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Performance of Broilers Fed a Broader-spectrum Antibiotic or a Narrower-spectrum antibiotic over Three Consecutive Grow-out Cycles

07 October 2013


New research from the US shows that broilers respond differently in terms of performance to broad- and narrow-spectrum antibiotics and that the response varies over successive flocks, possibly related to the evolution of different gut microflora rather than the development of any antibiotic resistance. Furthermore, bacteria involved in leg infections were thought to have originated in the gut.

Virginiamycin and bacitracin are antibiotics widely used in commercial broiler feed regimens to improve performance, an effect believed to be due to inhibition of intestinal clostridial populations, explained Mark LaVorgna of Zoetis Animal Health Global Poultry.

His paper - published in Journal of Applied Poultry Research with co-authors at Zoetis, Microbial Research Inc. and Colorado Quality Research - describes a floor-pen study conducted for three consecutive grow-out cycles using four feed regimens containing bacitracin (treatment 1), virginiamycin (treatment 2) or combinations of both after cycle 1 (treatments 3 and 4).

Virginiamycin is a broader-spectrum antibiotic, while bacitracin methylene disalicylate (bacitracin) is an example of a narrower-spectrum antibiotic.

Based on the data, the researchers suggest that broiler producers would be ill-advised to base multiflock antimicrobial programme 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 four per cent 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 the authors hypothesise 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.


The statistically significant end-of-flock advantages in feed conversion ratio and processing yield for virginiamycin-treated birds in cycle 1 were not maintained through cycle 3 (for treatments 2 and 4 versus treatment 1 in cycle 3), LaVorgna and colleagues reported.

During cycle 3, birds receiving virginiamycin (treatments 2 and 4) experienced higher mortality after seven days of age than those receiving bacitracin by more than four per cent. This late mortality, primarily due to Staphylococcus aureus and E. coli infections, as demonstrated by necropsy and microbiological culture, was unexpected but significant. Had replacement of dead birds not occurred, the feed conversion ratio for bacitracin could have been superior to that of virginiamycin in cycle 3.

The researchers commented that the increased Clostridium perfringens levels in the litter of the virginiamycin-treated birds indicate that significant changes in the microfloral populations occurred over time in the gastrointestinal tract of the virginiamycin-fed birds versus those fed bacitracin. Clostridium perfringens is the pathogen often associated with necrotic enteritis.

They also highlight that live production parameters responded differently to virginiamycin and bacitracin over the course of the three grow-out cycles.

The observed reduction in efficacy over time of virginiamycin was not due to antimicrobial resistance but rather to a shift in the microfloral populations, they proposed.

Based on the study data, the bacteria involved in leg infections may originate in the gastrointestinal tract, as neither antibiotic employed in the study can transport from the gastrointestinal tract, thus making any systemic action impossible, according to Lavorgna and co-authors. This reinforces the importance of maintaining good gut health.

They suggest that broiler producers would be ill advised to base multi-flock antimicrobial programme decisions on single-flock research studies or field trials.

LaVorgna and co-authors add that further research is necessary to determine whether broad-spectrum suppression of lactic acid-producing gut flora by virginiamycin is responsible for increased weight gain, as well as for increased vulnerability to bacterial infection and mortality.


LaVorgna M., J.L. Schaeffer, D. Bade, J. Dickson, K. Cookson and S.W. Davis. 2013. Performance of broilers fed a broader spectrum antibiotic (virginiamycin) or a narrower spectrum antibiotic (bacitracin methylene disalicylate) over 3 consecutive grow-out cycles. J. Appl. Poult. Res. 22(3):574-582. doi: 10.3382/japr.2012-00703

Further Reading

You can view the full report by clicking here.
Find out more about necrotic enteritis by clicking here.

 October 2013

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