Acid-Base Status, Renal Function, Water, and Macromineral Metabolism of Dry Cows Fed Diets Differing in Cation-Anion Difference

Dietary cation-anion difference was defined as the summation of the milliequivalents of Na and K minus the sum of the milliequivalents of Cl and S per kilogram of DM. Twelve Holstein cows were used in a crossover experiment to compare the effects of changing the cation-anion difference of a diet based on haylage. Two cation-anion differences, 481.8 and 327.2 meq/kg, were compared. Increased dietary cation-anion difference had no significant effects on BW or intake and digestibility of DM, ADF, NDF, and N. The diet with a cation-anion difference of 481 meq/kg of DM increased apparent absorption of water and urine volume. Fecal excretion of Na and absorption and urinary excretion of S were increased by a cation-anion difference of 327 meq/kg of DM. Although blood concentrations were unaffected, lower dietary cation-anion difference reduced concentrations of H+ and HCO3- in urine and total urinary excretion of HCO3-. Plasma volume, packed cell volume, glomerular filtration rate, and effective renal plasma flow were unaffected by diet. Small changes in dietary cation-anion differences, even within the positive range, affected acid-base status and water metabolism of dry pregnant cows without affecting renal function or blood volume.