Impacts of manure application timing and tillage practices on antibiotic resistant bacteria and genes in drainage water from manured fields
Agriculture is an economic cornerstone in Iowa and the surrounding states, as well as arguably the entire United States as a whole. However, there is growing concern about the use of antibiotics in the animal agriculture sector and its contribution to antibiotic resistance (Levy & Marshall, 2004). Yet it is still unclear how antibiotic resistance is affected by the environment at the field scale and what mechanisms can be employed to minimize the transport. This three-year study monitors both phenotypic and genotypic antibiotic resistance markers in subsurface drainage from research plots with varying manure application times and tillage practices. Enterococcus and tylosin and tetracycline-resistant Enterococcus were monitored as well as three antibiotic resistant genes (ermB, ermF, tetM) to 1) quantify antibiotic resistant bacteria (ARB) and antibiotic resistant genes (ARGs) in subsurface drainage 2) evaluate the impact of field management practices, manure application timing and tillage both the antibiotic resistant bacteria (ARB) and antibiotic resistant genes (ARGs), and 3) assess relationships between phenotypic and genotypic indicators of antibiotic resistance. Results of the study show overall low levels of both ARB and ARGs leading to further research questions with the targeted improvement of new methods. Although recommendations on specific field management practices best suited to minimize ARB and ARG transport cannot be drawn from the results of this study, it was shown that manure application in the spring had the strongest potential for future research.