Rates of Volatile Fatty Acid Production in the Bovine Rumen1

By means of an in vivo-in vitro technique, volatile fatty acid production rates for four rations fed twice daily to two 1500 lb. fistulated Holstein steers were determined at various times after feeding. Volume of rumen contents changes were also studied. The following results were obtained: The concentration of VFA in the rumen increased after feeding for from 4 to 6 hours. Rates of VFA production were greatest within the first 2 hours after feeding. The rates of production of propionic and valeric acid were proportionately greater than those of the other fatty acids for the first 2 hours after feeding. Volume of rumen contents increased greatly after the animals were fed and watered, but the percent dry matter of the rumen contents varied less than 2%. Volume of rumen contents and the total quantities of VFA in the rumen were greater after the evening feeding. Of the average 2,927 gm. of VFA produced per steer during a 24-hour period, valeric acid amounted to 108 gm., butyric acid 518 gm., propionic acid 559 gm. and acetic acid 1,743 gm. Of the 12.7 therms supplied by these VFA, 5.7% was from valeric acid, 24.3% from butyric acid, 21.9% from propionic acid and 48.1% from acetic acid. As the digested colories of the various rations increased from 29.1 to 36.3 therms the caloric value of the VFA production increased, but at a diminishing rate. The VFA produced in the reticulo-rumen would account for from 36 to 42% of the estimated digested calories of the four rations studied. These acids, on the average, accounted for 63% of the energy requirements of the steers for maintenance. Twenty-three per cent of the gross energy of the hay ration appeared as reticulo-rumen VFA. No preferential absorption of the various VFA occurred, absorption being proportional to concentration. Whereas only slight differences in the average concentrations of VFA in the rumen occurred between the four rations; the production differences were proportionately greater. Rumen pH decreased after each feeding. Urea did not markedly stimulate VFA production in vivo as it did in the previous in vitro experiments.