goat eating grass

Key points:

  • Refugia refers to maintaining a population of non-drench resistant worms on your land
    and within your animals in order to slow the development of drench resistance.

  • Leave 5-10% of your mob undrenched each time.

  • Graze adults behind the youngsters.

  • Leave resilient adults undrenched unless specifically required.

  • Drench animals two weeks before moving onto rested pasture.

  • Perform FEC tests on individuals prior to drenching.


With the rise of the super bug bacteria, resistant to all antibiotics, so too are we concerned with the development of super worms. If you find the concept confusing, let’s break it down in simple terms, before addressing one of the key concepts behind protecting our parasite drenches and our animals- Refugia.

Gut worms live inside the animal (let’s say “cow,” for example). When the cow takes a mouthful of grass she also takes in baby worms that have been waving about on the blade of grass, in the hopes of being eaten. Those babies (called “larvae”) find themselves a home in a specific part of the gastrointestinal tract. Here they set up camp, find another and make babies. The eggs laid by the female are encased in a protective capsule that keeps them snug and warm while they are passed out in the dung and sit on the pasture. The capsule allows them to sleep, dormant for some time (often to survive the cold of winter or the dry of summer), until environmental conditions are just right. When the warm and the wet line up, the larvae hatch, wriggle around to find a droplet of water and slowly ride that droplet of water up a blade of grass, where they wait for the next unsuspecting victim to wander by and take a bite. Wash. Rinse. Repeat.

So what happens when we drench our cow with a drug to kill those gut worms? We must first understand that, despite our best intentions, we will never truly rid her of the entire worm burden. Just as some children naturally outrun others on the athletic field, or are gifted in the classroom, we will always find some parasite larvae have lucked out with genetic mutations that allow them to outsmart the drenches. We accept that there will always be a very small percentage of gut worms remaining on the inside, after a drench.

The concern, however, is that every time we drench an animal the strongest worms not only survive, but then breed amongst themselves. All the ‘average Jo’s are removed from the party and we are leaving only resistant worms to mingle amongst themselves and create super-strong babies, thereby getting stronger every time we drench. Once the number of worms killed by a drench drops below 95% we are officially in “drench resistance” territory. Once that species of worm is resistant to multiple classes of drench, this is a superworm. Indeed this is the case on almost 30% of sheep farms across New Zealand, and is exceptionally high risk on lifestyle blocks.

Some of the key concepts behind preventing drench resistance include not over drenching, drenching preventatively and strategically, don’t underdose, and use combination products that have a better chance of killing a wider range of worms. More information on how to apply these to your block and your species, can be found here.

Thanks to wider education outreach and a growing public awareness, these concepts are becoming better understood. But one such concept that still baffles people is the concept of refugia. Let’s delve in.

What is Refugia?

Refugia is the concept of creating a “refuge” for the average Jo Blog worm, on the property and within the animals. By keeping worms around that do NOT carry the super-worm mutations that allow them to outsmart our drenches, we prevent the super strong worms from simply breeding amongst themselves and getting only stronger. Not only do the Jo-Blog worms dilute the number of resistant worms on pasture, but their “normal” genes will often mask any resistant mutations, slowing down the rise of drench resistance.

To maintain a healthy refugia, we must allow a safe space where some of the humble worms can survive. Here are some recommendations on maintaining refugia on your block.

Leave 5-10% of the mob undrenched

Traditionally, a farmer would draft an entire mob and drench each and every one as they ran through the stocks. Now, we know better. By choosing 5-10% of the heaviest, healthiest looking animals to leave undrenched, we allow a population of humble, non-super worms to reside. You see, every grazing animal will carry worms; the extent to which an individual is affected by them really comes down more to the individual's immune system than the actual number of worms inside. Therefore if a lamb, for example, is strong and healthy, this indicates that he is resilient to the worms inside him and managing just fine. Therefore, he may be a good candidate to be left alone.

If, of course, all individuals are sickly with heavy worm burdens, then certainly they will require drenching. But so far as a preventative drenching schedule goes, leave those 5-10% of the healthiest individuals untouched. These undrenched members of the group do not need to stay the same every time; remember their eggs will be coming out the dung and being spread throughout the group. So simply draft out the healthiest on each occasion and go from there.

Adults follow the youngsters

As animals grow, they tend to develop a good resilience to most species of worms. While there are some exceptions, such as barbers pole worm affecting skinny, stressed or heavily burdened adults, and goats being particularly high risk throughout their entire lives, generally this rule works well for sheep and cattle and means adults do not require drenching. This makes our adults the perfect safe space for our humble worms to reside.

We recommend having your young growing animals (under 1.5 years old) graze the pasture first. Not only do they have higher nutritional requirements and make good use of the longer, lush grass, but keeping them on the grass above 3cm long keeps them away from all those pesky worm larvae hanging out in the bottom 3cm. Move them out when the grass length drops low (below 3cm for sheep and below 8cm for cattle), then bring the adults through.

The resilient adults will move about, hoovering up all those gut worm larvae and tidying the pasture. But having the strong immune systems that they do, the worms are unlikely to do much harm. Furthermore, because the adults are usually altogether undrenched, they will be passing out humble, non-super worms onto the pasture to mix and mingle with the strong worms.

Rest the pasture before bringing the youngsters back through. For more advice on specific pasture length recommendations for your particular species, see our article ‘How much grass do livestock need? 

Leave them be for two weeks following drench

Traditionally, a mob would be drafted and drenched before being plonked in the next paddock immediately from the stock yard, with the idea of cleaning the animals out before moving on. This is now recommended against. You see, drenches do not work instantly. In fact, they take 7-10 days for all worms to be killed and expelled onto the pasture. During that time, the surviving super worms are coming out in the dung at increasingly concentrated doses.

Drench the animals, but leave two weeks before moving onto the next pasture. This allows the super worms to remain on the old pasture, where they will breed with the humble worms.

Move the mob onto fresh, rested pasture two weeks later, in a “clean” state.

Faecal egg count prior to drenching

Remember, how an animal responds to a worm burden is highly individual. In fact, 90% of the worms on the pasture are likely coming from just 10% of the mob. Usually these animals look poorly, but occasionally you may find an individual looks okay, but is a super-shedder of eggs. That’s where Faecal Egg Counts come in. By running a Faecal Egg Count test on your animals prior to drenching, you can isolate exactly who needs it and who doesn’t, thereby leaving a refugia of humble worms inside those animals that are otherwise unaffected.

A word of warning here, that as sheep and cattle approach 1.5-2 years old, the immunity they develop against gut worms also prevents the worms inside from producing as many eggs. This means Faecal Egg Count tests, which literally count the number of worm eggs in the dung, are less reliable for older animals. As previously mentioned, most goats mount little immunity as they age, meaning Faecal Egg Counts can be used more reliably right throughout life.