Secretion and absorption of phosphorus in the gastrointestinal tract of sheep fed on four diets

The effects of three phosphorus supplements on the metabolism of phosphorus in the gastrointestinal tract of sheep were examined. One group of sheep was fed on a low phosphorus (P-L) diet and three other groups on diets supplemented with monosodium orthophosphate (P-MSOP), meat and bone meal (P-MBM) or bran (P-ORG). The total phosphorus (TP), ultrafilterable phosphorus (UP) and nucleic acid phosphorus (NP) in the digesta were measured. There was a large net secretion of phosphorus into the rumen and the first quarter of the small intestine, and net absorption from the remainder of the small intestine of all sheep. The relative concentration of phosphorus in the omasum suggested considerable omasal secretion and a subsequent abomasal absorption of phosphorus, but this was considered to be largely an artefact. There were no consistent differences between diets in the net secretion and absorption of TP at sites along the gastrointestinal tract. Sheep fed on the P-L diet secreted more phosphorus per unit of phosphorus intake than sheep on the other diets. Most UP was present in the abomasum and cranial small intestine where the pH was lowest. The concentration of UP fell precipitously in the small intestine, which suggested that this was the major form of phosphorus absorbed and that there was a high degree of complex formation at the higher pH. The concentration of NP was relatively constant, and the lowest NP/TP ratios were found in the cranial small intestine. NP formed in the rumen was mainly degraded in the cranial small intestine. NP represented half the TP in the colon of sheep fed on diet P-L. The results indicate that the form of phosphorus supplement did not influence phosphorus metabolism in the gastrointestinal tract