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Drench Resistance Seminar April 2022

In April Tararua Vets in Pahiatua ran a parasite advisory session for clients who had completed a faecal egg count reduction test (FECRT) with the clinic in the last three years. 

The attending farmers heard from two of the leading experts in this field, Dave Leatherwick from AgResearch and Ginny Dodunski formerly of Totally Vets Taumarunui and now manager of the Wormwise programme.

This afternoon was arranged as an opportunity for farmers to not only hear from some experts in this field but also to network and share experiences. As in many other areas of the country, triple drench resistance in this area is on the rise, with an increasing number of farms being diagnosed with this challenge. The main worm that is causing issues is Trichostongylus, a round worm also known as black scour worm. This worm dominates in the autumn period and causes significant challenges for lambs.

Key messages from the day included:

  • We cannot continue as we are:
    • The long-term reliance that we have had on drenching animals as the main approach to controlling parasites cannot continue.
    • There are already cases of Startect and Zolvix resistance emerging in New Zealand.
    • The widespread and on-going intensive use of drenches without leaving a proportion of animals un-drenched is contributing to farms having a drench resistance status. It is important to note that there are more worms/larvae on the pasture than in the animals.
    • Refugia – the concept of having a susceptible population of worms on the ground by leaving a population of animals un-drenched, is very important.
  • No new drench families coming
    • There will be no new drench chemicals in the future. The sheep population is in decline, and we already have drench companies exiting the market (eg BI)
    • Already we are having challenges sourcing the current novel drenches – Startect is unavailable until next year and Zolvix is currently not easy to source (import issues).
  • Integrated Grazing
    • One of the key management principles of worm control is integrated grazing. Using one species or age class to groom or clean pasture for another species.
    • Cattle and sheep don’t share parasites so cattle can remove larvae from the pasture creating a clean pasture for lambs. Adult sheep have the ability to remove larvae from pasture and also have the ability to leave susceptible worms on the pasture, providing some refugia.
    • It is important that ensure that farms do not set up lamb-only areas, where only young stock graze.
    • There are a variety of ways this can be achieved. For example, some people graze lighter adult ewes with lambs, others have a finishing system with bulls and lambs being in rotation and at 50:50 ratio.
  • Cropping
    • Clean pasture can also be provided by cropping and re-grassing.
    • Many crops provide a high level of nutrition and can help minimise the impact of parasites on growth rates.
    • Feeding crops allows for the opportunity to increase the drench interval beyond the traditional 28-day interval.
    • Regular FECs should be used to assess parasite burden and to make sensible decisions regarding when to drench.
  • Introduction of susceptible worms
    • Buying in susceptible worms is an opportunity to change to parasite population dynamics.
    • When looking to do this, ensure that the animals you buy in are from a farm that is confirmed to have a good drench resistance status, not just assumed to have a good status.
  • Monitoring
    • Frequent monitoring via FECs is very important in order to understand the dynamics of your own property.
    • All farms should do regular FECs 10 days post-drenching to ensure drench effectiveness and also to monitor parasite burden later in the season, if you wish to stretch the drench intervals and prior to drenching adult sheep.