Prediction of Bovine Urine Specific Gravity and Total Solids by Refractometry

Abstract

Total daily urine was collected from 12 Hereford heifers during 4 days of water deprivation followed by 4 days of rehydration in a reversal experiment. The specific gravity of the urine samples, as determined by weight/volume, ranged from 1.0030 to 1.0413. Total solids concentrations, as determined by lyophilization, ranged from 1.70 to 9.90 gm./ 100 gm. urine. Refractive indices were determined with a temperature compensated refractometer and ranged from 1.3361 to 1.3493. The linear relationship between the independent variable, refractive index, and specific gravity or total solids was established, as was the relationship between specific gravity and total solids. The error in predicting total solids from specific gravity at the 95% confidence interval, could be 18.8% at the mean specific gravity. If specific gravity were predicted from refractive index, the deviation from the actual could be 22.8% at the mean refractive index. By contrast, total solids could be adequately estimated from refractive index. The prediction equation is Ŷ=625.19X−833.46, where Ŷ is the predicted total solids in gm./ 100 gm. urine and X the refractive index. The correlation between these two variables was .993 and the standard error of estimate 0.200. The prediction error (P=95%) at the mean refractive index would be only 6.2%. The determination of refractive index is simple and rapid, and of the techniques used offered the best method for evaluating renal concentrative properties.