title: A comparison of phosphorus speciation and potential bioavailability in feed and feces of different dairy herds using 31p nuclear magnetic resonance spectroscopy.
abstract: An experiment was conducted to examine how potential phosphorus (P) bioavailability (inferred from speciation) differs in feed and feces collected in spring from four dairy herds representing different management systems: (i) total confinement with cows fed total mixed ration (TMR), (ii) total confinement with TMR plus P mineral supplement, (iii) a hybrid of confinement with TMR and pastoral grazing, and (iv) predominantly grazing with supplemental grains. A treatment was included that air dried feces to simulate conditions after dung deposition. Wet chemical techniques and solution (31)P nuclear magnetic resonance spectroscopy ((31)P-NMR) were used to identify P concentrations and compounds present in water (a surrogate for P in overland flow), dilute acid (0.012 M HCl, an estimate of P utilization by cattle), or NaOH-EDTA (a solution that maximizes the organic P extraction) extracts of feed and feces. In general, P concentration in feces paralleled P in feed. Air drying feces decreased water-extractable P by 13 to 61% largely due to a decrease in orthophosphate, whereas NaOH-EDTA-extractable P increased by 18 to 48%. Analysis of dilute HCl was unsuccessful due to orthophosphate precipitation when pH was adjusted to 12 for (31)P-NMR. In water extracts, more P was in bioavailable diester-P forms, undetectable by colorimetry, than in NaOH-EDTA extracts. In feed, orthophosphate dominated (46-70%), but myo-IHP varied with feed (<10% in forage samples but 43% in a TMR sample). The proportion of myo-IHP decreased in feces compared with feed via mineralization but decreased less in systems with a greater proportion of available P input (e.g., orthophosphate and phospholipids). Feed and drying effect the concentrations and forms of P in feces and their potential impact on soil and water quality. Although bioavailable P in feces from pasture-based and confined systems can be similar in spring, dung-P is distributed on a lower kg P ha(-1) rate in grazing systems. The best method to mitigate P loss from feces is to decrease P in feed.
authors.affiliation: AgResearch, Invermay Agricultural Centre, Mosgiel, New Zealand.
authors.forename: R W
authors.initials: RW
authors.lastname: McDowell
authors.forename: Z
authors.initials: Z
authors.lastname: Dou
authors.forename: J D
authors.initials: JD
authors.lastname: Toth
authors.forename: B J
authors.initials: BJ
authors.lastname: Cade-Menun
authors.forename: P J A
authors.initials: PJ
authors.lastname: Kleinman
authors.forename: K
authors.initials: K
authors.lastname: Soder
authors.forename: L
authors.initials: L
authors.lastname: Saporito
mesh_terms: D000821:Animal Feed; D000818:Animals; D001682:Biological Availability; D002417:Cattle; D005243:Feces; D009682:Magnetic Resonance Spectroscopy / Q000379:methods*; D010759:Phosphorus Isotopes / Q000737:chemistry*
chemical_list: D010759:Phosphorus Isotopes