What are the first-choice antimicrobials for bovine respiratory disease in feedlot cattle?

The first-choice antibiotics for BRD are macrolides (tulathromycin at 2.5 mg/kg as a single dose, tilmicosin phosphate at 10 mg/kg as a single dose) or florfenicol (20 mg/kg intramuscularly repeated at 48 hours, or a single 40 mg/kg subcutaneous dose). These selections are based on field efficacy, duration of therapeutic concentration, route of administration, and ASTAG rating.

Why do most BRD treatment failures occur in feedlot cattle?

Most BRD treatment failures happen because the disease is recognized too late, not because the bacteria are resistant to antimicrobials. Australian isolates of BRD pathogens still show low prevalence of resistance to first-choice agents. Re-treatment should not occur until the therapeutic window of the first drug has fully expired.

What is the ASTAG rating system and why does it matter for feedlot cattle?

The ASTAG (Australian Strategic and Technical Advisory Group on Antimicrobial Resistance) rating system classifies antimicrobial agents by their importance to human and animal health as low, medium, or high. Agents rated as high importance, such as third-generation cephalosporins and streptogramins, require careful justification before use in feedlot cattle to preserve their effectiveness in treating refractory infections.

How is ruminal acidosis in feedlot cattle treated and prevented?

Individual cases are treated with oral drenching of sodium bicarbonate and magnesium oxide combined with parenteral penicillin to prevent liver abscessation. Prevention focuses on gradual grain adaptation, disciplined feed delivery, adequate effective roughage (5-10 cm particle length), appropriate grain processing, and inclusion of ionophores like monensin in feedlot diets.

What is the difference between domestic withholding period and export slaughter interval?

The export slaughter interval (ESI) is frequently longer than the domestic withholding period because export destinations apply lower maximum residue limits (MRLs). Veterinarians must calculate ESI for any off-label drug use in feedlot cattle destined for export, using the lower MRLs of the target export market to determine safe slaughter timing.

Which antimicrobials are recommended for footrot in feedlot cattle?

Systemic antimicrobials such as procaine penicillin (12 mg/kg every 24 hours for 3-5 days) or oxytetracycline (10-20 mg/kg depending on formulation) are the main treatments, supplemented by topical therapy using foot baths (e.g., 5% copper sulphate solution) when outbreaks affect multiple animals in a pen.

What are the 22 principles of appropriate antimicrobial use in feedlot cattle?

The 22 principles cover pre-treatment activities (disease prevention, professional intervention, alternatives to antimicrobials), diagnosis, therapeutic objective and planning, drug selection (justification, guidelines, ASTAG ratings, culture testing, spectrum preference, off-label considerations), drug use (dosage, duration, labeling, target animals, record keeping, compliance, monitoring), and post-treatment activities (environmental contamination, resistance surveillance, continuous evaluation and improvement).

What Every Feedlot Veterinarian Needs to Know About Antimicrobial Stewardship

Walk into any cattle feedlot and ask the manager what keeps them up at night. Chances are, it comes down to one thing: keeping animals healthy without losing the tools that make it possible. Antimicrobial resistance is already changing how veterinarians prescribe, how feedlots run their hospital pens, and how the industry keeps its social license.

For people working in intensive beef production, knowing when and how to use antimicrobials matters just as much as knowing which drug to grab. Here is a practical breakdown of what antimicrobial stewardship looks like for the diseases that turn up most often in Australian feedlots.

The Framework Behind Every Script

Responsible antimicrobial use comes down to 22 principles, covering everything from pre-treatment planning to post-treatment review. They are not bureaucratic checkboxes. They are what keeps the drugs working.

Before reaching for a bottle, the principles push you toward disease prevention first. Biosecurity, vaccination, nutrition, housing. Antimicrobials should never be a bandage for sloppy management. Every prescription needs a legitimate veterinarian-client-patient relationship, a real diagnosis, a clear goal, and a hard look at alternatives.

Drug selection has to account for the ASTAG rating system, which ranks antimicrobials by how important they are to human and animal health. Agents rated high importance, like third-generation cephalosporins (ceftiofur) and streptogramins (virginiamycin), need proper justification before you reach for them.

One principle deserves special attention: off-label use and the calculation of withholding periods and export slaughter intervals. When a vet prescribes outside the registered label, they carry the full responsibility of making sure residues fall below maximum limits in every edible tissue at slaughter. In feedlot cattle, where most animals are headed for both domestic and export markets, those limits are often different, and the math gets complicated fast.

Bovine Respiratory Disease: What Hits Feedlots Hardest

Bovine respiratory disease (BRD) is the biggest health problem in Australian feedlots. The peak hits 7 to 50 days after arrival. By that point, the animals have been through transport, comingling, handling, and adaptation to grain diets. Stress stacks up, and the pathogens move in.

The Bacteria You Are Dealing With

The main bacterial agents are normal residents of the upper airways that end up in the wrong place. Mannheimia haemolytica, Pasteurella multocida, and Histophilus somni are the usual suspects. Mycoplasma bovis is different. It does not normally live in the upper airways, and most Australian feedlot cattle have never been exposed to it before induction. Infection happens early in the feeding period. In chronic cases, Trueperella pyogenes and Fusobacterium necrophorum show up in lung abscesses.

