Food Safety Risks Associated with Sheep Grazing in Vegetable Stubble Fields

Recent outbreaks of E. coli O157:H7 linked to California produce have dramatically elevated concern that ruminants such as sheep grazing on crop residues may elevate levels of this and other pathogens in the soil and in subsequent fresh and fresh-cut vegetable crops grown at the grazed location. <P>
This concern is also being extended to the use of sheep as a non-chemical method for controlling unwanted vegetation in alfalfa crops, orchards and vineyards. In addition, sheep are often used to graze postharvest vegetable matter before the next crop is planted. Another concern related to sheep grazing is the movement of large numbers of sheep via public road that are also used for transit of vegetable harvesting equipment from field to field. <P>

Due to food safety concerns, over 99% of the volume of California leafy greens, are produced and marketed under the California Leafy Greens Marketing Agreement (LGMA). The participating companies have committed themselves to sell products grown in compliance with the food safety practices accepted by the LGMA board. The board recognizes the need for further research to validate or adjust these guidelines based on scientific evidence. In addition, certain large customers of leafy green products have imposed cumbersome and exhaustive requirements for production of leafy greens purchased by their company. One such customer has announced they would not purchase leafy greens from the largest vegetable production area in the Imperial Valley due to sheep grazing.<P> In order to assess the validity of these concerns and to develop science-based recommendations regarding sheep grazing and food safety, several key questions related to the prevalence and intensity of pathogen shedding, pathogen survival, and effect of crop system being grazed on pathogen prevalence must be addressed. <P>First, what is the prevalence of E. coli O157:H7 and Salmonella spp. in the feces of California sheep that graze crop residue and alfalfa? <P>Second, what is the range of concentration or intensity of these bacteria in sheep fecal material? <P>Third, does this prevalence or intensity of fecal shedding E. coli O157:H7 or Salmonella spp. shift upward or downward as sheep are rotated through different crop systems?<P> Finally, what is the rate of inactivation of E. coli O157:H7 and Salmonella spp. once it is deposited onto the soil surface or paved/graveled/dirt roads and subsequently exposed to solar radiation or tilled into the soil profile in preparation for the next crop? <P>

This research project will be directed by UC Davis, with collaboration from California State University, Chico, UC Cooperative Extension, Imperial County, and the California Wool Growers Association. <P>
This project has as its primary objective the identification of the extent to which sheep grazing in areas where fresh vegetables are harvested represents potential food safety risks and what management strategies can be implemented to ameliorate these risks.