Dissipation of Antimicrobials in Feedlot Manure Compost after Oral Administration versus Fortification after Excretion

Fortification of manure with antimicrobials is one approach to studying their dissipation. However, fortified antimicrobials may not accurately model dissipation that occurs after antimicrobials have been administered to livestock in feed and excreted in manure. This study examined the dissipation of antimicrobials excreted in manure versus those added directly to manure (fortified). Steers were fed a diet containing (kg(-1) feed) (i) 44 mg chlortetracycline, (ii) 44 mg each of chlortetracycline and sulfamethazine, (iii) 11 mg tylosin, and (iv) no antimicrobials (control). Fortified antimicrobial treatments were prepared by adding antimicrobials to control manure. Manure was composted for 30 d, sampled every 2 to 3 d, and analyzed for antimicrobials and compost properties. Antimicrobial dissipation followed first-order kinetics. The dissipation rate constant was significantly greater (based on 95% confidence limit) for excreted (0.29-0.54 d(-1)) than for fortified chlortetracycline (0.11-0.13 d(-1)). In contrast, dissipation rate constants were significantly greater for fortified sulfamethazine (0.47 d(-1)) and tylosin (0.31 d(-1)) than when the same antimicrobials were excreted (0.08 and 0.07 d(-1), respectively). On average, 85 to 99% of the initial antimicrobial concentrations in manure were dissipated after 30 d of composting. The degree of dissipation was greater (P < 0.0001) for fortified (99%) than for excreted tylosin (85%). Composting can be used to reduce environmental loading of antimicrobials before field application of beef cattle manure. Dissipation rates of fortified antimicrobials during manure composting may not accurately reflect those of antimicrobials that are consumed and excreted by cattle.