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Do Antimicrobial Growth Promoters Really Improve Performance, or Just Improve Health?

This article written by Dr. Reynold Bergen, BCRC Science Director, originally appeared in the May 2015 issue of Canadian Cattlemen magazine and is reprinted on the BCRC Blog with permission of the publisher.


Ionophores (Rumensin, Bovatec, Posistac) are not medically important because the ionophores approved for use in cattle are not used in human medicine. Other antimicrobials used in livestock are medically important. Concerns around antimicrobial resistance in both human and veterinary medicine have led to increased scrutiny regarding how medically important antimicrobials are used in livestock production. In response, pharmaceutical companies throughout North America are removing production claims (i.e. growth and feed efficiency) from products containing medically important antimicrobials. Some of these products also have health claims, but four Canadian products only have production claims (two Aureo S-700G products, Chlor S-700, and Neo-Terramycin). These products may disappear unless the companies pursue new health-related label claims.

But do medically important antimicrobials really promote growth? Or do they just keep calves healthy, which grow better than sick calves? Alberta Agriculture and Rural Development’s Dr. Kim Stanford led a research project at AAFC’s Lethbridge Research Centre to help answer this question. This research has been submitted for publication in the Canadian Journal of Animal Science.

What they did

240 freshly-weaned steer calves were obtained from the same ranch in each of two years. Steers averaged 553 (year 1) and 602lbs (year 2), and were not treated with antimicrobials before arriving at the research feedlot. Steers were vaccinated and treated for parasites on arrival. No preventative antimicrobials were given, and no ionophores were fed. Calves were individually treated for illness when needed.

Steers were divided into five experimental groups and fed in small pens. Natural steers (five pens) were not fed sub-therapeutic antimicrobials.CT+S700 steers (five pens) were fed chlortetracycline and sulfamethazine (350 mg of each antimicrobial per head per day). This corresponds to Aureo S-700G and Chlor S-700 label doses to maintain weight gains and feed efficiency in calves stressed by weaning, shipping and handling. CT11 steers (five pens) were fed 11 mg chlortetracycline per kg of diet dry matter every day. This corresponds to Aureomycin 110 G and 220G label claims to stimulate growth rate and improve feed efficiency in calves up to 300 lbs. CT350 steers (four pens) were fed a higher level of the same product as the CT11 steers. This off-label dose provided the same amount of chlortetracycline as the CT+S700 steers (350 mg per head per day). A fifth group of steers (five pens) were fed 11 mg tylosin per kg of diet dry matter. Tylan is approved to reduce liver abscesses but does not have a label claim for improved growth rate or feed efficiency. Tylan results won’t be discussed here.

Calves were backgrounded on 58% barley silage for 84 (year 1) or 74 days (year 2), then stepped up over 21 days to a 91% barley-based concentrate diet and finished for 123 (year 1) or 92 days (year 2).

What they learned

No differences between the Natural, CT+S700, CT11 and CT350 steers were seen for respiratory disease, foot rot, pinkeye or bloat treatment rates.

Growth rate: During backgrounding, the CT+S700 and CT350 (but not CT11) steers grew faster than the Natural steers in both years. CT11 steers grew faster than Natural steers in year 2 but not year 1. In both years, all four groups grew at the same rate during the finishing period, as well in the combined backgrounding + finishing period.

Feed efficiency: CT+S700 steers were most efficient during the backgrounding period in year 1. Natural and CT350 steers were intermediate, and CT11 steers were the least efficient. In Year 2, CT+S700 and CT350 steers were more efficient than CT11 or Natural steers during the backgrounding period. No statistically significant differences were found between the four groups during the finishing period or the combined backgrounding + finishing period in either year.

What it means

Using low-risk steers from a single source and fed in small pens minimized the risk of respiratory disease and other illnesses. This means that differences between the four groups were most likely due to the antimicrobials’ effects on growth, not animal health.

Steers fed the higher levels of sulfamethazine and/or tetracycline grew faster than steers fed the low level or no tetracycline during the backgrounding period, but these benefits did not translate to improved growth rate over the entire feeding period. Differences in feed efficiency were inconsistent.

Medically important antimicrobials fed at label doses did not consistently promote growth in low-risk feeder steers during backgrounding, and had no effects when the overall feeding period was considered. In higher risk calves, medically important antimicrobials may provide health benefits rather than directly promote growth and efficiency.

Removing production-related label claims for medically important antimicrobials will probably not affect the competitiveness of Canada’s beef industry, but it will help demonstrate the beef industry’s ongoing commitment to responsible antimicrobial use.

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