Calf drenching: do’s and don’ts

As we head towards weaning time, we need to consider the importance of responsible drench use in calves.

Worming is an essential stage of weaning to control the exposure of calves to several roundworm species, as well as lungworm commonly found on our pastures.

Cooperia, which live in the small intestines, and Ostertagia, which live in the abomasum, are the two most common roundworm species affecting our youngstock here in NZ.

The larvae live in the lower 2.5cm of grass and top 1cm of the soil, requiring warm, wet conditions to survive.

Calf growth rates can be limited by up to 30%, if burdens are unmanaged, due to loss of appetite and poor nutrient absorption, secondary to parasitic gastroenteritis, as well as pathology associated with lungworm infestation.

When should you first drench calves?

The first drench should be given to calves at weaning. Subsequent drenching can be carried out at 28 day intervals, if using oral drenches, or 42 days if using pour-ons or injectables.

Oral drenches should be repeated at a shorter interval as they have no persistence, so only work to kill worms currently present in that animal.

Worm life cycles tend to be 21 days long. Allowing an extra week before drenching ensures maintenance of your pasture’s refugia population so that calves develop immunity to adult worms, as well as limiting anthelmintic resistance.

Pour on drenches and injectables have persistence, therefore kill worms present in that animal for a prolonged period after drenching.

In some cases, drenching may be required before weaning, please discuss this with your KeyVet.

Every farm's refugia worm population is unique, so it is important to safely expose calves to promote early resilience to worm burdens as they begin to take in less milk and graze more.

Drenching intervals are important to ensure we control calves' exposure to worm burdens gradually. In general, by 15 months old a good immunity should have been established against worm burdens if effective worming protocols are in place.

Product choice and changes to product availability at VetSouth

Several Boehringer Ingelheim-branded drenches are no longer being produced and therefore are not available to purchase this season at VetSouth.

Delisted products include Arrest C. See alternatives for calves less than 120kg listed below:

  • Turbo Initial: Eprinomectin, Levamisole and Diclazuril. Also contains Diclazuril which kills coccidia.
    Coccidiosis affects a lot of calves resulting in decreased growth rates, scours and death. The biggest risk factor is if you graze calves on the same paddocks year on year as levels build up between the years.
  • Turbo Advance: Eprinomectin, Levamisole.
    For use after calves have adequate immunity to coccidiosis and therefore no longer require Turbo Initial.
  • Corporal: Levamisole, Oxfendazole

It is important to remember that Abamectin toxicity poses a risk to calves less than 120kg. This occurs due to the immature blood brain barrier allowing Abamectin to enter the brain causing neurological signs and death.

Always check product labels before using them. 

See listed below products containing abamectin suitable for calves over 120kg only:

  • Converge: Abamectin, Levamisole
  • Matrix: Abamectin, Oxfendazole, Levamisole
  • Alliance: Abamectin, Levamisole, Oxfendazole
  • Triple A: Oxfendazole, Abamectin, Levamisole

Oral drenches should be used in youngstock for as long as is safely practicable. After this point injectables and pour-ons should be used. For further advice on product choice, speak to our team of experienced vets.

Prevention of anthelmintic resistance

Anthelmintic resistance is defined as the loss of susceptibility of worms to our drenches. This is becoming a global problem that is now commonly seen in cattle parasites in NZ.

Cooperia is known to be resistant to several different anthelmintic classes, especially macrocyclic lactones (e.g. abamectin, eprinomectin). It is important to act now by following the guidance listed below to preserve our effective drenches.

Drenching technique

  • Before drenching calves, weigh them individually and dose to the heaviest animal. If there is a large variation within the group, then group them by weight and dose to the heaviest in each group.
  • When using oral drenches, ensure the entire dose is received.
  • Avoid drenching too regularly to allow immunity to develop gradually.
  • Check your drench guns are accurately dosing before use.
  • Don’t mix drenches into milk feeds as this can give variable dose rates. Drinking milk mixed with drench means that the rumen is bypassed, increasing the rate of drench absorption in the abomasum, potentially resulting in toxicity or death.

Pasture management

  • If you have a limited number of paddocks appropriate for calves, then try feeding silage such as lucerne to ease pressure on pastures and reduce overgrazing.
  • It is a good idea to turn over the soil every few years and leave fallow over summer to desiccate larvae. This reduces the larval challenge your calves are exposed to.
  • Try to return calves to the same paddock they were on pre-drenching and then move them to a new paddock roughly 3 days after drenching. This allows any resistant worms remaining in the calves to be shed before moving to new paddocks, therefore reducing resistance selection pressure.

Faecal worm egg counts

  • Start performing faecal egg counts to understand levels of resistance to different wormers in your herd. This can help us recommend the best drench to use.
  • Faecal samples can be collected before drenching to confirm worm burdens and then repeated after drenching to assess levels of resistance. Timings vary with drench so please discuss testing further with your KeyVet.

- Tottie Allingham


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