#16-039

Complete

Date Full Report Received

08/30/2019

Date Abstract Report Received

08/30/2019

Investigation

Institution:
Primary Investigator:

Swine production is a major economic activity in Iowa and the Midwest, and accounts for a significant portion of the gross farm income in the region. The swine industry is also among the most regulated agricultural operations in many states. Environmental stewardship is being increasingly recognized by swine operators as an important criterion for profitable operation. However, contamination of surface water and groundwater from swine manure continues to be a concern for producers. Here we report the results of our study assessing the environmental fate of antibiotics, antibiotic-resistant bacteria, and antibiotic-resistant genes on swine farms.

This work is important to producers because impairment of surface water by pathogens could become a significant roadblock for future expansion of the pork industry. While pathogen contamination of surface waters is the leading cause of water quality impairments in Iowa, the emerging concern about antimicrobial resistance (AMR) is likely to increase scrutiny of the animal production industry, specifically industry-related manure management practices. Our research improves understanding of the occurrence and transport of antibiotic resistant bacteria (ARB) and antibiotic resistant genes (ARGs) from fields receiving swine manure versus those treated with chemical fertilizer. This information is provided for a range of manure application timing (early fall (EFM), late fall (LFM), spring (SM)) and management practices (chisel plow, no till) to provide practical management information to farmers. Further we considered different manure management strategies (anaerobic digestion, two-phase storage, centrifugation, and ionophore addition) for treatment of manure prior to land application for their potential to reduce ARGs and ARBs.

In considering different manure application timings, we found that swine manure applications in early fall, late fall, and spring resulted in mostly similar patterns of ARB and ARG dissipation in soil. For the ARG, there was evidence for shorter persistence after spring manure applications; however, the spring period is also the season with greatest precipitation and tile drainage, factors which increase risk of transport to downstream waters. The analysis of resistant genes in soil showed that soils receiving swine manure approximately one year previously (continuous corn) or two years previously (corn soybean) to which had not had antibiotics for many years (controls) were not different in the abundance of ARG in soil. This is evidence suggesting that these ARG are not accumulating in the soil under these farming operations.

For tile drainage samples, previous research has reported significant differences in ARGs in drainage from plots receiving manure and plots not receiving manure in the fall. This study confirms that LFM and SM had a higher percent detection of ARGs when compared to control, however, the same cannot be said for EFM, whose percent detection was comparable to the control. It is possible that this could be attributed to the sampling scheme, as EFM sees the greatest lag time between manure application and when drainage flow sampling commences. Finally, this study saw the largest percent detection of all three ARGs selected (ermB, ermF, and tetM) in drainage from plots receiving spring manure (SM). Greater transport of ARG in tile drainage after SM occurred despite the shorter persistence of ARG in soil after SM application, suggesting that drainage predominates over ARG survival in soils in determination of ARG transport.

The reduction of both resistant Enterococcus and resistance genes in two-phase storage with no manure added suggests that this may be a promising management solution for reduction of AMR in manure storage. When compared to anaerobic digestion, our results suggest that two phase storage without manure addition may achieve similar treatment of antibiotic resistance bacteria and genes. This management strategy presents an opportunity for further research and development, and the practicality of installing storage in comparison to anaerobic digestion would need to be further evaluated.

This knowledge will assist farmers, researchers, and extension personnel in the development of better management strategies for manure storage and application for limiting the prevalence of ARGs and ARBs.

Contact:
Michelle L. Soupir, Ph.D.
Associate Professor & Associate Chair for Research/Extension
3358 Elings Hall * 605 Bissell Rd * Agricultural & Biosystems Engineering
Iowa State University * Ames, IA 50011
Office: (515) 294-2307