SUMMER DRENCHING, DON’T LET THE WORMS BURN YOU!

Written by Dr Tim Elliott for Virbac Australia.

Dr Tim Elliott is a large animal scientist with over 15 years’ experience. Tim has a particular interest in production animal parasitology and drug resistance. Tim’s work has been published in internationally renowned journals and has presented his work around Australia for scientific audiences and farmer groups.

The summer drench is a strategic drench which aims to reduce the worm population using the dry summer heat. Summer drenching is well known by sheep producers in Australia, in particular in regions which have uniform or winter rainfall.

The summer drench is very effective in lowering the amount of worms that are present in Autumn and Winter as the dry hot weather reduces the amount of worm eggs and larvae on the pasture. However, one of the issues that has arrived from this practice if not done correctly, is the selection for drench resistant worms. Any worms that survive the summer drench, can be carried by the sheep over the summer period into Autumn where they contaminate the pasture with drench resistant worm eggs that then develop into infective larvae.

The timing of the summer drench is not a set date or management event like other strategic drenches such as the pre-lambing drench and the weaning drench. The timing of the first summer drench will depend on when the weather conditions are no longer suitable for worm egg development and larvae survival. This is when the days become consistently hotter, the humidity drops, and the pasture begins to hay off.

The drench selection for a first summer drench will depend on a number of factors:

1. Selecting a drench that is currently highly effective on the property (ideally with efficacy above 98%).
2. Selecting a drench with a short withhold period (WHP) and export slaughter interval (ESI) if the animals are to be sold soon.
3. Using combination drenches are recommended due to their ability to obtain higher efficacy than single active drenches and slow down the development of resistance.

It is recommended to use a highly effective combination drench as your first summer drench. The table below provides my preferred options. The choice of the first summer drench will be affected by the drenches that were used pre-lambing and at weaning as ideally, they will be different drenches.

Recommended first summer drenches:

14 days after the first summer drench has been given, a faecal egg count (FEC) should be performed to confirm that all of the worms have been removed.

Second Summer drench

The need for a second summer drench is based on the season and on results of an FEC. If the summer has been hot with very little rainfall, then there is a good chance that the animals have not picked up many worms from the pasture over the summer period. It is important to have an FEC performed on the animals to determine if a second drench is required.

On the other hand, if there has been some good summer rainfall or storms, this can cause a spike in worm numbers especially for the highly pathogenic barber’s pole worm. If a rainfall event of over 12 mm has occurred in a day, combined with warm weather, this can be enough to allow the worm eggs to hatch on the pasture and infect the animals. If this does occur, perform a FEC 4 weeks after the rainfall event to assess if the animals have become infected.

Using an effective summer drench and monitoring your sheep via FECs during the summer and into the autumn will ensure that the pasture contamination from worms is lowered without any unnecessary drenching.

Your advisor or local sales representative should be consulted to ensure that you implement a sustainable approach to worm control on your property.