Abstract
Recovery studies in which chlortetracycline hydrochloride (CTC-HCl) standard was added to cattle and swine feed supplements at 4.09–9.99 g/ton showed lower antibiotic recovery turbidimetrically (80.6–98.7%) than by the AOAC modified standard as in 38.179(d) (91.2–98.7%) and the plain buffer as in 38.179(b) (93.8-133.0%) methods. Three feeds fortified with a commercial premix at the levels of 5.0 and 10.0 g CTC-HCl/ton showed an overall CTC-HCl recovery of 87.6–110.6% by manual turbidimetric assay. Results were 89.1–108.7% by the AOAC inactivated feed diluent standard and 95.4–125.4% by the plain buffer methods. For some sample extracts (as in cattle feed) the use of heat to stop bacterial growth in the turbidimetric method caused formation of a precipitate. Cooling of cultures to room temperature and rapid reading of sample turbidity followed by standard curve concentrations minimized this interference. The manual turbidimetric assay of low levels of CTC-HCl in feeds appears to offer advantages over other methods.