Hypocalcemia induced by intravenous administration of disodium ethylenediaminotetraacetate and its effects on excretion of calcium in urine of cows fed a high chloride diet.

Evidence supports the theory that a diet that is rich in nonmetabolizable anions fed to dairy cows during the dry period reduces the risk of hypocalcemic paresis puerperalis. When cows are fed a diet that is rich in anions instead of cations, more Ca is absorbed in the intestine and excreted in urine. We hypothesized that, in cows fed a diet that was rich in anions, the increased flow of Ca through the body could be drained to support the maintenance of plasma Ca concentration around parturition. The hypothesis was tested by binding plasma Ca through intravenous administration of Na2-EDTA and measuring excretion of Ca in urine. In a 2-period x 14-d crossover study, six, nonpregnant, nonlactating, multiparous cows were fed either a diet that was rich in cations (dietary cation-anion difference = +332 meq/kg of dry matter) or rich in anions (dietary cation-anion difference = -230 meq/kg of dry matter). On the last day of each feeding period, Na2-EDTA was infused intravenously until the amount of plasma Ca that was not bound to EDTA reached approximately 1 mmol/L. The amount of EDTA that could be infused was significantly greater when the cows were fed the diet that was rich in anions. During the infusion of Na2-EDTA the rate of Ca excretion in urine dropped to almost 0 when the diet that was rich in anions was fed. After feeding the diet that was rich in cations, excretion of Ca in urine was negligible and was not reduced further by Na2-EDTA infusion. Thus, in cows fed a diet that was rich in anions, the Ca intended for excretion with urine can be used when plasma Ca is under stress as would occur at the onset of lactation. However, the amount of Ca derived from plasma, interstitial fluid, and the skeleton during Na2-EDTA infusion was quantitatively much more important to the supply of Ca than was the reduction in excretion of Ca in urine. Most likely, this relationship would also be true when the production of colostrum begins.