Feeding antimicrobials to increase cattle growth may not be as effective as previously thought.
Their use in commercial cattle feeding has come under scrutiny as concerns widen about development of antibiotic-resistance bacteria.
Antimicrobials are commonly given to feedlot animals as a safeguard against illness. However, a growth promotion effect is thought to be an additional benefit that may have increased their use. Some products make a growth promotional claim on their labels.
That claim will be phased out by next year as part of the federal government’s efforts to potentially re-duce use of antimicrobials in animals and slow the development of antibiotic resistant bacteria that could affect human health.
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However, there have been questions about whether antimicrobials really do enhance growth or whether they simply keep animals healthy so they eat well and gain well.
A study by Alberta Agriculture beef research scientist Dr. Kim Stanford shed some light on the subject, at least as it pertains to small groups of cattle.
She found no consistent benefit in growth between cattle given antimicrobials and those that were not.
“It just shows that it is possible to get similar performance among all the groups, with or without sub-therapeutic antimicrobials, but it really depends on the sources of your cattle and how they’re managed.”
Stanford’s two-year study obtained weaned steers from a single source under relatively low stress, put them into pens of about 10 animals each and never mixed them.
Steers in five pens were given 350 milligrams each of chlortetracycline and sulfamethazine per head per day.
“That’s a fair whack,” said Stanford.
A second group of steers were given 350 mg of chlortetracycline per head per day and no sulfamethazine.
A third group of steers were given 11 mg of chlortetracycline per kilogram of dry feed per day.
A fourth group received 11 mg of tylosin per kilogram of feed and a fifth group served as controls with no antimicrobials.
Several groups showed better growth in the early days of feeding, but no significant differences were seen over the entire feeding period.
“The study showed that it is possible to not need these antimicrobials, but it takes increased management, which would cost quite a bit more to do,” said Stanford.
Steers in the research study were healthy at the start, kept in small groups and never mixed, but that combination of circumstances is unlikely to occur in commercial feedlot conditions.
Feedlots frequently sort and mix cattle to achieve uniform weights, which creates stress on the animals and also allows greater opportunity to share illness.
Stanford also said the cattle were not given ionophores, which are almost always given in commercial cattle feeding situations.
“It’s quite possible that having an ionophore … by itself might have increased performance in these animals too. That can’t be ruled out.”
Dr. Reynold Bergen, science director for the Beef Cattle Research Council, said in an article about the study that it could have important implications.
“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,” Bergen wrote.
He also said their use in higher risk calves “may provide health benefits rather than directly promote growth and efficiency.”
Stanford’s study is over, although she said questions remain.
“It would be nice to know how big of pens you could get before you needed to add in sub-therapeutic antimicrobials,” she said.
The increased management that would be needed to replace antimicrobial use in commercial feedlots would likely be a big factor.
“If people say, ‘thou shalt not feed antimicrobials,’ the beef industry can do that, but people just have to be prepared to pay a lot more for their meat.”
