Effect of dietary modifications of calcium and magnesium on reducing solubility of phosphorus in feces from lactating dairy cows

Unintentional movement of P from dairy cow manure to off-farm locations has been an environmental concern for some time. The objective of this study was to evaluate the effects of increasing the dietary concentration and solubility of Ca and the dietary concentration of Mg on lactation performance and solubility of fecal P from lactating dairy cows receiving diets formulated to the same concentration of P (0.38% of dry matter). Eight dietary treatments were evaluated in a 2 × 2 × 2 factorial arrangement involving 2 dietary sources of Ca (CaCO3 having a moderate solubility of Ca vs. CaCl2 having an excellent solubility of Ca) and 2 dietary concentrations of Ca (average of 0.64 and 0.95% of dry matter) and Mg (0.25 and 0.4% of dry matter). Twenty-four multiparous cows in mid lactation were fed the 8 diets in three 21-d periods. Dry matter intake and milk production were measured daily, and milk composition was measured on the last 6 milkings of each period. Fecal samples were collected twice a day during the last 5 d of each period, composited within cow, dried at 55°C, and subjected to 10 successive water extractions, and soluble P, Ca, and Mg were determined. Excretion of fecal P (g/d) was correlated positively with intake of P but not with intake of Ca or Mg. A smaller proportion of fecal P was extracted when dietary concentration of Ca increased (37.5 vs. 47.7%) and when CaCl2 instead of CaCO3 was fed (40.3 vs. 44.9%). Feeding more Mg reduced water-soluble P in feces but only when CaCO3 and not CaCl2 was fed. Increasing the amount of soluble Ca in the diet produced a relatively stable Ca-phosphate compound (hydroxylapatite) in ashed fecal samples, whereas feeding less soluble Ca resulted in a more soluble P phase (Mg-substituted whitlockite). Energy-dispersive X-ray elemental spectroscopy in conjunction with scanning electron microscopy showed spatial association between Ca-Mg and P. A reduction of approximately 5 g of soluble P/cow per d was detected when dietary concentration of Ca increased from an average of 0.64 to 0.95% of dry matter. Supplemental CaCO3 would be a preferred source of Ca over CaCl2 because cows fed CaCO3 tended to produce more 4% fat-corrected milk, more milk fat, and milk with a greater concentration of fat and protein. Current prices would also favor feeding CaCO3 over CaCl2. Increasing dietary intakes of Ca and Mg beyond current recommendations may increase formation of insoluble phosphate complexes (Ca-P rather than Ca-Mg-P associations), which result in decreased solubility of P in dairy-cow feces and reduce losses of P from agricultural areas where feces are applied.