I know there are a number of you intending to conduct this test to determine the extent (if any) of drench resistance on your farm this season.  To get the most out of the test results we need:

The presence of all worm species that we wish to test our drenches against and in this respect timing of the test is very important.  Too early in the season and Trichostrongylus may well be absent and too late and Ostertagia and Nematodirus can be hard to find.  The best timing for South Otago seems to be between the end of December and the beginning of April. 

FEC’s of at least an average of 500epg, but preferably nearer 1000.


The best procedure would be at weaning to mark 80-100 lambs, don’t drench them and either run them separately or they can run with the rest of the mob – it doesn’t matter which.  Starting 6-8 weeks after their pre-weaning drench do the occasional FEC to see how counts are going and when they reach a suitable level we can run the test.  The length of time it takes the lambs to get suitable egg counts obviously varies between farms & seasons.  In a drier year it will take longer.  Putting some lambs aside and not drenching them at the pre-weaning drench (where you do one) will often mean the FEC’s in the lambs will be at a high enough level to conduct the test too early in the season meaning, as I indicated above some worm species may not be present hence you haven’t tested the drench against them.  Therefore you have spent the money but have an incomplete set of results and are faced with having to repeat the test or put up with an incomplete set of results.  Assuming a weaning date between early December and early January this should mean that in most cases we will end up conducting the test in the desired January – March window.


I have analysed the results of our local FECRT results  – see the table below.  I have done this at two pass/fail levels.  Firstly the <95% level - in other words the % of farms where the particular drench failed to achieve a >95% reduction in FEC – this is the accepted national standard for determining drench resistance but is in my humble opinion bullshit.  It gives a very conservative estimate and can leave some farmers with a “rosy spectacled” view of their situation.  The second column headed <99% (in other words to get a pass the drench has to reduce the FEC by 99% or above) gives a more realistic snapshot of the situation.



Failure Level


<95% Reduction

<99% Reduction

Albendazole (white)



Levamisole (clear)



Dual (Arrest)



Half ivermectin



Full ivermectin




In other words on 63.3% of farms white drench failed to reduce the FEC by 95% or greater and on 80% of farms tested by 99% or greater and so on.  It would appear that anthelmintic resistance is alive and well in South Otago.  Of particular concern is the apparent rapid escalation of “mectin” resistance.  Using ineffective drenches is probably one of the best ways to make the situation worse so it is important to know your resistance status – hence conduct a FECRT.

Written by John Smart BVSc

Extract from October 2007 - Sheep Newsletter