Management-intensive grazing impacts on total Escherichia coli, E. coli O157:H7, and antibiotic resistance genes in a riparian stream

Authors: RUBECK, LAURA - Natural Resources Conservation Service (NRCS, USDA); Wells, James - Jim; HANFORD, KATHRYN - University Of Nebraska; Durso, Lisa; SCHACHT, WALTER - University Of Nebraska; Berry, Elaine

Submitted to: Science of the Total Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/18/2021
Publication Date: 4/15/2022
Citation: Rubeck, L.M., Wells, J.E., Hanford, K.J., Durso, L.M., Schacht, W.H., Berry, E.D. 2022. Management-intensive grazing impacts on total Escherichia coli, E. coli O157:H7, and antibiotic resistance genes in a riparian stream. Science of the Total Environment. 817. Article 152611. https://doi.org/10.1016/j.scitotenv.2021.152611.
DOI: https://doi.org/10.1016/j.scitotenv.2021.152611

Interpretive Summary: Riparian areas are the zones that occur along streams and other water bodies, and are characterized by unique vegetation and soil that is influenced by the presence of the water. Among a number of valuable functions, riparian areas are important for protecting stream water quality by stopping sediment, nutrients, and other contaminants that can be carried by runoff from the adjacent land. Vegetation in riparian zones can provide nutritious forage for cattle and other livestock. A benefit of grazing riparian zones is the potential for the removal of palatable weeds and other undesirable vegetation. However, unmanaged grazing of livestock can degrade riparian zones and negatively impact water quality. The goal of this research was to determine the impacts of management-intensive grazing (MIG) of cattle on the water quality of a stream flowing through the grazing area. Cattle were grazed for two-week periods in May in each of three years, and moved to new pastures daily, thereby reducing overgrazing of the forage and protecting the streambank. Cattle grazing and heavy rain events increased concentrations of the fecal bacteria E. coli in the stream water. However, MIG appeared to limit the level of E. coli concentrations compared to continuous grazing systems. In addition, the turbidity of the water, as measured by the concentration of total suspended solids, increased during grazing. However, MIG prevented large increases in water turbidity and nitrogen concentrations during heavy rain events. These results indicate that short-term MIG can limit the negative effects of cattle grazing on water quality. The ability to utilize this forage resource without negatively affecting water quality would be advantageous to livestock producers and the riparian environment.

Technical Abstract: The impacts of management-intensive grazing (MIG) of cattle on concentrations of total Escherichia coli, total suspended solids (TSS), and nitrate-nitrite nitrogen (NO3 + NO2-N), and occurrence of E. coli O157:H7 and selected antibiotic resistance genes (ARGs) in stream water and/or sediments were evaluated. Cattle were grazed for two-week periods in May in each of three years. Overall, grazing increased total E. coli in downstream water by 0.89 log10 MPN/100 mL (p < 0.0001), and downstream total E. coli concentrations were higher than upstream over all sampling intervals. Downstream TSS levels also increased (p = 0.0294) during grazing. In contrast, there was a main effect of treatment for downstream NO3 + NO2-N to be lower than upstream (3.59 versus 3.70 mg/L; p = 0.0323). Overwintering mallard ducks increased total E. coli and TSS concentrations in January and February (p < 0.05). For precipitation events during the 24 h before sampling, each increase of 1.00 cm of rainfall increased total E. coli by 0.49 log10 MPN/100 mL (p = 0.0005). In contrast, there was no association of previous 24 h precipitation volume on TSS (p = 0.1540), and there was a negative linear effect on NO3 + NO2-N (p = 0.0002). E. coli O157:H7 prevalence was low, but the pathogen was detected downstream up to 2½ months after grazing. Examination of ARGs sul1, ermB, blactx-m-32, and intI1 identified the need for additional research to understand the impact of grazing on the ecology of these resistance determinants in pasture-based cattle production. While E. coli remained higher in downstream water compared to upstream, MIG may reduce the magnitude of the downstream E. coli concentrations. Likewise, the MIG strategy may prevent large increases in TSS and NO3 + NO2-N concentrations during heavy rain events. Results indicate that MIG can limit the negative effects of cattle grazing on stream water quality.