Turbidimetric assay of tylosin in animal feeds containing urea

A turbidimetric method is described for determination of tylosin in animal feeds containing urea. This method includes several modified or new steps to existing turbidimetric and AOAC plate assays that improve the extraction of tylosin, remove interferences from feeds, free tylosin activity, concentrate tylosin from low-level feeds, and reduce variability of assay results. A larger analytical sample size has been incorporated into the assay to decrease variability of assay results. A methanol-phosphate buffer extraction solution has replaced the hot buffer and methanol extraction solution. A hydrolysis step, which is not contained in the AOAC plate assay, was developed to free tylosin from the tylosin urea adduct that forms over time in feeds containing urea. A disposable C18 column was used to concentrate tylosin from feeds at levels less than 15 ppm. By increasing the analytical sample size from 25 to 100 g, the coefficient of variation for 12 weighings of cattle feed was reduced from 28.4 to 9.3%. Average recoveries from cattle rations containing tylosin at levels of 8, 10, and 100 ppm were 94, 94, and 91%, respectively.