Assessment of the calcium and phosphorus nutrition in horses by analysis of urine.

Studied were made to determine if a practical assessment of the calcium and phosphorus nutrition of horses could be obtained from an analysis of urine samples. The concentrations of Ca and P in urine samples changed markedly when groups of 4 mares were fed diets containing from 1.0 to 3.9 g Ca/kg and from 1.5 to 6.1 g P/kg, but serum concentrations of Ca and P remained relatively constant. The concentrations in single urine samples were considered unreliable indicators of excretion of the minerals because of variations in water excretion, and two methods to overcome this problem were examined. In one method, the creatinine clearance ratios (%Cr) of Ca and P were calculated. The other method was simpler, and concentrations of the minerals were expressed as the ratio of the total solute concentration. Both of these methods appeared to give reliable estimates of the excretion of Ca and P in urine, and there were significant relationships between the values obtained for horses fed diets containing a wide range of Ca and P. When the horses were fed adequate Ca, the Ca-%Cr was greater than 2.5% and P-%Cr was less than 4%. The corresponding ratios of mineral concentration to solute concentration were greater than 15 mumole Ca/mosmole and less than 15 mumole P/mosmole. When a low Ca (1.0 g/kg) and high P (4 g/kg) diet was fed, the excretion of Ca and P had changed markedly after 3 days and could be monitored in single samples of urine by either method. It was concluded that a practical assessment of the Ca and P nutrition of horses could be obtained from an analysis of urine samples to determine if the diet is either low, adequate or high in Ca, and high in P.