Antimicrobial resistance in beef feedlot manure as impacted by forage concentration and essential oil in finishing cattle diets

Antibiotic resistant (AR) infection is a serious threat to public health and growing evidence demonstrates its linkage to antibiotic use in livestock. Because manure represents a potential vector for antibiotic resistant bacteria (ARB) to environmental matrices and foods of animal origin, research is underway to assess the effectiveness of feedlot manure management practices on antibiotic resistance. This research focused on evaluating the impact of beef cattle diet management strategies on AR prevalence in freshly excreted manure and consolidated feedlot surface material. Two treatments - forage concentration and essential oils (EO) - in cattle diets were tested in a factorial design for their impact on the concentrations of ARB in beef feedlot manure. Freshly excreted manure and feedlot surface material were analyzed for generic and tetracycline- and azithromycin-resistant Escherichia coli (E. coli), and for generic and tetracycline- and tylosin-resistant Enterococci spp. Results indicate that the inclusion of EO in cattle diets does not significantly impact AR characteristics of manure. Moreover, in the cases where a significant difference was observed due to the effect of forage, increasing forage concentration in finishing diets resulted in increased concentrations of resistant bacteria or had no significant effect when compared to a standard finishing diet (14% forage). The results of this study indicate that standard and low forage finishing diets produce manure with concentrations of ARB that are equal to or lower than those for high forage diets. Future research will be focused on identifying alternative management practices to mitigate the risk of AR in beef finishing manure.