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Drench choice and rotation to minimise resistance

Rotating drenches and drench efficacy (or effectiveness) are fundamental to minimising the risk of developing drench resistance in sheep flocks. Resistance to various drenches is widespread throughout southern Australia, but relatively few producers know which drenches are no longer effective on their farm.

Research has demonstrated that internal parasites (‘worms’) in sheep are much more likely to develop resistance to drenches if they are repeatedly exposed to the same chemical during their lifetime. Given that the average worm life span is 3 months and they are mostly located in the sheep’s gut during summer it is critical that sheep do not receive the same drench chemical (‘active’) more than once during this period.

In contrast, during the green (winter/spring) period more worm larvae are likely to be present on the pasture than in the sheep’s gut. As a consequence, using the same drench active twice during this period will have a minimal impact on developing drench resistance as the infective larvae on the pasture will not be exposed to the drench.

Drench resistance develops when worms survive exposure to a chemical and their progeny inherit the ability to ‘resist’ the chemical. The consequent drench failure can occur if the chemical actives are not rotated frequently or their efficacy is less than 100%. If this occurs over summer when few worm eggs/larvae remain viable on the ground, the resistant worms in the gut will largely be responsible for contributing to the resistant eggs/larvae available on the pasture for pickup after the autumn break and drench resistance will then become more widespread on that farm.

Frequent rotating between drench groups or ‘chemical actives’ has been the recommendation for many years, but this advice has advanced to include: 
• ideally do not drench during summer, except for lambs/weaners, unless worm egg count monitoring indicates it is critical to do so. Never drench twice during summer unless it is essential and then ensure different chemical actives are used on each occasion; 
• preferably always use highly effective double or triple actives ie the more chemicals included in a drench the lower the risk of worms surviving to develop drench resistance. There are many combination drenches (ie multiple chemical actives) commercially available; and 
• if a mob is to be drenched and there are adult sheep present in good condition with no evidence of a worm burden leave them undrenched. This will ensure any worms surviving in their gut remain sensitive to the drench active(s) and help to maintain a population of ‘drench sensitive’ worms on the farm. This is termed managing “refugia”.

Research has shown that if a drench active or combination of actives is only killing 95% of the worm population in the gut, it is necessary to leave one third of the mob undrenched to encourage refugia and ensure drench resistance is not increased.

Ideally do a drench trial every 3 years to check the efficacy of various drench groups on your farm. As a minimum, do a drench check by monitoring the worm egg count 10 days after using a drench to see if any worm eggs remain. This will at least indicate whether an alternative drench or combination needs to be considered for next time.

The critical task underpinning your drenching program is to frequently monitor the worm egg count in several mobs and especially in young sheep. This is integral to planning worm control strategies in contrast to inappropriate or unnecessary drenching. Monitoring needs to be done every 1-3 months in higher rainfall grazing enterprises or 1-3 times a year in mixed farming operations. Seek more specific advice from your animal health advisor.

For further information contact Dr Colin Trengove, Pro Ag Consulting or refer to WormBoss