A Comparison of Phosphorus Bioavailability in the Manure of Different Dairy Systems using 31P NMR.
Richard McDowell1, Zhengxia Dou2, John D. Toth3, Barbara Cade-Menun4, Peter J. Kleinman5, Kathy J. Soder6, and L. Saporito6. (1) AgResearch, Invermay Agricultural Centre, Private Bag 50034, Mosgiel, New Zealand, (2) University of Pennsylvania, 382 West Street Rd., Kennett Square, PA 19348-1692, (3) Univ. of Pennsylvania, 382 West Street Rd, Kennett Sq, PA 19348, (4) Bldg. 320 Rm. 118, Stanford University, Stanford University, Dept. Geological & Environmental Sciences, Stanford, CA 94305-2115, (5) USDA-ARS, Bldg. 3702, Curtin Rd., University Park, PA 16802-3702, (6) USDA-ARS, Building 3702 Curtin Road, University Park, PA 16802
An experiment was conducted to examine how P forms and bioavailability differ in feed and dung collected from four dairy systems, 1) total confinement with adequate diet P supplied by a total mixed ration (TMR); 2) total confinement with excess diet P; 3) a hybrid of confinement and pastoral grazing; and 4) predominantly grazing with supplemental grain feed. Liquid state 31P nuclear magnetic resonance spectroscopy (31P-NMR) was used to identify phosphorus compounds present in water, dilute acid and NaOH-EDTA extracts of feed, forage and feces. While NaOH extracts have been used with 31P-NMR to estimate organic P forms, water and dilute acid extraction are better suited to determine P forms that are potentially vulnerable to surface losses. The concentration of P in feces was generally proportional to that in feed; however, the feces of animals on pasture had higher P content than those fed TMR at the same P concentration. A greater proportion of P in water extracts was labile diester P than in NaOH-EDTA extracts. Water extractability of P decreased when fecal samples were dried; however this did not occur in extracts of dilute acid and NaOH-EDTA. Type of feed input and drying of feces had strong effects on changing the distribution of fecal P forms and on the potential impact of P on soils and surface water bodies. Although P availability in spring pasturage was greater than in TMR, the larger land area used for grazing, in combination with less diet supplementation with mineral P and reduced P chemical fertilisation of pasture, was shown to reduce the potential for negative impacts of P in manure on agricultural land and associated water bodies.