Effects of Dietary Cation-Anion Difference on the Acid-Base Status of Dry Cows

¹ Department of Animal, Dairy, and Veterinary Sciences, Utah State University, Logan 84322-4815
² Department of Medical Sciences, School of Veterinary Medicine, University of Wisconsin, Madison 53706

Responses in dry matter intake (DMI) and acid-base balance to three sources of anionic salts (dietary cation-anion difference = –63 to –40 meq/kg of dry matter), an acidified fermentation by-product, MgSO₄·7H₂O + NH₄Cl, and MgSO₄·7H₂O + CaCl₂·2H₂O + CaSO₄, were evaluated relative to the responses of cows fed a control diet (dietary cationanion difference = 203 meq/kg of dry matter) that did not contain anionic salts. Diets were fed for 1-wk periods to eight nonlactating Holsteins 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 to 7 was reduced by dietary anionic salts. Diets containing anionic salts induced a mild metabolic acidosis that was completely compensated by nonrespiratory mechanisms (decreased blood bicarbonate and base excess; pCO₂ and pH values were unaffected). Urinary pH values and bicarbonate 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 salts because of the relatively greater excretions of Cl^- and S^{2 –} versus Na⁺ and K⁺ by cows fed these diets. Dietary anionic salts decreased mean ruminal pH by 0.12 units, possibly because of the reduced strong ion difference of ruminal fluid. Dietary anionic salts increased mean ruminal NH₃ concentration by 2.2 mM, probably because of the higher nonprotein N content of these diets. The strong negative relationship (r² = 0.95) between urinary pH and net acid excretion by cows fed the diets containing anionic salts suggested that urinary pH measurement might be a useful tool to assess the degree of metabolic acidosis that was imposed by dietary anionic salts.

Key Words: acid-base balance • anionic salts • dairy cows

Submitted on July 11, 1997
Accepted on January 21, 1998

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