Tips for an effective worming strategy

How to plan your worming strategy during the grazing season

With increasing concerns about development of resistant parasites, cattle wellbeing, the environment and biodiversity, promoting the responsible use of anti-parasitics is essential. An outbreak of lungworm or parasitic gastroenteritis caused by gutworms can have a huge effect on both the health and performance of all cattle, in particular first-season grazers and yearlings. Gutworms can reduce cattle thrive, suppress feed intake, and alter feed conversion efficiency, which will inevitably affect cattle’s daily live weight gain and failure to meet growth targets..


What about anthelmintic resistance?

The threat of anthelmintic resistance (the ability of worms to survive a dose of wormer that should kill it) is ever increasing and requires a change in our worming habits. Best practice for anthelmintic usage need to be integrated into farm health plans to help protect your herd’s health and productivity, whilst protecting long-term sustainability in parasite control on your farm.


Considerations to bear in mind when you are planning your worming strategy:


  1. Tools to assess parasite challenge and inform treatment and control strategies.

    Without identifying the presence of worms, it is impossible to make a sound decision regarding control strategies. You can identify worms by:

    Testing Faecal Samples

    • Faecal egg counts - testing 6-8 weeks after turnout of first season grazers can provide an  indicator of worm burden and assess pasture contamination.
    • Baermann test - can identify lungworm larvae in faecal samples.

    MOO Testing

    Detects high antibody levels to the gutworm Ostertagia ostertagi in milk.

    Weight Assessment

    Can identify animals not reaching targets - when nutrition and health status are otherwise good poor thrive is likely to be a result of a worm burden, a gutworm challenge.


    Vigilant stockmanship can detect signs of parasitic infection, such as illthrift, scouring and coughing, early in the course of disease allowing prompt action.

    Blood Pepsinogen Levels

    Provide an indicator of the tissue damage caused by gutworm. Testing youngstock at the end of the season can be used to assess the parasite challenge faced, and the effectiveness of control strategies.

  2. Identifying the animals to treat.

    When cattle are grazing, some are more susceptible than others are to worms. Your worming strategy will depend on their age and level of immunity:


    Research shows that it takes approximately 8 months of exposure to parasites for cattle to develop effective immunity against gutworm. This means that animals may remain susceptible into their second season at grass. However, whilst immunity renders cattle more resistant to infection, they may still harbour worm burdens that lead to production losses so monitoring is important in all ages.1 However, it is good practice to monitor cattle regularly to identify if worm challenge has overcome immunity leading to disease.


    First season grazing (FSG) cattle are naïve to worms and are susceptible to infection, which can affect animal performance and lead to disease. Strategic treatment during the grazing season is usually necessary for FSG cattle. With second grazing season cattle, diagnostics should be done to determine if worming is required.

  3. Grassland Management

    95% of worms are estimated to be located on grassland while only 5% are present in cattle.2 A holistic approach to control should control exposure to pasture challenge alongside worm burdens in animals, whilst reducing practices that may select for resistant worms. Approaches include:

    • Implement treatment strategies that reduce selection for resistance and preserve a genetically diverse population of worms on your farm:

    • When implementing strategic group treatments leave 10% of the fittest animals untreated.
    • Implement a targeted approach to treatment based on an assessment of parasite risk: strategic treatment of animals in their first grazing season
    if faecal egg counts performed 6-8 weeks after turnout are high; selective treatment of animals within a group that are failing to meet growth targets.

    • Rotational grazing rather than set stocking.
    • Avoid grazing bare pastures - 80% of worms are concentrated in the first 5cm of grass.2
    • Lowering stocking rates will reduce the need for cattle to graze close to dung pats, decreasing the risk of ingesting worms.
    • Incorporating break crops into grassland rotation e.g. tillage, silage and graze the most susceptible animals on low risk pastures e.g. after-grass, re-seeding.


  4.  Use wormers correctly:

• Select an appropriate product with efficacy against the parasites and parasite stages that you are targeting.
• Dose accurately - assess animals’ weights as accurately as possible, using scales or a weigh tape.
• Use an appropriate, well maintained application device and calibrate before use.
• Monitor product efficacy - perform regular post treatment faecal egg counts and engage with your vet to ensure any incidences of suspected lack of efficacy are investigated further and reported to the product manufacturer. This approach is key to understanding the reasons why an anthelmintic treatment may not have been effective and determining whether resistance may be an issue on your farm; and report any suspected lack of efficacy to the product manufacturer.


  1. Ravinet, N. (2014) Change in milk production after treatment against gastrointestinal nematodes according to grazing history, parasitological and production-based indicators in adult dairy cows. Veterinary Parasitology.
  2. Herd, R. (1985) Strategies for Nematode Control in Cattle. Modern Veterinary Practice. 66:10, 741-744

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