Dissipation of Antimicrobials and Resistance Genes in Compost Originating from Cattle Manure after Direct Oral Administration or Post-Excretion Fortification of Antimicrobials

Our study compared the dissipation of antimicrobials and resistance genes (ARG) during composting of cattle manure fortified with antimicrobials versus manure from cattle administered the same antimicrobials in feed. Manure was collected from cattle fed diets containing (kg^-1 dry feed): (1) 44 mg chlortetracycline (CTC), (2) a mixture of 44 mg each of chlortetracycline and sulfamethazine (CTCSMZ), (3) 11 mg tylosin (TYL) or (4) Control, no antimicrobials. Fortified treatments were generated by adding antimicrobials to control manure. All manures were composted for 30 d with a single mixing after 16 d. Quantitative PCR (qPCR) measured the concentration of 16S rDNA and genes associated with tetracycline (tet), erythromycin (erm), and sulfamethazine (sul) resistance. Over 95% of antimicrobials dissipated over 30 d of composting, but CTC and SMZ remained higher in fed than fortified treatments, with a reverse trend observed for tylosin. Copy numbers of 16S rDNA decreased (P < 0.05) over 30 d, but were not altered by the presence of antimcrobials. Levels of all ARG except tet(L) decreased by 0.1-1.6 log₁₀ copies g dry matter^-1 in the first compost cycle, but some genes [tet(B), tet(L), erm(F), and erm(X)] increased (P < 0.05) by 1.0-3.1 log₁₀ copies g dry matter^-1 in the second. This pattern aligned with the positive correlation of these ARG with residue concentrations in the first cycle and the negative correlation in the second. During 30 d of composting, levels of tet(M) and tet(W) in CTC, erm(A), erm(B) and erm(X) in TYL, and sul(1) in CTCSMZ remained higher (P < 0.05) in fed than fortified treatments. Our results suggest that dissipation of ARG during composting of manure fortified with antimicrobials differs from manure generated by cattle that are administered antimicrobials in feed and does not always align with the dissipation of antimicrobial residues.