The fecal resistome of beef cattle from conventional grain-fed and grass-fed systems in the Western United States

Bacteria in the gastrointestinal tract of cattle may develop antimicrobial resistance (AMR) due to the use of antibiotics in live animals and can be excreted in feces, posing a risk of contamination. However, it remains unclear whether different beef production systems influence the levels of AMR in cattle feces. The objective of this study was to characterize and compare the fecal resistome of cattle raised in grass and grain-feeding systems in the Western United States. Fecal samples were collected from individual cattle at 14 months of age and two days before their respective harvest date. Groups included: (1) Conventional grain-fed (CON, n = 10), (2) Grass-fed for 20 months (20GF, n = 10), (3) Grass-fed and then grain-finished for 45 days (GR45, n = 10), (4) Grass-fed for 25 months (25GF, n = 10). According to metagenomic analysis, grass-feeding systems, particularly the one with extended grass-feeding, are associated with a less diverse resistome. The 25GF group had smaller (P < 0.05) Chao1 value than the other groups at the harvest time. Antimicrobial resistance genes (ARGs) richness and evenness were higher in CON and GR45 than in 20GF and 25GF (P < 0.05). Additionally, the resistome of GR45 and CON differed from 25GF (P = 0.018). In grass-feeding systems where antibiotics were not administered, animals' feces exhibited greater (P < 0.05) diversity in transferable biocide and metal resistant genes (BMRGs) compared with the grass-fed but grain-finished system. Greater ARG diversity in grain-finished feeding systems may enhance the spread of antimicrobial-resistant bacteria (ARB) during production, posing additional risks to food safety. Similarly, higher BMRG diversity observed in grass-fed systems may promote ARB spreading through co-selection mechanisms, which could also contribute to potential food safety concerns.