Do you know whether that drench you just gave your stock is working? They may look to be free from signs of parasitism, but a drench can have lost a significant amount of its efficacy before you will notice any clinical impact. Continued use of a poorly effective drench is a perfect recipe for building up high levels of resistant parasites. So what can you do?
A drench check is a faecal egg count performed after drenching to confirm that the product has actually worked. With the lifecycle of worms completed within 21 days, a drench check is typically done 7-10 days post drenching. At this stage drenched stock will have taken up infective larvae off the pasture, but these will not yet have had time to mature and lay eggs. Hence, a faecal egg count, at this point in time, should yield no eggs at all. If eggs are present, they have been produced by a parasite that was not killed by the drench given and therefore carries resistance genes.
Young stock are the best class to use for drench checks, as, from 6 months onwards, immunity towards worms starts to develop, resulting in natural suppression of the production of eggs. Your weaning drench would be the ideal opportunity to do a check, as it is early enough in the season that you will obtain the full spectrum of worm species whilst allowing time to make changes for the rest of the season or investigate further if the result is questionable.
For each drench that you use on property, a drench check should be performed at least once a year. This way any potential issues can be detected and addressed at an earlier stage than waiting until your next reduction test, by which stage your property may have been contaminated with vast numbers of resistant parasites. That said, a full blown reduction test is still going to give you the most comprehensive information about which drenches are working on your property and drench checks are not a substitute for this.
So, next time you drench your stock, get them back in 10 days later to collect 10 faecal samples. These can be taken from the paddock so long as they are fresh. We can have results to you within 24-48 hours and give you more confidence when making your drench choices. With drench resistance a serious emerging issue that has significant production limiting effects, you cannot afford not to have this information at hand.