Phosphorus and calcium retention in steers fed a roughage diet is influenced by dietary 25OH-vitamin D

Active absorption of phosphorus (P) from the alimentary tract is promoted by increased concentrations of active vitamin D [1,25(OH)₂D₃] in blood; however, the production of this active vitamin is determined by plasma concentrations of calcium (Ca) and not P. As a consequence, diets of adequate Ca content, but insufficient P, will not promote active P absorption. Dietary supplements of 25OH-vitamin D (25OHD), the precursor of 1,25(OH)₂D₃, may stimulate P absorption and P retention of ruminants consuming diets marginally deficient in P. To evaluate this hypothesis, steers (n = 18) were fed a pelleted low-quality roughage diet containing an adequate Ca concentration (0.68%). Nine steers received a supplement of 25OHD mixed into their feed, at a rate of 3.25 mg/head.day, before pelleting. The other nine steers were fed the Control diet without supplementation. All steers were individually housed for 10 days before being moved into metabolism crates for a further 3-day period. The steers which received the diet containing 25OHD exhibited increased retention of P and Ca, ~4 and 3 g/day, respectively. Blood samples were taken before the adaptation period and then daily during the period in the metabolism crates. Plasma concentrations of both P and Ca were increased by 25OHD supplementation throughout the collection period. Addition of 25OHD to the diet of grazing animals may reduce the need for P supplementation programs and improve productivity.