Preference for inorganic sources of magnesium and phosphorus in sheep as a function of need.

We determined whether sheep discriminate among different mineral supplements containing P and Mg and if they modify their selection as a function of the basal diet received. Forty lambs were offered 4-way choices among inorganic sources of P and Mg: Magnesium oxide in its coarse (1-C-MgO) and granular (2-MgO) forms, and Magnesium Phosphate (3-Mag33; 33%Mg-2.7%P, and 4-MGP; 25%Mg-15%P), and 2-way choices between MGP and each of the other minerals, and between MgO and C-MgO (Baseline). Subsequently, lambs were randomly assigned to 4 groups (10 lambs/group) and fed rations such that the levels of Mg and P were low (LMg-LP), adequate (NMg-NP), low in Mg and adequate in P (LMg-NP), or adequate in Mg and low in P (NMg-LP). After 29 d, choice tests (Post-deficiency) were repeated. During Baseline, lambs ate and preferred Mag33 > C-MgO = MGP > MgO (P < 0.05). This pattern remained during post-deficiency tests, but lambs in LMg_LP, LMg_NP increased their preference for MGP and C-MgO, respectively, whereas lambs in NMg_LP increased their preference for C-MgO and MGP relative to Baseline (P < 0.05). Serum concentrations of P and Mg increased after preference tests, and preference for MgO and MGP in low-Mg groups increased as the serum concentration of Mg declined (P < 0.05). Thus, lambs discriminated among different minerals and some groups modified their preferences based on the basal diets received, which rectified mineral imbalances.