Application and comparison of two models to study effects of calcium sources in sheep

The aim of this research was to assess and compare two compartmental models by studying effects of different Ca sources on Ca and P metabolism of sheep. Brazilian male sheep (20) were fed a basal diet supplemented with different sources of Ca, being: limestone (L), alfalfa hay (AH), shell meal (OSM) and citrus pulp (CTP). After 21 days, each sheep was given, as a single dose via the right jugular vein, 7.4 MBq of radio-calcium (45Ca) and 7.4 MBq of radio-phosphorus (32P). Calcium and P metabolism were evaluated by comparing the Vitti–Dias (VD) and Fernández–Lopes (FL) models [Dias, R.S., Kebreab, E., Vitti, D.M.S.S., Roque, A.P., Bueno, I.C.S., France, J., 2006. A revised model for studying phosphorus and calcium kinetics in growing sheep. J. Anim. Sci. 84, 2787–2794; Lopes, J.B., Vitti, D.M.S.S., Abdalla, A.L., Haddad, M.L., Figueredo, A.V., Moraes, R.C.B., 2001. Modelo do fluxo biológico do fósforo de fontes de fosfato em suínos, usando o 32P como marcador. Rev. Bras. Zoot. 30, 165–173], by contrasting flows between gut and plasma, plasma and bone, and plasma and tissue. There were no differences in Ca and P intakes for the treatments. Ca flows from tissue and bone to plasma and vice versa were similar among treatments, though net bone and net tissue Ca retentions were higher for treatments L and OSM and lower, as well as negative, for AH and CTP (P<0.05). Net bone Ca results were consistent between the VD and FL models, although net tissue Ca retention was slightly higher for the VD. The presence of pectin in CTP and oxalate in AH could have affected Ca balance on these treatments. Total Ca absorption was higher (P<0.05) for L with both models. The chemical form of Ca in the different sources affected its metabolism, but did not affect P metabolism. Both models had higher P resorption than P absorption in bone, suggesting that the sheep were mobilizing P. It could be inferred that impaired digestion induced P mobilization from bone to supply P for metabolic needs. Both the VD and FL models had the same pattern for the P flows, and for net bone and tissue P retentions. Both models can be used to assess Ca and P kinetics in ruminants, and both suggest that our sheep tended to be deficient in Ca and P as well as that the inorganic sources of Ca were better utilized.