THE ADDITION OF BUFFERS TO RUMINANT RATIONS: I. EFFECT ON WEIGHT GAINS, EFFICIENCY OF GAINS AND CONSUMPTION OF RATIONS WITH AND WITHOUT ROUGHAGE

The effect of potassium on the sodium requirements of growing steers with and without alpha-tocopherol supplementation

1. Hereford steers, housed in concreted yards and given demineralized water, were offered ad lib. a low-sodium (0.07 g Na and 4.5 g potassium/kg) basal diet of sorghum grain, urea and minerals, to which 0, 15 or 30 g K as KHCO3/KG diet had been added. Mean daily intakes of K were 26, 107 and 168 g/steer respectively. 2. The steers were given daily supplements of 0, 3.25 or 6.50 g Na as NAHCO3/steer, added to the food; mean daily intakes of Na (food plus supplemental NaHCO3 minus residues) were 0.41, 3.06 and 6.26 g/steer respectively. 3. Rate of gain of body-weight was positively related to the rate of Na supplementation and negatively related to the K content of the diet, but there was no interaction between Na and K contents of the diet. 4. the Na:K ratios for the parotid saliva from steers not given Na supplements was 0.3, whereas steers ingesting either 3.1 or 6.3 g Na/d had Na:K ratios of about 12. Increasing the amount of K in the diet had no consistent effect on the saliva Na:K ratio. The Na:K ratios for rumen fluid reflected changes in the saliva Na:K ratio in response to Na supplements and were inversely related to the amount of K in the diet. 5. The width of the adrenal zona glomerulosa from steers receiving the basal diet, without Na supplements, was significantly greater than that from steers given the Na supplements, but it was not affected by the amount of K in the diet. 6. Both Na supplements and the diet containing 30 g added K/kg significantly increased the Na:K ratio of the plasma. 7. The addition of 200 mg DL-alpha-tocopheryl acetate/steer per d to the diet had no significant effects on the growth rate of the steers. 8. It was concluded that the Na requirement of steers for growth was not signficantly affected by the amount of K in the diet up to 168 g/d.

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

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.