Current guidelines for the minimum distance between cattle feed lots and fresh produce growing fields are likely inadequate to ensure leafy greens are not contaminated with E. coli from dust and manure. “Additional research is needed to determine safe set-back distances between cattle feedlots and crop production that will reduce fresh produce contamination,” according to scientists who conducted a two-year study.

Food safety guidelines currently call for a minimum distance of 120 meters, about 394 feet, but the researchers found E. coli contamination on leafy greens growing 180 meters (590 feet) away from a cattle feed lot. The team of nine plant pathologists and produce specialists from several academic entities published their findings in the March 2016 edition of the journal Applied and Environmental Microbiology. The scientists from across the country used the 6,000-head beef cattle feedlot at the U.S. Meat Animal Research Center (USMARC) in Clay Center, NE, for the two-year research project. It was funded in part by the Center for Produce Safety and a California Specialty Crop Block Grant. “Leafy greens typically are consumed raw, so protecting these products from microbial contamination in the pre-harvest environment is critical to reducing the risk for foodborne illness,” according to the report. In agricultural regions it is not unusual to find produce growing in fields adjacent to concentrated animal feeding operations (CAFOs), which present ample contamination potential. Cattle are particularly prone to shedding E. coli via their feces and the nature of feed lots, where animals’ movements continually kick up contaminated dust and debris, makes airborne contamination of nearby food-growing fields a possibility. The researchers chose to examine the potential for contamination of leafy greens because those vegetables are more frequently associated with Shiga toxin-producing E. coli (STEC) outbreaks than any other food group, except beef. In addition to checking for E. coli contamination on the spinach, mustard greens and turnip greens in the test plots, the scientists checked air samples collected in the growing fields for the pathogen. They checked for total E. coli bacteria and specifically for the potentially deadly E. coli O157:H7. Researchers collected hundreds of individual leaves from the produce plots six different times in 2011 and 2012. They planted nine plots each year, with three plots each at distances of 60, 120 and 180 meters from a cattle feedlot. The team collected 270 leafy greens samples — of 10 leaves each — and 100 feedlot manure samples six times from June through September. “Both E. coli O157:H7 and total E. coli bacteria were recovered from leafy greens at all plot distances,” the scientists and extension researchers reported. “E. coli O157:H7 was recovered from 3.5 percent of leafy green samples per plot at 60 (meters), which was higher than the 1.8 percent of positive samples per plot at 180 (meters), indicating a decrease in contamination as distance from the feedlot was increased. “Although E. coli O157:H7 was not recovered from air samples at any distance, total E. coli was recovered from air samples at the feedlot edge and all plot distances, indicating that airborne transport of the pathogen can occur.”   (To sign up for a free subscription to Food Safety News, click here.)