Performance of broilers fed a broader spectrum antibiotic (virginiamycin) or a narrower spectrum antibiotic (bacitracin methylene disalicylate) over 3 consecutive grow-out cycles

Abstract

SUMMARY Virginiamycin and bacitracin are widely used in commercial broiler feed regimens to improve performance, an effect believed to be due to inhibition of intestinal clostridial populations. We conducted a floorpen study of 3 consecutive grow-out cycles using 4 feed regimens containing bacitracin (treatment 1), virginiamycin (treatment 2), or combinations of both after cycle 1 (treatments 3 and 4). Based on our data, we suggest that broiler producers would be ill advised to base multiflock antimicrobial program decisions on single-flock research studies or field trials. Virginiamycin-fed birds experienced feed conversion and processing weight advantages over bacitracin-fed birds during cycle 1, but had a 4% higher mortality rate in cycle 3 (for treatments 2 and 4 compared with treatment 1). The higher mortality, resulting primarily from bacterial infection, may be incidental, but we hypothesize it could be due to the broad-spectrum suppression of beneficial microflora in the gut by virginiamycin, allowing opportunistic bacterial growth. Microbial suppression may help channel energy to bird growth rather than to microbial proliferation; however, over consecutive grow-out cycles, it may also create gut dysbiosis that makes birds vulnerable to opportunistic infection. Additional research is necessary to determine whether broad-spectrum suppression of lactic acid-producing gut flora is responsible both for increased weight gain and increased vulnerability to bacterial infection and mortality.