Managing worms & drench resistance on small farms

⚡ The short answer

More drench is not better worm management.

Drenches are valuable tools for controlling internal parasites, but using them more often is not the same thing as managing worms better.

Every drench treatment creates selection pressure. Susceptible worms are killed, while worms that survive treatment have a greater opportunity to reproduce. Over time, those survivors can become a larger proportion of the worm population until a product that once worked well becomes partly or seriously ineffective.

That is drench resistance.

The aim is not to avoid drenching animals that genuinely need treatment. It is to build a system where:

• animals are well fed;
• worm challenge is monitored;
• drenches are used when they are useful;
• the product being used is actually effective;
• animals are dosed correctly;
• and treatment records help inform the next decision.

Beef + Lamb New Zealand's Wormwise programme treats drenching as one part of a much broader parasite-management system, alongside nutrition, grazing management, monitoring and refugia.

🧬 The mechanism

What is drench resistance?

A drench — also called an anthelmintic or dewormer — is a medicine used to control internal parasites. Drench resistance occurs when worms survive treatment at a rate that means the product no longer performs as expected.

The process is straightforward evolution. Imagine a worm population where most worms are susceptible to a particular active ingredient, but a small number have genetics that let them survive it.

You drench the animals. Most susceptible worms die. Some resistant worms survive. Those worms reproduce.

Repeat that selection often enough and resistant genetics can gradually become more common. Eventually the farm can reach the point where a drench is no longer controlling worms adequately. Wormwise describes resistance as a major reason farmers need to test efficacy rather than assume a familiar product still works.

🔁 The trap

Why simply drenching more often can make the problem worse.

It is understandable to think: more worms = more drench.

But repeated treatment can create two problems. First, it places stronger selection pressure on the worm population. Second, it can hide the underlying reason animals are continually facing high worm challenge.

That reason might include:

• susceptible young stock;
• poor nutrition;
• heavily contaminated pasture;
• repeatedly grazing the same livestock class over the same ground;
• animals being forced to graze pasture too low;
• an ineffective drench;
• or a treatment programme based mainly on the calendar rather than monitoring.

Wormwise specifically identifies frequent preventive treatment, and systems that rely heavily on drench, as risk factors for resistance.

A useful question is therefore: why do these animals keep needing treatment?

Sometimes the answer is that treatment genuinely is required. Other times, the better long-term answer lies partly outside the drench bottle.

🔍 Efficacy

Don't guess whether your drench still works.

One of the dangerous things about drench resistance is that the bottle looks exactly the same.

You administer the correct-looking product. The animal is treated. Nothing obvious tells you whether every important worm species was actually controlled.

That is why treatment records alone are not enough. Wormwise recommends using diagnostic tools such as:

• faecal egg counts;
• drench checks;
• larval cultures where appropriate;
• and more formal faecal egg count reduction testing.

These can help determine both worm burden and whether treatment has been effective. A veterinarian or animal-health adviser can help decide which test is appropriate and how to interpret it.

🧪 Monitoring

What is a faecal egg count?

A faecal egg count, or FEC, measures the number of worm eggs present in a dung sample.

It does not tell you everything. Different worm species produce different numbers of eggs, and some important parasite problems are not perfectly represented by a simple count.

But FECs can still provide useful information about whether animals are carrying a significant worm burden and whether treatment might be required. They can also be used as part of a drench check.

Instead of saying:

"It's been four weeks, so they must need another drench,"

you can ask:

"What is actually happening?"

That shift from calendar treatment to evidence-based treatment is one of the most important changes a small farm can make.

✅ Checking

What is a drench check?

A drench check is designed to answer another important question: did the treatment actually work?

The basic idea is to compare parasite evidence before and after treatment at an appropriate interval. If substantial egg production continues after a product should have controlled the worms, that can indicate that the drench is not adequately effective.

The exact timing and interpretation depend on:

• livestock species;
• drench family;
• parasite species;
• product;
• and testing method.

For that reason, this guide does not prescribe one universal testing timetable. The important habit is simpler: don't assume efficacy forever just because the drench worked in the past.

Wormwise recommends regular checks because continued use of an ineffective product can quickly increase the proportion of resistant worms in the farm system.

🏷️ Products

Drench families matter more than brand names.

Changing bottles does not necessarily mean changing the type of drench.

Different commercial products can contain active ingredients from the same drench family. That matters because resistance is associated with the active compounds and their modes of action, not the colour of the label.

When recording treatments, it is useful to know:

• product name;
• active ingredients;
• drench family where known.

Wormwise maintains current information on the different anthelmintic classes used in New Zealand and their resistance considerations.

For product selection on a specific farm, use current local veterinary or animal-health advice rather than relying on a generic internet recommendation.

⚖️ Dosing

Under-dosing is not a safer compromise.

Giving less drench than required does not reduce resistance risk. It can do the opposite.

A sub-lethal dose may kill the most susceptible worms while allowing less-susceptible worms to survive.

Wormwise therefore recommends weighing animals appropriately, calibrating drench equipment, and avoiding accidental or deliberate under-dosing.

The registered label remains the authority for:

• dose;
• route;
• species;
• withholding periods;
• and other product-specific requirements.

🔧 Before the drench gun

Good dosing starts before you pick it up.

Useful practical checks include:

• know the approximate weight of the animals;
• calibrate the drench gun;
• check the equipment is actually delivering the intended volume;
• read the label;
• check expiry and storage;
• record the treatment at the time it is given.

If you are treating a group, the system needs to ensure the largest animals are not accidentally being under-dosed.

