News & Advice

Drenching deliberations

Oct 26, 2022 | Biosecurity, Dairy, Grazing youngstock, Young Stock

Hanneke Officer, Veterinarian, Anexa Vets Gordonton

As calves are weaned and spending more time on pasture, their exposure to worm eggs increases. With rapid daily growth rates, the impact of a worm burden on weaned calves can be detrimental. On top of this, calves have low immunity to worms, which makes frequent drenching even more important. Most drenches are licensed to be used every 4 weeks, which is based on the lifecycle of the worms as well as susceptibility of calves under 1 year old.


Frequent drenching however, can enhance resistance to the drenches used. Drench resistance refers to the ability of a worm species to survive a drench treatment.  As susceptible worms are killed by the drench, only resistant worms are left to breed with other worms that also survived the drench.  Once the resistant worms are abundant enough to be noticed by the farmer, managing them is difficult. This poses a bit of a dilemma: we need to drench every month, but we don’t want to promote the growth of resistance in worms on the farm.


What options are there to limit drench resistance?

  • Refugia: One way to slow the development of resistance is to allow some worms to avoid exposure to drench, providing non-resistant worms for the resistant ones to breed with. You can do this by not drenching a small number of calves per mob.
  • Quarantine drench for bought-in stock or stock returning from grazing: drench on arrival and leave in a quarantine paddock for two to three days.
  • Use an effective drench with multiple families: there are three drench families, all targeting worms in a different way. This means combining families will increase efficacy and decrease chance of resistance. Choose a drench with at least two different families. The three families are:
    • Macrocyclic lactones (MLs): better know as ‘mectin’ drenches (ivermectin, abamectin etc.)
    • Benzimidazoles: also known as ‘white’ drenches although they’re not necessarily white (albendazole, fenbendazole etc.)
    • Levamisole: this is a clear drench
  • Accurate dosing is essential to minimise the development of resistance (underdosing will contribute to resistance as the worms won’t all be killed). Maximise the efficacy of your drenching by weighing stock and drenching animals according to weight (don’t just guess!). Calibrate your equipment and check that it is delivering the correct dose.


It is possible you might already have some form of drench resistance on your property. You might see this when the drench doesn’t seem to be working as well, or for as long as it should; for example dirty bums that never fully disappear or come back faster, or coughing is ongoing if lungworm is an issue. 


The best way to find out is to do a faecal egg count 7 to 10 days after drenching calves.  If no eggs are present, the drench was effective. If eggs are present, these are then counted to give a quantitative estimate of the resistance issue. Following this, a larval culture can be done to identify which worm species are resistant and a plan can be made for future drenches to deal with this.


It is worth bearing in mind that worms are not the only cause for calves to have dirty bottoms and/or poor growth rates. If faecal egg count results are good then you may need to rule out what else could be going on. Other causes of ill thrift can include:

  • nutritional issues caused by poor quality, insufficient quantity or a sudden change in diet
  • infections such as Yersinia, Coccidia, Salmonella or BVD
  • trace mineral deficiencies

A chat with your vet will help you decide if you should drench or if some other testing is required. With a range of tests, treatments and drenches available to you, it is important to make an informed decision which will serve your youngstock well.



To summarise:

  • DO drench youngstock under 1 year old frequently: every 4 weeks
  • DO weigh animals and calibrate equipment to ensure accurate dosing
  • DO use a drench which combines at least two drench families
  • DO check faecal egg counts after drenching if you suspect an issue with efficacy
  • DO use a quarantine drench on returning stock or bought in stock
  • DO consider other causes of ill-thrift


Other Anexa resources you may find helpful:

Podcast: Calves – from weaning to grazing

Youngstock Services

Share This