If you’re rearing calves for beef, or as dairy replacements, you’ll likely be treating them at intervals for internal parasites, or worms.
Great feeding, alongside grazing and management strategies for avoiding worm intake are really important tools for reducing the amount of drench you need to use. But when you do drench, you need to be sure the product you’re using is actually doing its job.
In a survey of cattle drench performance results completed in 2025, we found that resistance to combination drenches appeared to be very common on farms raising heifers and dairy beef.
Of 19 tests of mectin/levamisole drench products, 12 (63 percent) showed Cooperia surviving the treatment, and four (21 percent) showed the more damaging Ostertagia surviving. A high percentage of these particular tests didn’t have enough Ostertagia there to begin with, so 21 percent is likely quite an underestimation.
As many as 55 percent of tests of an oral triple combination showed Cooperia surviving, and 45 percent of these tests had Ostertagia left behind after treatment.
These are worrying numbers, and they mostly represent an unseen iceberg on farms. There are an increasing number of cases at the tip of this iceberg showing up – calves that are sick and growing poorly, despite having been regularly drenched with what was thought to be an effective, combination product. In most cases, by the time this is seen, there are very high numbers of resistant worm larvae on pasture, drench treatment options are limited, and affected calves can take long time to turn around.
So don’t let this be you!
A really simple action you can take this season is to do some faecal egg counts after your routine calf drenches.
A faecal egg count (FEC) is a physical count of the number of worm eggs in the faeces and is used to determine if parasites are still present after drenching. If your drench killed all the worms, there will be none left to lay eggs, and the FEC should be zero.
A ‘drench check’ is where a FEC is taken at 10-14 days post drench. If parasite eggs are still present in the faeces at the drench check, it is possible that the drench is not working and the situation requires further investigation.
A ‘larval culture’ where the eggs present are hatched out and grown into larvae, allows identification of which worms are surviving your drench. Advice can be tailored around this.
Drench formulation matters too. For resistant Cooperia, an oral drench will commonly kill a lot more worms than will an injection or a pour-on. Pour-ons are our least favourite way to drench calves, as the absorption of these drugs can be highly variable and result in a sub-optimal worm kill. Absolutely DO NOT EVER administer drench to calves in their milk. At best, it may not work very well, and at worst, toxicity can result from calves absorbing too much drug, too quickly.
You should check how well your drench is working early in the season and again in the autumn.