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 (Pi) net absorption in dairy cows were investigated. Three non-lactating, non-pregnant, rumen-fistulated Swedish Red breed dairy cows were studied in a 3x3 Latin square design. Monosodium dihydrogen orthophosphate dihydrate (NaH2PO4*2H2O) was continuously infused into the abomasum for 4days. The solutions provided 0, 14.4 or 28.8g Pi/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 Pi in collected samples of rumen fluid, blood, faeces and urine were determined. Pi flow into the small intestine increased (p<0.05) with Pi infusion. Pi net absorption tended to increase (p=0.08) but proportion of absorbed Pi tended to decrease (p=0.08). Urinary Pi excretion was negligible and did not affect P homoeostasis (p=0.50). There was no change in plasma Pi concentration (p=0.45) in response to Pi infusion. The increase in total faecal P excretion (p<0.05) with increasing level of infused Pi was solely because of increased soluble faecal Pi (p<0.05). It is suggested that at P overfeeding, intestinal Pi net absorption is saturable in dairy cows.