Short-term effects of dietary cation:anion balance on bone mineral homeostasis in the bovine.

Fifteen Friesian oxen between 12 and 18 months of age with a mean body mass of 240.7 kg, were randomly assigned to diets containing 0.25% phosphorus (P) or less, to evaluate the acute effects of a dietary cation:anion balance (DCAB) of either -11.1, +16.5 or +25.6 mmol 100 g-1 diet dry matter calculated as (Na+K) - (Cl+S), on blood, bone and faecal P, calcium (Ca) and magnesium (Mg) for a period of 9 weeks. Bone and faecal P concentrations were significantly (P < 0.05) higher in the anionic treatment group compared with the cationic and control treatment groups. The same was true for blood P, but significant (P < 0.05) only when compared with the cationic treatment group. Bone Ca was lower (NS) and blood Ca was significantly (P < 0.05) higher in the cationic compared to the anionic and control treatment groups, but faecal Ca was significantly (P < 0.05) higher in the anionic compared to the cationic, and control treatments groups. There was little difference in blood and bone Mg content among treatment groups, but faecal Mg was significantly (P < 0.05) higher in the anionic treatment group compared to the other 2 treatment groups. Whether this was due to the anionic nature of the diet or dietary concentration of Mg is unclear. Results from this research indicate a P sparing effect in which a low DCAB may improve the availability of P in animals where dietary P is limiting. In critical evaluations, weekly sampling of rib bone should be used to give a more accurate assessment of acute changes in bone mineral.