Application of strong ion difference theory to urine and the relationship between urine pH and net acid excretion in cattle

Objective-To develop an equation expressing urine pH in terms of independent variables, derive an equation relating urine pH to net acid excretion (NAE), and apply this new knowledge to determine the role that monitoring urine pH should play when diets with low cation-anion difference are fed to dairy cattle. Animals-11 Holstein-Friesian cows. Procedures-A physicochemical strong ion approach was used to develop a general electroneutrality equation for urine that involved urine pH and strong ion difference (SID [difference between strong cation and strong anion concentrations]), Pco₂, the concentration of ammonium ([NH ₄⁺]) and phosphate ([PO₄]), and 3 constants. The general electroneutrality equation was simplified for use in bovine urine and applied to 321 data points from 11 cows fed different diets. Results-Urine pH was dependent on 4 independent variables (urine SID, [NH₄ ⁺], Pco₂, and [PO₄]) and 3 constants. The simplified electroneutrality equation for bovine urine was pH ≈ {pK ₁′ - log₁₀(S Pco₂)} + log ₁₀([K⁺] + [Na⁺] + [Mg²⁺] + [Ca ²⁺] + [NH₄⁺] - [Cl^- - [SO ₄^2-]). The relationship between urine pH and NAE (in mEq/L) for cattle fed different diets was pH = 6.12 + log₁₀(-NAE + [NH₄⁺] + 2.6). Conclusions and Clinical Relevance-A change in urine SID, [NH₄⁺], Pco₂, or [PO ₄] independently and directly led to a change in urine pH. Urinary [K⁺] had the greatest effect on urine pH in cattle, with high urine [K⁺] resulting in alkaline urine and low urine [K⁺] resulting in acidic urine. Urine pH provided an accurate assessment of NAE in cattle when pH was > 6.3.