The utilization of phytate-phosphorus by the transition dairy cow
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.
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