Specific dosing advice should come from the label and professional guidance rather than a general farm guide.

🚚 Incoming stock

Bought-in livestock can bring resistant worms with them.

Resistance does not have to evolve on your property. It can arrive in a stock truck.

Bought-in lambs and calves can be particularly important because they may have already received several treatments before arriving. Wormwise recommends thinking carefully about quarantine parasite management for incoming stock so that highly resistant worms are not introduced into the existing farm population.

The exact quarantine programme is farm- and species-specific. This is not an area where a generic online drench recipe is appropriate.

A useful general principle is: treat unknown parasite status as something worth investigating, not something to ignore.

🐑 Individuals

Don't treat all animals as identical.

Animals differ in their susceptibility to parasites. Young animals are generally more vulnerable. Older animals may have developed stronger immunity. Even within one mob, some individuals can carry heavier burdens or perform more poorly than others.

That is why targeted selective treatment can form part of some modern worm-management systems. The idea is to treat the animals that require treatment while leaving suitable animals untreated, helping maintain a population of drench-susceptible worms known as refugia.

🌾 Nutrition

Nutrition matters more than it first appears.

A worm problem can sometimes look like a drench problem when nutrition is also limiting performance.

Young animals that are poorly fed may:

• grow slowly;
• take longer to develop immunity;
• cope less effectively with parasite challenge.

This is why Wormwise puts feeding and farm system management alongside drenching and diagnostics. The objective is not merely to kill worms. It is to raise healthy animals that are less dependent on repeated chemical intervention.

Feed supply on a small block is largely a grazing question — rotational grazing for small farms covers how much pasture to leave behind and how to tell when an area has actually recovered.

📝 Records

What should you record?

For each treatment, useful records include:

• animal or mob;
• date;
• reason for treatment;
• product;
• active ingredient;
• drench family where known;
• animal weight or treatment weight;
• dose administered;
• withholding period;
• follow-up test result;
• treatment outcome.

Over time, those records can answer questions such as:

• How often are these animals actually being treated?
• Are the intervals getting shorter?
• Which animals repeatedly need treatment?
• Has this drench ever been checked for efficacy?
• Did poor performance improve after treatment?
• What product did we use last time?

Those are much better questions than: "Didn't we drench them sometime last month?" We made the same argument in Bulletin #1: Why good records matter more than another drench.

🔢 A worked example

A simple small-farm example.

Imagine a group of lambs that has been drenched every month since weaning. At first everything appears fine. Then growth begins to slow before each scheduled treatment.

The instinct might be to shorten the interval. But a better investigation could include:

• checking feed availability;
• weighing the lambs;
• running a FEC;
• checking whether the current drench still works;
• looking at where the lambs have been grazing;
• considering whether the same young stock are repeatedly contaminating their own grazing block.

If the drench is only partly effective, giving it more often can make the resistance problem worse. If nutrition is poor, another treatment may not solve the performance problem.

The record of previous treatments is the starting point, not the final answer.

🚫 Mistakes

Common worm-management mistakes.

Drenching because the calendar says so. A fixed date does not prove the animals need treatment.

Never checking whether the product still works. An ineffective product can continue selecting for resistant worms.

Changing brands without checking the active ingredients. Different brands can belong to the same drench family.

Under-dosing. Sub-lethal treatment can favour less-susceptible worms.

Treating every animal automatically. Not every animal has the same parasite burden or resilience.

Ignoring nutrition. Poor growth is not automatically proof of worms.

Forgetting incoming livestock. Bought-in stock can introduce resistant worms.

Keeping incomplete records. If you do not know what was used previously, future treatment decisions become guesswork.

🛠️ In the app

How My Block Hand can help.

My Block Hand can keep together:

• livestock treatment history;
• product and active-ingredient records;
• withholding periods;
• animal weights;
• health observations;
• reminders;
• grazing history;
• notes from FECs or drench checks.

The app does not decide whether an animal needs a drench. It helps answer: what happened last time? That is valuable when discussing the next treatment with a veterinarian or animal-health adviser.

👣 Next steps

Practical next steps.

• Review how often your young stock are being drenched.
• Check that current treatments are being dosed correctly.
• Record product and active ingredients, not just "drenched".
• Consider FEC monitoring rather than relying entirely on fixed intervals.
• Check drench efficacy periodically.
• Review nutrition and grazing pressure.
• Have a quarantine plan for bought-in livestock — see our guide to buying and quarantining new stock.
• Discuss suspected resistance with your veterinarian or parasite adviser.

✅ In short

The aim is not fewer drenches at any cost.

An animal that genuinely needs treatment should be treated.

Sustainable parasite management is not about refusing to use drench. It is about making sure the right animals are treated, with an effective product, correctly, for a reason.

And then checking what happened.

The longer effective drenches remain effective, the more useful they remain when animals genuinely need them.

📚 Further reading

Where this page's guidance comes from.

Beef + Lamb New Zealand's Wormwise programme is the primary authority used throughout this guide:

• Wormwise programme overview — the whole system: nutrition, grazing, monitoring, refugia and drench.
• Drench resistance — who cares? — risk factors, and how to assess your own situation.
• Drench families — the anthelmintic classes available in New Zealand, what they target, and what is known about resistance to each.
• Quarantine drenching — managing the parasite status of incoming stock.
• Refugia — leaving some worms unexposed to drench, and the ways to achieve it.
• Worm diagnostics — faecal egg counts, drench checks, larval culture and reduction testing.
• Research papers and additional tools — where the Wormwise handbook and the supporting research sit.