Characterization of acid-base disturbances and effects on calcium and phosphorus balances of dietary fixed ions in pregnant or lactating does.

Effects of fixed cation-anion balance on acid-base status and calcium and phosphorus balances were examined. Pregnant and lactating goats were fed a diet of alfalfa hay, concentrate and minerals to vary the cation-anion balance [meq sodium (Na) + meq potassium (K)-meq chloride (Cl)]/100 g diet dry matter (DM) over the range found in ruminant feeds. Small but significant effects on ruminal pH, fermentation and dilution rate were observed. Metabolic acid-base status of pregnant and lactating goats was normal when (Na + K - Cl) balance was 40 to 50 meq/100 g DM. The other treatments drastically altered plasma electrolyte concentrations, causing metabolic acid-base disturbances and profound changes in calcium and phosphorus metabolism. Subclinical hypernatremic, hypochloremic metabolic alkalosis was induced by a dietary fixed cation excess (Na + K - Cl) of greater than 85 meq/100 g DM (typical of buffered, alfalfa diets) and caused hypocalciuria, diminished calcium and phosphorus absorption, and possibly diminished dietary calcium absorption and resorption of calcium from bone. Subclinical hyperchloremic, hyponatremic metabolic acidosis from a diminished dietary fixed cation-anion balance (Na + K - Cl) of less than 10 meq/100 g DM (typical of nonbuffered corn silage or grain diets) caused hypercalciuria, enhanced calcium and phosphorus absorption and apparently enhanced calcium resorption from bone. Apparent effects on absorption and resorption depended on calcium and phosphorus intakes. Alterations in goats performance were not demonstrable. Dietary excesses of fixed cations over anions (meq Na + K - Cl/100 g diet DM greater than 50) cause metabolic alkalosis in ruminants, whereas fixed anion excesses (meq Na + K - Cl/100 g diet DM less than 40) cause metabolic acidosis. Content of electrolytes in diets should be reported in all nutrition trials with ruminants for assessment of metabolic acid-base status.