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

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Abstract
1. Two wethers fitted with permanent rumen cannulas were used to study the effect of two mineral supplements, Na2HPO4 and a 1:1 mixture of Na2CO3 and NaHCO3, 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), CO2 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 CO2 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 CO2 tension in the rumen fluid.5. The Na2HPO4 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 Na2HPO4 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.