Phosphorus excretion in beef steers as impacted by increasing levels of dicalcium phosphate supplementation

Wednesday, July 23, 2014
Exhibit Hall AB (Kansas City Convention Center)
Elizabeth A Riley , Virginia Tech, Blacksburg, VA
Deidre D. Harmon , Virginia Polytechnic Institute and State University, Blacksburg, VA
Jason K. Smith , Virginia Tech, Blacksburg, VA
Abigail L Zezeski , Virginia Tech, Blacksburg, VA
Scott P. Greiner , Virginia Tech, Blacksburg, VA
Katharine F. Knowlton , Virginia Tech, Blacksburg, VA
Mark A. McCann , Virginia Polytechnic Institute and State University, Blacksburg, VA
Abstract Text:

Feeding phosphorus (P) to grazing cattle in excess of requirements can lead to increased P in surface water.  The objective of this study was to determine the impact of increasing levels of dicalcium phosphate as a supplemental inorganic source of P.  Eight Hereford steers, 304±22 kg of BW, were randomly assigned to one of four dietary P treatments in a 4 x 4 replicated Latin square.  Steers were fed a ration containing 80±1% chopped grass hay (0.10%P) and 0, 33, 65 or 95 g/d of dicalcium phosphate.  All steers were supplemented with 0.79 kg/d beet pulp, 0.23 kg/d rumen-protected fat supplement and 20 g/d trace mineral salt.  Diets were formulated to approximate 50, 100, 150, and 200% of dietary P requirement for a growing beef steer.  Steers were housed in individual pens and fitted with total fecal collection bags.  Each period consisted of a 9-d adjustment period followed by a 5-d collection period.  Following the final collection of each period, a 10 ml blood sample was collected via jugular venipuncture and analyzed to determine plasma inorganic P (Pi) concentration.  Samples were analyzed for Pi and total P (Pt) using the molybdovanadate yellow and blue methods, respectively.  Data were analyzed using the PROC GLIMMIX procedure of SAS with a model that included diet and period.  Linear, quadratic and cubic treatment effects were evaluated using preplanned contrasts.  Dietary P increased linearly (P < 0.05) across diets at levels of 3.88, 10.45, 16.47 and 22.28 g/d, respectively.  Total P excretion increased linearly with increasing dietary P content (7.73, 11.41, 15.50, and 21.51 g/d; P < 0.05) as did Pi excretion (3.19, 6.51, 10.50, 14.67 g/d, P < 0.05) and thus Pt excretion was highly correlated (P < 0.05; r = 0.94) with Pi excretion.  Apparent P digestibility increased quadratically with increasing dietary P (-110.67, -9.32, 5.83, and 3.75%, P < 0.05), with negative digestibility’s suggesting dietary P levels below requirement.  P solubility ([Pi / Pt] x 100) increased quadratically with increasing dietary P (39.71, 56.64, 66.60, and 67.85 %, P < 0.05), indicating that the percentage of water soluble P increases when feeding P levels above requirements. Similar to fecal P trends, blood plasma Pi increased linearly with increasing levels of dietary P (6.40, 8.35, 8.72, and 9.19 mg/dL, P < 0.05).  Beef cattle operations can reduce environmental impacts by closely matching P supplementation to P requirements.