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

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 twoweek periods in May in each of three years. Overall, grazing increased total E. coli in downstream water by 0.89 log(10) 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 log(10) 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 21/2 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.

Automated summary

The study evaluated the impacts of management-intensive grazing (MIG) on water quality, specifically the concentrations of total Escherichia coli, total suspended solids (TSS), and nitrate-nitrite nitrogen (NO3 + NO2-N), as well as the occurrence of E. coli O157:H7 and antibiotic resistance genes (ARGs). The results showed that grazing increased the concentrations of total E. coli and TSS in downstream water, while reducing the concentration of NO3 + NO2-N. furthermore, precipitation and overwintering mallard ducks were found to also affect water quality. Overall, MIG was found to have a positive effect in reducing the negative impacts of cattle grazing on stream water quality.