Phytate phosphorus (pP) availability in rumen fluid was analyzed during the transition period of two heifers by in vitro and in situ incubations. Apparent digestibility (AD) of P during the transition period was measured and P balance determined. The study included three periods, a prepartum period (P1), including the 2 wk prepartum, a postpartum period (P2), including a 4 wk postpartum and a post-transition period (P3) which corresponds to d 47 to 56 for cow 1854 and d 64 to 73 for cow 1847. In vitro [Subscript p]P availability of corn, soybean meal (SBM), whole cottonseed (WCS), a total of mixed ration (TMR) and phytic acid (PA) were determined. Percent [Subscript p]P loss was determined by in vitro incubation for 15, 30, 60, or 120 min following precipitation with ferric chloride. Percent [Subscript p]P loss, averaged across periods, was 90.78% in corn, 85.35% in SBM, 75.61% in WCS, 88.80% in TMR and 84.73% in PA. A significant period effect was observed for [Subscript p]P when PA was incubated, but not for corn, SBM, WCS, or TRM. In situ incubation of feeds and TMR showed that 99% of [Subscript p]P was either solubilized or phosphates were hydrolyzed after 72 h of incubation. Equations were developed to predict [Subscript p]P loss after a given incubation time. Apparent digestibility was determined in each period, using 24 h total fecal collection. Apparent digestibility of P was 56.03% in P1, 29.81% in P2 and 9.05% in P3. Period 1 was different than both P2 and P3. Phosphorus consumed (g/d), fecal P (%) and fecal DM (kg/d) were showed period effects. To determine P balance, TMR, intake data and fecal data from the AD study were utilized along with urine, milk, growth, and pregnancy P requirements. Urinary output was estimated and urinary P concentrations were determined. Phosphorus balance was -37.72 g/d in P1, 0.15 g/d in P2 and -24.09 in P3. Due to large standard deviations no difference was observed between periods for P balance. A cow effect occurred when urine P (mg/L and g/d) was analyzed. Results indicate that ruminants possess the ability to efficiently utilize [Subscript p]P.