Using fecal phosphorus, calcium and ash excretion to predict total and inorganic phosphorus intake of beef cattle consuming a forage-based ration

Wednesday, July 23, 2014
Exhibit Hall AB (Kansas City Convention Center)
Deidre D. Harmon , Virginia Polytechnic Institute and State University, Blacksburg, VA
Jason K. Smith , Virginia Tech, Blacksburg, VA
Mark A. McCann , Virginia Polytechnic Institute and State University, Blacksburg, VA
Abstract Text:

In order to mitigate the environmental and economic impact of phosphorus (P) supplementation in excess of beef cattle requirements, producers are currently being encouraged to re-evaluate P supplementation strategies.  One of the major limiting factors in doing so is that P intake of grazing cattle is generally unknown.  The objective of this evaluation was to determine the ability of fecal total P (Pt) and fecal inorganic P (Pi), calcium (Ca) and ash fractions to explain variation in Pt and Pi intake.  Results of dietary and fecal Pt, Pi, Ca and ash analyses were obtained from two previous experiments that quantified P excretion of beef cattle supplemented with P from different dietary sources.  Steers included in each of the two experiments were fed a basal ration of low P chopped grass hay (0.10 and 0.13%) and supplemented with increasing levels of dicalcium phosphate (0, 33, 65, 95 g/d) or increasing levels of corn gluten feed (0, 0.5, 1.0, 1.5 kg/d) to provide increasing levels of dietary P that were below, met or exceeded NRC recommended P requirements.  Steers in the dicalcium phosphate trial were 304±22 kg BW and steers in the corn gluten trial were 427±79 kg BW.  Daily feed intake measurements and total fecal collections were used to quantify Pt, Pi, Ca and ash content (expressed as a percentage of DM) of feed offerings, orts and feces.  Values were then used to determine respective nutrient intake and excretion.  All statistical analyses were conducted using JMP Pro.  Interpretation of the results of an initial factor screening indicated that fecal percentage of Pt, Pi, Ca and ash explained a significant (P < 0.05) portion of the variation in Pt and Pi intake.  Full four-way factorial regression models were generated to predict Pt (R2 = 0.79; P < 0.0001) and Pi (R2 = 0.83; P < 0.0001) intake expressed in g per d using the Fit Model procedure.  Refinement of the full factorial model using the reverse stepwise personality supports the use of the full factorial design, as removal of higher-order interactions resulted in a reduction of the R2 value.  These results indicate the ability of fecal Pt, Pi, Ca and ash percentages and their interactions to explain the majority of the variation in Pt and Pi intake.  These and additional measurements could be utilized to develop a fractional P intake prediction model for beef cattle.