The objective of this study was to determine how the method and timing of swine manure application may impact the movement of multiple manure constituents in agricultural runoff. The manure constituents included nutrients, antimicrobials, antimicrobial resistant bacteria, and antimicrobial resistance genes. To achieve the objective, a series of field rainfall simulation tests were conducted. In the tests, swine manure slurry from a commercial farm was broadcast on or injected into plots by a commercial manure applicator. A set of 3 simulated rainfall events were initiated on the manure amended plots 1 day, 1 week, 2 weeks, or 3 weeks after the manure application. Such experimental design allowed us to systematically study the impacts of both the application method and the timing of application relative to rainfall on the movement of manure constituents in runoff. Results show that manure application method had no significant impact on the transport of nitrogen, but had significant impact on dissolved and total phosphorus in runoff. The levels of three antimicrobials, chlortetracycline, lincomycin, and tiamulin, were higher in runoff from broadcast plots than from injected plots. Other than E. coli, the two application methods did not yield significantly different levels of microbial constituents in runoff. On the other hand, the timing of land application relative to rainfall events exhibited significant impacts on the levels of nearly all manure constituents in runoff. Longer intervals led to less load in runoff. The only noticible exception was the antimicrobial resistance gene tet(X), whose relative abundance in runoff increased with longer time between application and rainfall. These findings show how the movement of different manure constituents in runoff may be affected by different manure land application strategies, highlighting the complexity in designing best management practice for manure application.
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