Selecting The Right Antibiotic

With most of the older antibiotics still available and several very powerful long-acting antibiotics being released, the choice for the rancher or feedlot owner has never been greater. The difficulty is with so many factors hinging on the outcome, deciding which antibiotic to use or if one is necessary at all, can be very difficult.

One article cannot hope to clarify every possible combination, but in the end producers need to work with their veterinarian to decide on a strategy and list of product choices for at least the common diseases. Reading the label is always beneficial as diseases for which clearance has been granted are written on the label. This alone gives you a start as to what types of diseases and subsequently what organ systems the antibiotic targets.


Veterinarians consider several points before we decide what is the appropriate course of treatment. First, are antibiotics even necessary or will convalescence be all that is required? If there is no bacterial infection present or expected in the future, antibiotics may be unnecessary. On large mature cattle or feedlot animals, the withdrawal time needs to definitely be considered. If a condition becomes chronic slaughter may be an option. We don’t want to burden ourselves and the critter with a long slaughter withdrawal. If daily care is better, can we accomplish that or is the next best long-acting product the way to go. Other considerations will be the syringability (especially important in winter) dosage amount, safety, means of administration (subcutaneous, oral, intravenous) and cost of the product per treatment day. The cost per day is really the way to truly compare treatment prices. The longer-acting products will cost more because they last longer. The upside is less labour necessary and subsequently less stress on the cattle processing them. This may be nullified if other procedures or painkillers must be given on a daily basis regardless.


The main decisions we as veterinarians and you as farmers make, are what is the condition, what organ system is primarily involved and the causative likely bacteria. From these three main things the most appropriate first, second and third choice of treatment is made. These three choices might be made in different order on specific farms based on farmer preference, previous results, or current research results. Veterinarians will even have different “favourites.” There is almost never a specific single choice.

A few antibiotics are what we call “broad spectrum,” meaning they work against a wide array of bacteria in different organ systems. The older sulphonimides as well as newer drugs like Nuflor or Excenel are fairly broad in their effect. The long acting form of Excenel is Excede and again is broad spectrum.

Other drugs are very specific for say treatment of pneumonia. The macrolide antibiotics are a class of drugs which specifically get into the lungs. Drugs such as micotil or draxxin fit into this category and, as an example, are excellent for pneumonia, but only a few other things. Veterinarians, on your behalf, may prescribe them for specific things such as seminal vesiculitis in young bulls. There will never be a label claim against these oddball infections. That is where the veterinarians experience will become invaluable.

There are two big classes of bacteria gram positive and gram negative. Clostridial infections such as blackleg or foot rot are caused by gram-positive organisms. We were always told at veterinary school “P” for positive and “P” for penicillin. This older antibiotic is still quite effective against conditions like foot rot. Diseases such as blackleg produce toxins and the animal succumbs quickly, so prevention in the form of vaccination is the only effective way to prevent this disease.


In order to be effective, we must pick the right drug, administer it in time and at the right dosage. Weight must be estimated correctly. These antibiotics have been formulated to be effective at the appropriate dosage. Twice as much as necessary will not be more effective and will only cost you more and result in an increased drug withdrawal period. The safe rule is if you double the dosage you double the slaughter withdrawal. Always keep that in mind.

I would be remiss if I didn’t mention supplemental drugs to act as pain killers, anti-inflammatorys, appetite stimulants, and other treatments. In specific disease entities they are often given in conjunction with antibiotics to quicken or improve the response. Again your veterinarian can advise what works best for them.

The selection of the appropriate medication for the specific disease takes some thought. The biggest step-saver is recording what products you use (record either the active ingredient or trade name) and list the diseases it is good against as well as the dosage. Have a first and second choice. This will go along ways to making sure the appropriate product is given especially by new workers. Have the slaughter withdrawal period listed as well so drug residues don’t become an issue. Write it down, as it is all too much information to keep in ones head. Have epinephrine handy in case you get a drug reaction and have the appropriate syringes and needles on hand to complete your treatment kit.

Roy Lewis is a practicing large animal veterinarian at the Westlock Veterinary Center, north of Edmonton, AB. His main interests are bovine reproduction and herd health.

About the author


Roy Lewis is an Alberta-based veterinarian specializing in large-animal practice. He is also a part-time technical services vet for Merck Animal Health.



Stories from our other publications