Drenching Livestock

Drenching, why, how and when?

Whether a commercial or lifestyle farmer, drenching and parasite control is an ongoing and vital part of animal health. Controlling parasite numbers and preventing the development of resistance to drenches are the key aims of any parasite control program.

Parasite numbers are primarily dependent on environmental conditions and stock density. However, other factors that can complicate the issue are:

  • Species-specific metabolism of drugs

  • Cross-grazing of different species (such as sheep and goats)

  • Buying in stock with unknown parasite burdens

  • Stock access to long-standing water sources

  • Dench resistant parasites

  • Repeat grazing of young stock on the same pastures year after year.

For this reason, a "one size fits all" approach to drenching is inappropriate and can be deleterious if implemented long term. Here at Warkworth Vets, we recommend a three-step process for drenching.

  1. Assess the parasite burden (drench only when needed)

  2. Drench wisely (appropriate product and dose)

  3. Perform a drench check (ensure the drench is working)


A Faecal Egg Count (FEC) is a cheap, quick and effective method that will answer steps ONE and THREE of this drenching approach. All you need to do is collect a fresh faecal sample from your stock (4-6 samples in large mobs is enough) and drop it into the clinic. We can usually have a result the same day and then advise if drenching is appropriate for your stock class.

1. Assessing the parasite burden

In New Zealand, most parasites of significance in livestock have similar lifecycles and will flourish under warm, wet weather.

Early spring has traditionally been the parasite's friend, any parasite eggs on pasture that have survived the winter will start to develop and be infective to grazing stock. Once ingested, they take around three weeks to develop and produce eggs that fully re-infect the pasture.

The traditional approach to drenching has been to drench a couple of times in spring at one-month intervals, then once in autumn to limit parasite numbers that overwinter. However, with the emergence of drench resistance and a changing climate, it is no longer that simple, and you may need to drench any time of year.

When to Drench

The parasite season starts as warm and wet weather breaks the winter cycle, occurring in august. Therefore, collecting faecal samples for testing before drenching is essential to measure efficacy post drenching.

Depending on your stock type and density, you may need to drench every month through spring and into summer if it remains wet due to high parasite loads on pasture. This is not ideal due to cost and the risk of parasite resistance, so performing another FEC before each drench or at least after the 3rd or 4th drench in spring is recommended, so you know that continuing monthly drench cycles is warranted.


Perform an FEC whenever parasitism is suspected, or weather conditions favour development. In the Rodney region, this may be any time from late winter through to early winter the following year if wet, warm weather persists for 2-3 weeks or more.


2. Drench wisely

Choosing the correct drench can be confusing due to the many and varied brands and drug combinations offered on the market. The three key considerations you need to assess are:

• What drug active is appropriate (have I proven efficacy in the past?)

• What route of administration should I use?

• What dose should I use?

Drug Actives

There are only five drug actives on the market, they are packaged as either single or multi-drug formulations, and in general, you should use an old generation active before a new generation and the least number of actives required for efficacy.

As a broad guideline, this means using the "fewest actives that work" (confirmed by doing a drench check as outlined in step 3). Although, the reason for this approach is due to drenching resistance, using a multi-combination or new generation drench comes with the risk of developing resistance to those actives. Once this occurs there a few options available.

Route of administration

Injectable, oral and pour-on formulations are available. Double-check your equipment and administration method labelled on the packaging. Also check the expiry date on the product (Do not use the expired product). With pour-on drenches avoid wet weather.


Drench resistance primarily develops due to under dosing, so getting this right is very important. When drenching, always drench the entire mob of any stock class to the weight of the heaviest animal in that mob. If in doubt about weights, estimate higher rather than lower.


  • Use the "fewest actives that work"

  • Does all animals to the weight of the heaviest in the mob

  • Strictly follow the labeled with-hold periods for milk and meat going for consumption

3. Perform a drench check

Ensuring your drench has actually worked is very important, particularly for the first drench of the spring season and the last drench in autumn. Without this you may be wasting money on a drench that is not effective or an expensive drench that could be replaced by a much cheaper one.


Perform FEC 7-10 days post drenching to confirm efficacy. Record results, use this to aid drench choice at the next dose.

Additional notes

  • Keep an eye out for "barbers pole" or Haemonchosis in sheep, goats and alpacas. It is an internal parasite that can rapidly increase in numbers on pasture, causing anaemia (look for lethargy and white gums). Call the clinic ASAP as it can kill.
  • Liver Fluke is a parasite of livestock with a particular lifecycle. It does not fit the above rules and requires veterinary intervention for correct diagnosis and treatment.
  • Ticks require a specific drench for control. Call the clinic for advice.
  • Quarantine bought in stock and performed a pre and post-drench FEC to confirm you don't import resistant parasites onto your property.

Warkworth Veterinary Services Ltd

We are a seven vet mixed practice, serving the greater Warkworth and Wellsford regions, from Puhoi in the south to Maungaturoto in the north and from east to west coast in between.