If you farm sheep on any kind of productive scale you will be well-aware of the need to be able to effectively control internal parasites. A significant factor in our ability to reduce parasite challenge is the high prevalence of internal parasites that have become resistant to the chemicals we have relied on to control them (drench resistance).

In previous articles I have highlighted how farm system factors, stocking policies, and grazing management practices make some farms heavily reliant on chemicals to control parasites, with a consequence of this high chemical use being an increase in the rate that drench resistance develops.

Combating the problem of drench resistance requires a multi-pronged approach that should involve discussion and farm policy development around sheep genetics (resistance/resilience), sustainable sheep:cattle ratios, appropriate young stock numbers, forage selection to reduce larval challenge, refugia management, and appropriate chemical use.

To remain productive into the future, farms that have already developed highly resistant parasites to multiple chemicals will need to undertake far more aggressive and disruptive changes than farms that have very little current drench resistance. Developing an appropriate strategy is reliant on a robust understanding of the current drench resistance profile of the parasites on your farm.

The current industry standard test is the Faecal Egg Count Reduction Test (FECRT).

This test involves:

  • Taking a pre-drench faecal sample for a faecal egg count (FEC) and then culturing (hatching) these eggs to assess what species of parasites are present
  • Drenching groups of lambs with accurate doses of different drench families
  • 14 days later taking post-drench samples for FEC and larval culture

From the results collected, the efficacy of each tested drench can be calculated for each species of parasite present.

Some critical points around planning for your FECRT this season:

Timing – The strongylid parasites we are most commonly talking about are; Nematodirus, Teladorsagia (brown stomach worm), Trichostrongylus (black scour worm), Haemonchus (barbers pole), and Cooperia. You want to run the FECRT during the time of year when the critical production limiting parasites relevant to your farm are going to be present in the samples. The timing of when to test will vary across the country.

In North Canterbury (where we see almost no barbers pole) I have focused on getting tests completed early in the season before summer dry conditions limit my ability to get egg counts to rise in lambs. If I used this timing in the Waikato I may not have enough barbers pole in the sample to get an accurate test for a critical parasite in this district.

Starting egg counts – The pre-drench samples need enough eggs in them to calculate an accurate reduction percentage. For my North Canterbury tests I need to plan very early and mark a group of lambs to leave completely un-drenched from birth to ensure enough eggs are present early in the summer.

Lamb age – The FECRT must be run on lambs, it will not work in ewes. In my experience it can be a struggle to reliably get egg counts high enough in lambs older than nine months.

Number of lambs – I use 15 lambs per drench group and typically run 5 drench groups in my standard FECRT = 75 lambs. You need to plan alongside your advisor well in advance of your FECRT how many drenches you need for the test to get a robust understanding of your farms current drench resistance status.

Pre-test management of lambs – In my rush against the dry I want to see FECRT lambs pushed hard, on low covers, grazing behind other groups of lambs. If my 75-100 drench test lambs have it easy, on good feed we never get egg counts high enough to start.

Pre-test monitoring – We usually do a sample FEC a couple of times prior to the test to check we are on track and have sufficient eggs in the sample before we pull the trigger.

Equipment – Your vet will typically arrange all the drench to use for the tests and bring with them the equipment to mark lambs into groups, and to collect faecal samples. It is important that you have arranged for a working set of scales to weigh each lamb prior to their dose of drench.

The FECRT is going to be relied on for a number of critical decisions. You need to make sure it is accurate, that the results are truly representative of the parasites on your farm, and that the timing of the test ensures that critical parasites are present in the test sample. This takes planning, and it takes an experienced adviser to ensure all the boxes are ticked prior to the test starting. I strongly advise you contact your vet well before weaning to get the plan for your FECRT sorted.

  • Ben Allott is a North Canterbury veterinarian.