The effect of phosphate and carbonate-bicarbonate supplements on the rumen buffering systems of sheep

Abstract

1. Two wethers fitted with permanent rumen cannulas were used to study the effect of two mineral supplements, Na 2 HPO 4 and a 1:1 mixture of Na 2 CO 3 and NaHCO 3 , on the buffering system in the rumen of sheep given a pelleted roughage-concentrate diet. 2. Measurements were made of rumen pH, buffering capacity value (β), total volatile fatty acids (VFA), CO 2 and inorganic phosphorus concentrations as well as rumen cellulase activity. 3. The addition of either mineral supplement to the diet significantly (P < 0.001) increased the rumen pH values during the critical post-feeding period, there was a difference of only 0.5 pH unit between the minimum and maximum pH values. 4. Both mineral supplements significantly increased the concentrations of CO 2 in the rumen fluid and rumen cellulase activity. They resulted in a general reduction in total VFA concentrations. It is suggested that this may possibly arise from increased absorption of VFA associated with increased CO 2 tension in the rumen fluid. 5. The Na 2 HPO 4 supplement significantly ( P < 0.001) increased the β values in the pH range 6–8, whereas the bicarbonate supplement contributed to raising the β values in the pH range 5–6. 6. The bicarbonate supplements raised cellulose digestibility from 39.7% for the control diet to 45.6% and the addition of Na 2 HPO 4 further raised the digestibility to 51.1%. It would appear that low rumen pH impairs rumen cellulase activity and this could be offset by the addition of mineral supplements.