Abstract/Summary

The objective of the study was to evaluate the effect of inorganic phosphorus (Pi) infusion on P absorption in large intestine, milk production and phosphorus excretion. Four ruminally- and ileally-cannulated crossbred cows were used in a 4×4 Latin Square with 21 d periods. Cows were fed a total mixed ration containing 0.21% P, providing 50% of the cows’ P requirement. Cobalt-EDTA (Co-EDTA) was used as marker to measure large intestine digesta flow. On d 13 to 21 of each period, each cow was infused daily with 0, 20.1, 40.2, or 60.3 g Pi into the abomasum and total collection was conducted on d 18 to 21. Ileal samples were collected every 9 h on d 18 to d 21. Feed, digesta, and fecal samples were analyzed for total P and Pi using the molybdovanadate yellow method and blue method, respectively. All data were analyzed using PROC GLIMMIX in SAS 9.3 using contrasts to evaluate linear, quadratic and cubic effects of Pi infusion dose. Dry matter (DM) intake, apparent DM digestibility, milk yield and milk total P were unaffected by Pi infusion. Ileal flow and fecal excretion of total P and Pi increased linearly with increasing infused Pi. In the large intestine, net absorption of TP and Pi were increased linearly with increasing infused Pi. The magnitude of absorption from the large intestine was greater than reflected in current models and raising questions that could be evaluated with longer infusion periods or dietary alteration.