EFFECTS OF DIETARY CATION-ANION DIFFERENCE ON THE ACID-BASE STATUS OFDRY COWS

Responses in dry matter intake (DMI) and acid-base balance to three so urces of anionic salts (dietary cation-anion difference = -63 to -40 m eq/kg of dry matter), an acidified fermentation by-product, MgSO4. 7H( 2)O + NH4Cl, and MgSO4.7H2O + CaCl2. 2H(2)0 + CaSO4, were evaluated re lative to the responses of cows fed a control diet (dietary cation-ani on difference = 203 meq/kg of dry matter) that did not contain anionic salts. Diets were fed for l-wk periods to eight nonlactating Holstein s assigned to two replicated 4 x 4 Latin squares. Daily DMI increased as time of access to the diet increased up to d 5; mean DMI over d 5 t o 7 was reduced by dietary anionic salts. Diets containing anionic sal ts induced a mild metabolic acidosis that was completely compensated b y nonrespiratory mechanisms (decreased blood bicarbonate and base exce ss; pCO(2) and pH values were unaffected). Urinary pH values and bicar bonate excretion were reduced, and urinary NH4+ and titratable acidity excretion were increased, for cows fed diets containing anionic salts . Strong ion difference in urine was decreased by dietary anionic salt s because of the relatively greater excretions of Cl- and S2- versus N a+ and K+ by cows fed these diets. Dietary anionic salts decreased mea n ruminal pH by 0.12 units, possibly because of the reduced strong ion difference of ruminal fluid. Dietary anionic salts increased mean rum inal NH3 concentration by 2.2 mM, probably because of the higher nonpr otein N content of these diets. The strong negative relationship (r(2) = 0.95) between urinary pH and net acid excretion by cows fed the die ts containing anionic salts suggested that urinary pH measurement migh t be a useful tool to assess the degree of metabolic acidosis that was imposed by dietary anionic salts.