Corn stalk residue may add antibiotic-resistant bacteria to manure composting piles

Manure is commonly used as a fertilizer or soil conditioner; however, land application of untreated manure may introduce pathogens and antibiotic-resistant bacteria (ARB) into the soil, with harmful implications for public health. Composting is a manure management practice wherein a carbon-rich bulking agent, such as corn (Zea mays L.) stalk residue, is added to manure to achieve desirable carbon/nitrogen ratios to facilitate microbial activities and generate enough heat to inactivate pathogens, including antibiotic-resistant pathogens. However, when comparing compost piles and stockpiles for ARB reduction, we noticed that bulking agents added ARB to composting piles and compromised the performance of composting in reducing ARB. We hypothesized that ARB could be prevalent in corn stalk residues, a commonly used bulking agent for composting. To test this hypothesis, corn stalk residue samples throughout Nebraska were surveyed for the presence of ARB. Of the samples tested, 54% were positive for antibiotic-resistant Escherichia coli or enterococci using direct plating or after enrichment. Although not statistically significant, there was a trend wherein the use of pesticides tended to result in a greater prevalence of some ARB. Results from this study suggest that bulking agents can be a source of ARB in manure composting piles and highlight the importance of screening bulking agents for effective ARB reduction in livestock manure during composting.