Calves on good pasture

Worms, also known as internal parasites, are an issue that all farm animals have to deal with.

At times they can cause problems, especially in young, vulnerable animals. While there are effective treatments for worms, such as anthelmintic drenches, relying on these chemicals to control worms can bring headaches. Drench resistance is just like the overuse of antibiotics, in that overuse of drenches can mean the clever parasites adapt to the effects of drenches and produce drench-resistant offspring.

You are probably aware of the effects worms can have on livestock - scours (diarrhea), ill thrift, itchy bottoms, pale gums, anaemia and general malaise are a few symptoms of a high worm burden. It’s interesting to note, that if you see flat tapeworm segments in your lamb faeces, there is no need to be alarmed. It is the other worms that do the damage.

If we don’t want to rely totally on chemicals to control worm burdens, what can we do to help prevent them? There are several management tools we can use that will help decrease the number of worms and increase the health of our animals.

Understanding the worm life cycle

Internal parasites live in the stomach and intestines of animals. They lay eggs, which are passed out in faeces onto the paddock. The eggs hatch into larvae.  If these larvae are then eaten by an animal they grow into adults, reproduce within the animal and so begin the next generation.

The larvae in the grass need moisture to survive so they are most commonly found at the base of the grass and higher up grass when the grass is wet. Eggs and larvae can survive for weeks or in some cases months in pasture.


While the likes of Otago, Canterbury and Marlborough are usually dry and hot during summer which kills off a lot of worm larvae, areas that receive regular higher rainfall will be more susceptible to high worm populations as the continued growth and damp conditions favour larvae survival.

During winter, many parts of the country are too cold and the larvae don’t survive, but in more temperate areas, worms may be a problem throughout winter.

What animals are mainly affected?

The biggest impact worms have is on young animals. This is because they haven’t developed an immunity to the effects of worms. Adult animals appear to create an environment in their gut that is not hospitable for worm reproduction. The exception is goats and alpacas who remain highly susceptible to worms no matter what age.

Using pasture to control worms


Paddock plants may be one of the most effective ways of helping to control worms. Plants with condensed tannin in them make the rumen of the animal inhospitable to worms. Dock is a weed that most people curse when they see it in their paddocks, but don’t go grubbing out or spraying Dock in a hurry as it is an effective natural wormer and most animals will eat it. Trefoil, burdock, plantain and chicory are also good sources of tannin. Trees such as willow, poplar and tagasaste (tree lucerne) also have condensed tannin and are good for stock, not only for worm control but for their general health.

Pasture length

Pasture length is an important part of controlling worm burdens. Worm larvae live in the bottom three centimetres of pasture and in times of mild temperatures and green grass, those worm eggs will be hatching and larvae will be rife. It's important to keep the grass for sheep above 3 cm and it should be longer for cattle. Of course, this is not always possible in winter, when grass growth slows down. In an ideal situation, there should be saved paddocks that stock can be shifted to. Rotational grazing certainly helps.

Cross grazing

Cross-grazing sheep, goats and alpacas with cattle is a very effective way of controlling worm burdens. The cattle mop up the larvae and eggs the sheep leave behind without being affected and the sheep do the same to cattle worms. Goats and sheep share the same worms. Horses are also an effective dead end for most cattle and sheep worms.

Other control methods


By choosing to breed only from animals that have a proven low worm burden or faecal egg count during times of pressure, we can breed animals that will be more resistant to worms when they are young. The genes that determine the animal’s ability to resist worms will be passed on and eventually, you will have a group of animals that are more worm resistant.

Parasitic fungi

Parasitic fungi can be added to animal feed. When the animal ingests the fungi, it is passed out and the fungi feed on the worm larvae, so while it doesn’t have a direct impact on the worms like a drench, it kills the larvae, so the animals don’t ingest them.  

Stocking rates

Overstocking is one of the major causes of a high worm burden. When animals are constantly eating in close competition, the ingestion of larvae will be much higher than animals that are spread out over a wide area. Good stocking rates are better for the animal, the environment and the farmer’s mental wellbeing!


There are times when drenching will need to be done, but for it to be effective, there needs to be a few rules.

Always drenched to the weight of the heaviest animals. Under-drenching is a major cause of drench resistance.

If you are bringing new animals onto the property, give them a quarantine drench. You don’t know what the animals might be carrying in terms of worms. Keep them separated for at least a couple of weeks before introducing them to the rest of the flock or herd.


When drench-resistant worms mate with each other, their offspring are not killed by drenches and they in turn will produce drench-resistant offspring.

To slow up the process of all worms becoming drench-resistant, we need drench-resistant worms to mate with susceptible (non-drench-resistant) worms, so that at least some of their offspring will be killed by drenching.

The process of making sure that you have susceptible worms on your property is called refugia.

You do this by:

  • Drench only when needed
  • Leave the healthiest 20% of animals un-drenched
  • Let drenched and un-drenched animals mix
  • Leave animals in a paddock for 1-2 weeks after drenching before moving to fresh pasture

Faecal egg counts

Faecal egg counts are a good indicator of what is going on with your animals. To test the mob it’s best to get at least 10 samples. The samples can be collected fresh off the paddock, so long as the samples are going to get to the vet quickly. You can also stand animals in the yards and collect samples from there. If the samples aren’t going to get to the vets quickly then make sure that they are refrigerated. This is to stop the eggs from hatching and giving a false reading.

For an individual animal, you can get a sample from fresh droppings or you can insert a finger into the rectum and remove a sample directly.

So, to recap:

  • Most livestock carry worms.
  • Adult livestock aren’t as affected by a worm burden
  • Young livestock are at the most risk from the detrimental effects of worms
  • Goats and Alpacas of any age are particularly susceptible to worm burdens
  • Paddock plants can help with worm control
  • Pastures should be maintained at a length that is above 3cm
  • Chemical drenches sometimes need to be used, but they need to be used correctly
  • Climate affects worm numbers
  • Faecal egg counts are a useful tool in assessing a flock or herd’s worm burden