| Summary: | Antimicrobials are widely used in North America’s livestock industry. Field application of livestock manure disperses antimicrobials in to the environment. Antimicrobials can increase the level of antimicrobial resistance development in human and livestock pathogens. Three main studies were conducted to: i) quantify the simulated rain runoff losses of chlortetracycline, sulfamethazine and tylosin following surface application vs. soil incorporation of beef cattle feedlot manure; ii) quantify the dissipation kinetics of these antimicrobials in a seasonally-frozen soil following field application of manure; and iii) quantify and compare the dissipation of excreted vs. fortified (that is, added to antimicrobial-free manure) antimicrobials during indoor composting. Manure was sourced from cattle (Bos taurus) receiving no antimicrobial (control), 44 mg chlortetracycline, 44 mg each of chlortetracycline and sulfamethazine, or 11 mg tylosin per kg-1 feed. Antimicrobial concentration in simulated rain runoff following field application of manure generally reflected the corresponding concentrations in manure. Mass loss ranged from 1.7 to 6.5% for chlortetracycline and was 4.8% for sulfamethazine and 0.24% for tylosin. Incorporation of manure into the top 10 cm of soil significantly reduced the mass loss of chlortetracycline and the concentration in runoff of both chlortetracycline and sulfamethazine. Both chlortetracycline and sulfamethazine were persistent in the seasonally-frozen Canadian prairie soil tested. The first-order dissipation half-life (DT50) for chlortetracycline added along with sulfamethazine was 77 d during the growing season and 648 d during the non-growing season. The DT50 of chlortetracycline added alone did not differ significantly between the two seasons (mean DT50 = 121 d). Sulfamethazine was detected throughout the 10-mo monitoring period (mean ≤ 16 ± 10 µg kg-1). Composting dissipated 85–99% of initial concentrations of chlortetracycline, sulfamethazine, and tylosin in manure within 30 d, indicating the potential of composting to minimize the dispersal of these antimicrobials in agroecosystems. The first-order dissipation constant (k) was significantly greater for excreted chlortetracycline (0.29 d-1 - 0.54 d-1) than for the fortified (0.11 d-1 - 0.13 d-1) compound. In contrast, dissipation was significantly greater for fortified sulfamethazine (0.47 d-1) and tylosin (0.31 d-1) than for the excreted antimicrobials (0.08 d-1 for sulfamethazine and 0.07 d-1 for tylosin). === October 2016
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