A variety of antimicrobials are added to swine feed or water by the swine industry, often at sub-therapeutic levels for growth promotion. These pharmaceuticals can boost the daily growth of pigs and reduce animal death rates at swine facilities, thereby enhancing overall production efficiency and increasing profitability. Tylosin is among the most widely used of the macrolide class of veterinary antibiotics by the swine industry.
The occurrence of antibiotic-resistant bacteria and antibiotic-resistant genes near swine operations has been reported. However, very little is known about their release from tile-drained fields receiving swine manure application. Data from full-scale field experiments will provide valuable information about the relative retention and transport of ARB and ARGs in manure-amended, tile-drained agricultural fields in the corn/soybean cropping systems prevalent in the Upper Midwest.
Studying the Fate and Transport of Antibiotics, Antibiotic-resistant Bacteria and Resistance Genes
Four plots were sampled at Iowa State University’s Northeast Research Farm near Nashua, Iowa. These agricultural plots are instrumented with a subsurface water-quality monitoring system operational since 1988. Each 1-acre plot is drained separately with subsurface drain lines installed in the center of the plot.
The plots encompass two tillage practices: chisel plow and no-till. Liquid swine manure was injected on one of each tillage type while the second of each type received urea ammonium nitrate and served as a control for assessing background levels. A summary of the plots selected for sampling is presented in Table 1.
Table 1. Northeast Research Farm plots selected for sample collection, including ID numbers, crop, tillage practice and nutrient-management history
|23||Corn||CP||Fall injected swine manure at 150 lb total N/acre|
|24||Corn||CP||Spring preplant spoke injected UAN at 150 lb N/acre|
|25||Corn||NT||Fall injected swine manure at 150 lb total N/acre|
|34||Corn||NT||Spring preplant spoke injected UAN at 150 lb N/acre with cover crop|
Manure was obtained from a commercial swine facility currently including tylosin in the feed at sub-therapeutic levels for growth promotion. Manure was injected with a shank, forming a band of treated soil at agronomic rates of 150 pounds nitrogen per acre. Following manure application, composite soil samples were collected from each manure plot — three from the manure band and three from the area between the manure bands — and three samples were also collected from the control plots.