The use of antibiotics by the swine industry to increase production efficiency and treat disease is thought to contribute to antibiotic resistance in the environment. When manure from hog operations is applied to fields with subsurface drainage, it is possible that the antibiotics and bacteria with resistance will be transported through tile systems and discharged into surface waters. To investigate this, tylosin, enterococci (a pathogen indicator organism), and antibiotic resistance genes (ARGs) were assessed in manure, soil and tile water samples. The ARG examined in this study were the erm genes which confer resistance to macrolide antibiotics, including tylosin and erythromycin. Manure from a swine facility which administers tylosin at sub-therapeutic levels was applied to chisel plow and no-till plots with separate tile drains. The use of tylosin in swine production caused an increase in erm genes in manure and in manured-treated soil above the background levels of erm genes in soils not receiving manure. This increase in soil is greatest immediately after manure application; and ermB, ermC, and ermF persist in manure injection band in concentrations greater than in non-manured soils over winter. However, the manure band concentrations eventually decreased to levels equivalent to the non-manured control soils. This is potentially due to a reduction in erm-hosting bacteria in the soil following manure application. The same trend was observed in the decline of total enterococci populations over time potentially due to die off and other environmental factors, but enterococci were less persistent than erm genes after manure application. Tylosin concentrations are very low in the soil and water, and do not likely impact the selective pressures on erm genes in either matrix. Erm gene concentrations in tile water were not different between tillage or manure treatments during the dry to average years of precipitation (2011 and 2012), but in 2013 and 2014 when above average precipitation occurred significantly higher concentrations of ermB genes were observed in tile drainage from the manure amended plots. However, concentrations of erm genes observed in 2013 and 2014 are similar to levels observed in our previous years of study. A significant reduction of erm genes in drainage water samples was observed in the second year of the corn-soybean two year rotation. However, the increasing prevalence of crop rotations with two or more years of corn and manure indicates the need to monitor corn-corn rotations that receive manure application yearly.
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Associate Professor, Agricultural and Biosystems Engineering
Iowa State University, 3358 Elings Hall
Ames, IA 50011