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A Better Understanding of Antimicrobial Resistance

Updated: Dec 4, 2023

By Mikaela Baker, (B. Ag Sci, M. Anim. Sci) TRAC Ruminant Productivity Consultant

Addressing antimicrobial resistance on dairy farms is an area that is critical for making informed management decisions, but it can also be a daunting path to take. Learning about antibiotic resistance is important to realizing the implications it has for animals’ responses to antibiotics, and the efficacy of antibiotics in humans. Having a farming business that focusses on reducing antibiotic use does not equate to withholding treatments from sick cows or allowing suffering. On most farms, cows are healthy with low disease incidence and have excellent preventative management practices already, however it is worthwhile taking a quick stock take and amending any areas that could be improved. Preventing resistance is a holistic approach to the way we interact with animals, the quality of the infrastructure on farms, feed quality and consistency to prevent animals requiring antibiotics.

What is AB resistance and stewardship?

Antibiotics are used extensively in human and veterinary medicine and animal production. Since the discovery of penicillin, resistance has been a concern. When microbes are exposed to doses of antibiotics not sufficient to completely kill the bacteria, or non-lethal doses that allow the microbes to adapt, resistance can occur. In some cases, an organism can become resistant to multiple antimicrobials in a single step through plasmid-mediated conjugation, the most common way for genes to spread through a bacterial population. Bacterial genetic mutations develop slowly over multiple steps. Antibiotic resistance hasn’t only occurred through overuse of drugs, natural mutation can occur and resistance has even been found in ancient Mummies. The consequences of increased resistance in both humans and animals can increase the number of infections seen, severity of infections and treatment failure. While resistance can occur naturally, reducing dependence on antimicrobials will go a long way in mitigating challenges on farm and in the home.

Antimicrobial stewardship is the concept where everyone considers their role in reducing resistance. It is currently estimated that 700,000 human deaths globally per year are due to Antimicrobial Resistance (AMR) with that number expected to overtake cancer to 10 million deaths. A key approach to reducing resistance is through “Prudent use” of antibiotics and evidence based therapeutic decisions. Prudent use involves selecting drugs to specifically treat problems, using the correct dose and duration by completing the full course of antibiotics. Therefore, blanket treatments and treating animals with insufficient, inappropriate, or excessive amounts is advised against. Prudent use is achieving an effective and best outcome, while reducing the chance of resistance. Prophylactic use of drugs has already started to be advised again in some countries, while vaccines are still strongly encouraged. Antimicrobial stewardship is about protecting microbials for the future.


There are many ways that we can adapt the use of drugs on farm to be more effective. The first step is preventative measures and addressing the core issue, rather than a ‘Band-Aid’ approach to treat the symptoms. A great place to start is the calf rearing shed, there are many challenges we face in young calves, whom do not have a well-adapted immune system. Many illnesses that affect calves are not responsive at all to antibiotics, and antibiotics can increase shedding of the microorganisms with some diseases. When antibiotics are given in these circumstances, they are treating secondary diseases, not the root cause. Scourban for example is excellent at saving calves and helping to manage scours, however it is a medium importance drug to human health, and therefore should be used with consideration and not relied upon. Through vaccination programs, calf shed hygiene and nutrition, calves will build their immune system and handle many challenges that will significantly reduce the reliance on antibiotics. These steps include feeding high quality tested colostrum that has been appropriately collected, stored and fed within 6 hours, feeding quality milk that does not contain antibiotic residues and treating with hydration therapy at first signs of disease. Antibiotics are still a key tool in keeping calves healthy. Treating calves quickly and appropriately will also reduce the chance of further infections, or other calves becoming ill.

