Sejin Cheong ¹ Carolyn Chandler-Khayd ¹ Sequoia Williams ² Amélie CM Gaudin ² Peiman Aminabadi ³ Michele Jay-Russell ³ Emily Evans ⁴ Lee Klossner ⁴ Paulo Pagliari ⁴ Patricia D Millner ⁵ Annette Kenney ⁶ Fawzy Hashem ⁶ Amber R Sciligo ⁷ Alda F. A. Pires ^1*

• ¹ Department of Population Health and Reproduction, School of Veterinary Medicine, University of California, Davis, Davis, California, United States
• ² Department of Plant Sciences, College of Agricultural and Environmental Sciences, University of California, Davis, Davis, California, United States
• ³ Western Center for Food Safety, University of California, Davis, Davis, California, United States
• ⁴ Department of Soil, Water and Climate, College of Food, Agricultural and Natural Resource Sciences, University of Minnesota, Saint Paul, Minnesota, United States
• ⁵ Environmental Microbial and Food Safety Laboratory, Beltsville Agricultural Research Center, Agricultural Research Service (USDA), Beltsville, Maryland, United States
• ⁶ Department of Agriculture, Food and Resource Sciences, University of Maryland Eastern Shore, Princess Anne, Maryland, United States
• ⁷ The Organic Center, Lexington, Kentucky, United States

Integrated crop-livestock systems (ICLS) use animals to graze crop residues or cover crops before planting fresh produce and provide ecosystem services to support organic production. However, there is a risk of foodborne pathogen transfer to fresh produce because grazing may introduce foodborne pathogens into the soil and produce. To examine the effect of cover crop use and the risk of cover crop grazing on the contamination of soil and produce by foodborne pathogens in ICLS, a three-year (2019-2021) study was conducted in organically managed plots, which were assigned three different treatments (fallow without cover crop or grazing, cover crop without grazing, or cover crop with grazing by sheep) in a maize/tomato rotation. A total of 184 pre-and post-graze fecal samples and 96 samples of tomatoes were cultured for foodborne pathogens (Escherichia coli O157, non-O157 Shiga toxin-producing Escherichia coli (STEC), and Listeria monocytogenes). Soil samples were collected monthly until 126-171 days after grazing (824 in total) and tested for foodborne pathogens and generic E. coli (MPN/g). We did not detect any foodborne pathogens from harvested tomatoes. One non-O157 STEC positive soil sample (0.1%, 1/824) was detected in the fallow treatment, and one L. monocytogenes-positive (1.1%, 1/92) was detected from the post-graze fecal samples. Grazing cover crop by sheep has a low risk of foodborne pathogen contamination for cash crops. Mixed effect zero-inflated negative binomial models were used to compare the treatment effect on generic E. coli. Soil samples collected in the graze cover crop treatment showed significant increases in the counts of generic E. coli until 61-82 days post grazing, but no difference was observed after 96-123 days, compared to the baseline of the fallow treatment. Findings from generic E. coli counts support the use of the USDA National Organic Program (NOP) 90-or 120-day interval rule between applying raw manure and harvesting in organic farming into ICLS. This longitudinal field trial confirmed that the effect of sheep grazing on foodborne pathogen contamination in ICLS is minimal but further studies comparing the genetic associations between fecal and soil samples would be necessary to distinguish the source of foodborne pathogens.