Phosphorus net absorption in dairy cows subjected to abomasal infusion of inorganic phosphorus – a pilot study

In this pilot study, the effects of phosphorus (P) supply on inorganic phosphorus (P_i) net absorption in dairy cows were investigated. Three non‐lactating, non‐pregnant, rumen‐fistulated Swedish Red breed dairy cows were studied in a 3 × 3 Latin square design. Monosodium dihydrogen orthophosphate dihydrate (NaH₂PO₄*2H₂O) was continuously infused into the abomasum for 4 days. The solutions provided 0, 14.4 or 28.8 g P_i/day. Rumen fluid volume and outflow rate were estimated at day four of each experimental period using cobalt‐lithium EDTA as an external marker. Acid insoluble ash in feeds and faecal samples was used to quantify P faecal excretion. Concentrations of P_i in collected samples of rumen fluid, blood, faeces and urine were determined. P_i flow into the small intestine increased (p < 0.05) with P_i infusion. P_i net absorption tended to increase (p = 0.08) but proportion of absorbed P_i tended to decrease (p = 0.08). Urinary P_i excretion was negligible and did not affect P homoeostasis (p = 0.50). There was no change in plasma P_i concentration (p = 0.45) in response to P_i infusion. The increase in total faecal P excretion (p < 0.05) with increasing level of infused P_i was solely because of increased soluble faecal P_i (p < 0.05). It is suggested that at P overfeeding, intestinal P_i net absorption is saturable in dairy cows.