Dissipation of antimicrobial resistance determinants in composted and stockpiled beef cattle manure

Windrow composting or stockpiling reduces the viability of pathogens and antimicrobial residues in manure. However, the impact of these manure management practices on the persistence of genes coding for antimicrobial resistance is less well known. In this study, manure from cattle administered 44 mg of chlortetracycline kg-1 feed (dry wt. basis) (CTC), 44 mg of CTC and 44 mg of sulfamethazine kg-1 feed (CTCSMZ), 11 mg of tylosin kg-1 feed (TYL), and no antimicrobials (control) were composted or stockpiled over 102 d. Temperature remained ≥55°C for 35 d in compost and 2 d in stockpiles. Quantitative PCR was used to measure levels of 16S rRNA genes and tetracycline [tet(B), (C), (L), (M), (W)], erythromycin [erm(A), (B), (F), (X)], and sulfamethazine [sul(1), (2)] resistance determinants. After 102 d, 16S rRNA genes and all resistance determinants declined by 0.5 to 3 log10] copies per gram dry matter. Copies of 16S rRNA genes were affected (P < 0.05) by antimicrobials with the ranking of control > CTC = TYL > CTCSMZ. Compared with the control, antimicrobials did not increase the abundance of resistance genes in either composted or stockpiled manure, except tet(M) and sul(2) in CTCSMZ (P < 0.05). The decline in 16S rRNA genes and resistance determinants was higher (P < 0.05) in composted than in stockpiled manure. We conclude that composting may be more effective than stockpiling in reducing the introduction of antimicrobial resistance genes into the environment before land application of manure.