Another common area is mastitis. Less than 2% infections are considered normal for mastitis. This equates to 4 cows per month in a 200-cow herd, 8 per month in a 400-cow herd and 10 per month in a 600-cow herd, but we consider those numbers very high. Treating mastitis quickly is important to cow recovery and reducing loss of milk, however the focus is preventative. Regular checks and quick amendments to cracked inflations, vacuum issues, water quality, teat spray quality, wearing gloves, animal stress, managing farm conditions, teat spraying accuracy and milking technique are critical to keeping mastitis low. In respect to prudent use of drugs, getting mastitis cultured every now and then can provide insight to using the correct drug to treat the infection present, sensitivity tests can also be performed to identify the appropriate drug to treat bacteria in your herd. Alternatively, culling is sometimes the most appropriate option. Decisions can also be made easier by using bench top meters to identify mastitis pathogens and devise a treatment plan.

Stephanie Bullen has given excellent presentations around Antimicrobial Stewardship (AMS) and one of the drugs she talks about is Excenel (Ceftiofur), which is part of a class of antimicrobials that is of high importance to human health as seen in table 1. In a survey she found that 39% of the farmers had used it. The drug is registered for respiratory disease in Australia, however 65% of those people had used it for foot rot. The foot rot bacteria, Fusobacterium necrophorum is highly sensitive to penicillin, which is both cheaper and of lower importance to human health. A good example of AMS is following label guidelines and considering the relevance of drugs and alternatives.

Ionophores are tools used in animal production and are fed to achieve a combination of; improved feed efficiency, aid in the control of coccidiosis, reduce the risk of bloat and acidosis and reduce liver abscesses. Ionophores alter the chemistry within the microorganism, causing them to enter a futile energy cycle, causing a shift in ruminal bacteria population for more efficient digestion and increase in energy status. Ionophores are not used in human health. Resistance to ionophores disappears once the ionophore is removed from the diet. Some farming systems in the sheep industry are choosing to go ionophore free to meet current and future market demands.

Table 1 Antibiotic Importance. Source: Stephanie Bullen, AARN presentation

What creates a healthy animal?

Consistency and balance. Healthy, happy and fully fed cows are more resistant to becoming sick. This is mostly due to an uncompromised immune system. Stress taxes the immune system, through increased inflammation from events such as the stresses of calving, handling, feed restriction, psychological stress, pain etc. The immune system requires glucose and calcium, which is partitioned away from milk production and when resources are low or exhausted, the animal is vulnerable to becoming ill. As an example, facing preventable health challenges such as lameness will also decrease the cow’s ability to respond to mastitis pathogens. Preventing cows from becoming ill and treating cows quickly will have flow on effects. For example, research by Megan Abeyta found that cows off feed for 6 hours per day had 22% loss in milk yield, while cows off food for 12 hours had a 39% loss in milk yield. The stress of feed restriction resulted in immune system activation, leaky gut (intestinal hyperpermeability) and therefore a high use of glucose and redistribution from milk yield. When the immune system is activated, it requires 1070g of glucose per 12 hours in a lactating dairy cow, which equates to 35 MJME lost. For a full immune activation, that works out to be the equivalent of 24-28L of milk in 24 hours. To prevent leaky gut and immune activation, having ad-lib access to food for as many hours as possible is number one. Consider the time off feed from milking, walking to and from the dairy, and how early they are cleaning up pasture or TMR before milking. Being observant for changes in behaviour, manure, milk production and transitioning the cows between feed and system changes will keep the cow happy and healthy.

In summary

Being aware of antibiotic use on farms improves decision making and training of employees. Getting the foundations correct will set the farm up to be in an excellent place to decrease antibiotic use, which will also save costs, especially considering the iceberg theory. The clinical cases of disease are just the tip of the iceberg of greater underlying health conditions and sub-clinical diseases in the herd, these are the ones that have the greatest cost to production. With existing emphasis on reducing disease on farms - adopting the extra precautions will fit easily into current practice. The Australian Dairy Industry Sustainability Framework’s first point is that the “dairy industry uses antibiotics responsibly, as little as possible, as much as necessary, to protect the health and welfare of our animals”.

For more information or to discuss this article further,

please get in touch with your local TRAC Expert In Ruminant Productivity

on 08 8733 1888 or email us at


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Tom Thorn

0427 243 319

Owen Rees

0429 437 823

Mikaela Baker

0457 243 319


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