A short-term study of calcium and phosphorus absorption in sheep fed on diets high and low in calcium and phosphorus

Compartmental modelling techniques were used to measure the absorption of calcium and phosphorus in young sheep which had been fed ad libitum for 6 months on one of four diets containing 1.35 or 8.2 g calcium and 0.72 or 4.5 g phosphorus per kg dry matter. In most of the compartmental models, absorption could be represented by a primary and secondary compartment. The percentage of 45Ca absorbed increased as calcium intake decreased so that the amount of calcium absorbed was not greatly affected by changes in the dietary intake or abomasal contents of calcium. Also, there were no differences due to treatment in the amount of calcium absorbed by primary or secondary absorption. The percentage of 32P absorbed did not vary between treatments, although the percentage absorbed by primary absorption was higher in sheep on low phosphorus diets. For secondary absorption, the percentage absorbed was higher in sheep on high phosphorus diets. The amount of phosphorus absorbed by primary absorption was the same for all treatments; however, sheep on low phosphorus diets absorbed very little phosphorus by secondary absorption, whereas sheep on high phosphorus diets absorbed 2.5 times as much phosphorus as those on low phosphorus diets. The amounts of mineral absorbed by primary absorption for phosphorus and calcium, and by secondary absorption for calcium, were not altered by changes in the amount of phosphorus or calcium mixed with the tracer, indicating that primary absorption may be saturable for phosphorus and calcium and secondary absorption for calcium. The large absorption of phosphorus by the secondary compartment was proportional to the amount of phosphorus in the lower small intestine and therefore the phosphorus was probably absorbed by a non-saturable mechanism.