the incubation. In the 5–9 cm uninhibited slurry the acetate concentrations increased from 45 to 93 μM after 3 days followed by a decrease to 3–8 μM. Propionate increased from to 3 μM after 2 and 3 days in the 0–2 and 5–9 cm slurry, respectively, and decreased again to values around 1 μM. Isobutyrate concentrations remained around μM throughout the incubation in both uninhibited slurries (data not shown).