Purpose: The objectives of this study focused on assessing the die-off of generic Escherichia coli in the soil, after fields were grazed by sheep; determining the prevalence of Shiga toxin-producing Escherichia coli (STEC) in the sheep; and assessing the genetic relatedness between E. coli isolates from soil and sheep feces.
Methods: Sheep grazed cover crops at an organic farm prior to planting Field A (onions) and Field B (melons). Fecal samples (n=24) were collected from sheep on day 0 and 56. Soil samples (n=192) were collected weekly for the first five weeks, then monthly for eight visits total. Escherichia coli strains were compared by pulsed-field gel electrophoresis (PFGE).
Results: Results showed a 3.0-log reduction of mean generic E. coli concentration (Most Probable Number, MPN) in the soil from the peak of 3.7 log MPN/g at 48 days post sheep (DPS) grazing to 0.67 log MPN/g by 111 DPS in Field A. Field B showed a 2.6-log reduction of mean generic E. coli concentration from the peak of 3.5 log MPN/g at 14 DPS to 0.95 log MPN/g by 84 DPS. STEC prevalence in the sheep flock was 4.2% (1 of 24). Closely related E. coli strains were found in soil and feces.
Significance: Developing research-based waiting periods between grazing and harvest is important to inform best practices for farmers and food safety stakeholders. Although conditions vary by season and region, this study indicates that the mean generic E. coli MPN/g levels in the soil after grazing are below 1.0 log MPN/g by the 120 day standard used by the USDA National Organic Program for raw manure application.