1. Three wethers fitted with permanent rumen cannulas were given a pelleted roughage—concentrate ration containing 45% ground barley straw as roughage. The animals were given 1 kg of the diet at 08.00 h and 500 g at 17.00 h. 2. The measurements made to determine the buffering components of the rumen fluid were: rumen pH, buffering capacity value (I), total volatile fatty acids (VFA), CO2 and inorganic phosphorus concentrations. I was expressed as the quantity of titrant (m-equiv./l.) required to bring about 2 unit changes in rumen fluid pH. The pH ranges in which the I values were measured were 4—6, 5—7 and 6—8. All measurements were made at hourly intervals over a 9 h period commencing 1 h before feeding. 3. Rumen pH values were high before feeding, decreased to a minimum 2 h after feeding and then increased to approximately the original values. The concentrations of total VFA followed an inverse pattern. There was diurnal variation in the concentrations of CO2 but inorganic P levels varied very little throughout the day. 4. Significant correlations were obtained between VFA, inorganic P and CO2 levels and the I values. It would appear that the bicarbonate and phosphate buffers were not effective in regulating the rumen pH when sheep were given a roughage—concentrate ration. © 1969, The Nutrition Society. All rights reserved.
