Effect of decreasing dietary phosphorus supply on net recycling of inorganic phosphate in lactating dairy cows

Five ruminally cannulated lactating Holstein cows, fitted with permanent indwelling catheters in the mesenteric vein, hepatic vein, portal vein, and an artery were used to study intestinal absorption and net recycling of inorganic phosphate (Pi) to the gastrointestinal tract. Treatments were low P (LP; 2.4 g of P/kg of DM) and high P (HP; 3.4 g of P/kg of DM). The dietary total P (tP) concentrations were obtained by replacing 0.50% calcium carbonate in the LP diet with 0.50% monocalcium phosphate in the HP diet. Diets were fed for 14 d and cows were sampled on d 14 in each period. Cows were fed restrictively, resulting in equal dry matter intakes as well as milk, fat, and protein yields between treatments. Net Pi recycling (primarily salivary) was estimated as the difference between net portal plasma flux (net absorption of Pi) and apparently digested tP (feed - fecal tP difference). Phosphorus intake, apparently digested tP, and fecal tP excretion decreased with LP. An effect of decreased tP intake on net portal plasma flux of Pi could not be detected. However, despite numerically minute net fluxes across the liver, the net splanchnic flux of Pi was less in LP compared with that in HP. Though arterial plasma Pi concentration decreased, net Pi recycling was not decreased when tP intake was decreased, and recycling of Pi was maintained at the expense of deposition of Pi in bones. Data are not consistent with salivary Pi secretion being the primary regulator of Pi homeostasis at low tP intakes. On the contrary, maintaining salivary Pi recycling at low tP intakes indicates that rumen function was prioritized at the expense of bone P reserves.