RENAL EXCRETION OF ACID IN CALVES FED EITHER ROUGHAGE OR CONCENTRATE DIETS

¹ Physiology Department, Rowett Research Institute, Bucksburn, Aberdeen, AB2 9SB

Calves fed a roughage diet (50 per cent hay + other components) excreted an alkaline urine rich in bicarbonate but poor in phosphorus. Infusion of HCl into the rumen led to a fall in urine pH, to a decrease in bicarbonate excretion and to the excretion of acid in the urine. Between 16-25 per cent of this acid was excreted as acid phosphate (H₂PO₄^-) and the remaining 75-84 per cent as ammonium ions.

In contrast, when the calves were fed a concentrate diet (85 per cent barley + other components) the urine was acid and contained appreciable amounts of phosphorus and ammonium ions. Infusion of HCl into the rumen produced a further increase in acid excretion, practically all of which appeared in the urine as ammonium ions.

Excretion of calcium in the urine increased during infusion of HCl and was correlated with urine pH.

There were few changes in faecal mineral excretion during acid infusion but less sodium and potassium were excreted in the faeces when the concentrate diet was fed compared to when the roughage diet was fed.

Note:

The authors would like to thank Mr A. Faulks and Mr W. Buchan for skilled assistance during these experiments and to Mr J. Mathieson of the Analytical Department.