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.

Availability of phytate-bound P as influenced by supplemental phytase was studied in eight horses consuming four diets in a 4 x 4 Latin square design experiment. The treatments were a control (containing a low P level, 18.4 g/d) and three high-P diets. These diets contained P as monocalcium phosphate (MCP; 43.7 g/d), myoinositol hexakisphosphate in the form of wheat and rice bran (MIHP; 41.8 g/d), or MIHP with microbial phytase (MIHPP; 42.5 g/d). The proportions of phytate-bound P were 3, 1, 55, and 56% for the control, MCP, MIHP, and MIHPP, respectively. The MIHPP diet was supplemented with 300 phytase units (FTU)/kg (as-fed basis). Feces and urine were collected quantitatively and analyzed for P, Ca, and Mg. Urinary P excretion was lower (P < 0.05) with the control diet (0 g of P/d) than with the MCP diet (1.0 g of P/d). The low urinary P excretion (0.3 g of P/d) for the MIHP diet suggested low P availability compared with the MCP diet, but apparent digestibility of P expressed as a percentage of intake did not differ (P = 0.065) between these diets. Apparent Ca digestibility was lower (P < 0.05) for the MIHP diet than for the MCP diet (26.4 vs. 42.4%). This difference may have been caused by the origin of the Ca in these diets. Phytase supplementation increased apparent Ca digestibility from 26.4 to 31.5% (P < 0.05). Magnesium was not influenced by the level of phytate in the diet. Our data indicate that phytase supplementation had more influence on Ca digestibility than on P digestibility and suggest that phytase supplementation may be beneficial for improving Ca digestibility for horses receiving a phytate-rich diet.

Literature data indicate that the form of K in the ration can affect its inhibitory influence on Mg absorption in ruminants. We tested whether identical amounts of K either intrinsically present in artificially dried grass or present in added KHCO3 have different effects on Mg absorption in dry cows. In a 3 x 3 Latin square design, six cows were fed rations consisting of low-K grass and concentrate with or without KHCO3 or a ration consisting of high-K grass with concentrate without added KHCO3. Each ration was given for a period of 4 wk. The ration low in intrinsic K contained 26 g of K/kg of dry matter, the ration low in intrinsic K plus KHCO3 contained 43 g of K/kg of dry matter, and the ration high in intrinsic K also contained 43 g of K/kg of dry matter. The three rations were balanced for crude protein, crude fat, crude fiber, Mg (2.2 g/kg of dry matter), Ca, P, and Na. Apparent Mg absorption was 10.8 +/- 1.54% of intake (mean +/- SE, n = 6) when the cows were fed the low-K ration, but dropped to 1.9 +/- 3.4 and 2.1 +/- 1.9% of intake, respectively, when the rations high in KHCO3 and high in intrinsic K were fed. The two high-K rations induced similar increases in ruminal K concentrations both before and after feed consumption. The feeding of KHCO3 did not influence ruminal pH. The intake of extra K may raise ruminal K concentrations, which increases the transmural potential difference so that Mg transport across the rumen epithelium becomes depressed. Thus, intrinsic and added K had identical effects on ruminal K concentrations and on Mg absorption. Feeding trials with ruminants in which K intakes are manipulated with the use of KHCO3 may reflect those cases when concentrations of K intrinsically present in feedstuffs may vary.

In vitro experiments with isolated rumen epithelium have shown that the relationship between the ruminal K concentration and either the apical membrane potential difference or the mucosal-to-serosal Mg flux reach plateau values at high ruminal K concentrations. Hence, it may be hypothesized that the inhibitory effect of supplemental K on Mg absorption becomes smaller at high initial K intakes. To test our hypothesis, 6 ruminally fistulated, nonpregnant dry cows were fed 6 experimental diets in a 6 x 6 Latin square design with a 2 x 3 factorial arrangement of treatments. Four cows were of a Friesian-Holstein x Holstein-Friesian cross, and the 2 remaining cows were of a Meuse-Rhine-IJssel x Holstein-Friesian cross. The diets provided either 40.6 or 69.1 g of Mg per day and contained 20.7, 48.0, or 75.5 g of K per kilogram of dry matter. The dietary variables were obtained by mixing KHCO(3) and MgO into the basal concentrate. Absorption of Mg and the urinary Mg excretion was significantly decreased by supplemental K and significantly increased after the intake of supplemental Mg. In contrast to apparent Mg absorption, the urinary excretion of Mg was not affected by the dietary K x Mg interaction. Postfeeding ruminal K and Mg concentrations were increased with increasing K and Mg intakes. Postfeeding ruminal K concentrations and the urinary excretion of Mg showed a linear negative correlation; the slope was not significantly affected by Mg intake. Therefore, our hypothesis was rejected. Furthermore, these data indicate that supplemental Mg can effectively counteract the suppressant effect of K on Mg absorption in cows.

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