Finding It Before It Gets Worse

Most BRD cases are caught by trained livestock staff looking for coughing, nasal discharge, labored breathing, and the vague signs of septicemia. Visual assessment alone has limited accuracy. Some studies put sensitivity and specificity below 70 percent. That is why ongoing training matters.

Crush-side tools help confirm the diagnosis. Chest auscultation, rectal temperature, and thoracic ultrasound all have their place. Ultrasound gives reliable results when an experienced operator spends at least five minutes on each side of the chest with the animal properly restrained.

First-Line Drugs for BRD

Based on field efficacy, duration of action, route of administration, and ASTAG rating, the first-choice antibiotics are macrolides or florfenicol:

Tulathromycin. A single subcutaneous shot at 2.5 mg/kg. Seven days of therapeutic concentration.
Tilmicosin phosphate. Single subcutaneous shot at 10 mg/kg.
Florfenicol. Either 20 mg/kg intramuscularly with a repeat 48 hours later, or a single 40 mg/kg subcutaneous dose.

Most BRD cases that do not respond fail because the disease was caught too late, not because the bacteria are resistant. Australian isolates still show low resistance to first-choice agents. That means re-treatment with something else should wait until the first drug has fully run its course.

When You Need a Second Round

If clinical signs have not improved after the first drug is done, oxytetracycline is the standard re-treatment. When the projected sale date falls inside the withholding period or export slaughter interval of the first-choice drugs, shorter-interval options like ceftiofur make more sense.

Other Conditions That Turn Up in Feedlot Pens

Tracheal Oedema and Necrotic Laryngitis

Both cause noisy breathing and audible stridor, but the pathology is different. Tracheal oedema, sometimes called “honker,” involves swelling of the tracheal mucosa and shows up later in the feeding period, past the main BRD window. Necrotic laryngitis is driven by Fusobacterium necrophorum causing ulceration and necrosis of the laryngeal lining.

For tracheal oedema, treatment pairs oxytetracycline with dexamethasone to cover both infection and inflammation. For necrotic laryngitis, the choice depends on severity. Mild cases do well on procaine penicillin with anti-inflammatory support. Moderate cases respond to oxytetracycline or ceftiofur.

Ruminal Acidosis

This is a management failure, not something antimicrobials alone fix. It happens when cattle eat too much rapidly fermentable carbohydrate and overwhelm the rumen’s buffering capacity. The cascade moves from sub-acute acidosis (rumen pH below 5.6 for more than three hours a day) to full clinical acidosis with dehydration, toxaemia, and sometimes death.

Treating individual cases means oral drenching with sodium bicarbonate and magnesium oxide, plus parenteral penicillin to head off liver abscessation. But the real answer is prevention. Gradual adaptation to grain diets. Disciplined feed delivery. Effective roughage with a particle length of 5 to 10 cm. Proper grain processing.

Ionophores like monensin are standard in feedlot diets. They lower the risk of acidosis while improving growth rate and feed efficiency. Virginiamycin is a different story. It belongs to the streptogramin class, rated highly important for human health. It should only be used when everything else has failed and the situation is extraordinary.

Lameness: Footrot and Septic Arthritis

Interdigital necrobacillosis, better known as footrot, is a major cause of lameness. Fusobacterium necrophorum is the primary culprit. Trauma to the interdigital skin during transport and handling, plus wet muddy pens, sets it up.

Treatment combines systemic antimicrobials (penicillin or oxytetracycline) with topical therapy through foot baths when outbreaks hit multiple animals. Catch it early and the outlook is good.

Septic arthritis is a different animal. It usually follows haematogenous spread from respiratory infections or a penetrating wound into a joint. The prognosis is poor. In most feedlot settings, euthanasia is the better call than treatment. If you start seeing more cases than usual, it is worth reviewing the BRD program.

Pink Eye and Ear Infections

Infectious bovine keratoconjunctivitis, or pink eye, is common in freshly arrived cattle. Moraxella bovis is the main agent, and corneal trauma from dust, UV light, or plant awns opens the door. Treatment runs from topical cloxacillin or cephalonium ointments to subconjunctival penicillin with dexamethasone. Systemic oxytetracycline is reserved for severe cases or when both eyes are involved.

Ear infections, otitis media, often follow BRD as a secondary complication. The infection climbs up the eustachian tube. The same drugs used for BRD work here, and most cases recover without drama.

The Withholding Period Complication

Every treatment decision in a feedlot has to balance clinical effect against residue risk. The export slaughter interval is usually longer than the domestic withholding period because export markets set lower maximum residue limits. Vets have to think about when the animal is likely to be slaughtered before picking a drug, especially toward the end of the feeding period.

Short-acting drugs like ceftiofur offer narrower withdrawal windows and become the practical choice when sale is close. Long-acting formulations like tulathromycin give better clinical coverage for most BRD cases but need more time before slaughter.

The Bottom Line

Antimicrobial stewardship in feedlot cattle is not about saying no to treatment. It is about treating with intention. The right drug, at the right dose, for the right duration, and only when it is actually needed. The 22 principles provide the structure, but the real work happens in the pens. Training staff to spot disease early. Building rations that keep rumens healthy. Keeping records that let you track what works. Having the discipline to let a good drug finish its job before reaching for another one.

The tools are still effective. Whether they stay that way depends entirely on how they are used.