TECHNICAL NOTE - AN ALGEBRAIC-METHOD FOR CALCULATING FECAL OUTPUT FROM A PULSE DOSE OF AN EXTERNAL MARKER

Fecal output estimates derived from a one-compartment, Gamma-2, age-de pendent model were compared with estimates derived algebraically by co mputing the area under the marker excretion curve for lambs given a pu lse dose of ytterbium-labeled forage. Lambs were fed one of four diets (as-fed basis): 100% alfalfa hay, 100% prairie hay, 50:50 alfalfa:sor ghum grain, and 50:50 prairie hay:sorghum grain. For the one-compartme nt model, fecal output was calculated as the dose of Yb (micrograms) d ivided by the initial concentration in the compartment (micrograms of Yb/gram of DM) multiplied by the age-dependent rate constant (hours-1) . For the algebraic method, fecal output was calculated as the dose of Yb divided by the area under the marker excretion curve ([micrograms of Yb/gram of fecal DM].hours), both with the full complement of fecal samples and with fecal samples collected at 12-h intervals. Fecal out put estimated by the three methods did not differ (P > .15) from measu red fecal output (total collection). Marker retention time calculated from the one-compartment, age-dependent model was numerically greater ( P > .10) than retention time calculated algebraically (sum of concen tration x time divided by sum of concentrations weighted for collectio n interval) for lambs fed all four diets- These results suggest that t he area under the marker excretion curve generated from a pulse dose o f Yb-labeled forage will provide estimates of fecal output that do not differ from those calculated from a one-compartment, age-dependent mo del.