Online learning: Western Australian Winter Rainfall—Deciding when to drench and what drench to use

This strategy describes when to use strategic drenches and how to decide when tactical/therapeutic drenches are needed.

Structured reading

For those who like to see all the information and simply read through it in order. Each heading is a link to a page of information—the dot point provides a summary of the page.

Tip: Keep this page open and open the links in new tabs.

Western Australian winter rainfall: When to test and when to drench
The times for routine worm testing and drenching in this region. Not all testing or drenching is routine; other times to do these are recommended by the Drench Decision Guide, according to details you provide about your mob of sheep.

Western Australian winter rainfall: Drench Decision Guide
This tool recommends whether a mob should be drenched, the length of protection warranted and when to worm test again. It is your day-to-day tool on drenching decisions that should be used in conjunction with the annual program of routine testing and drenching times.

The DDG tool steers you through a series of questions about your sheep; choose the answer that applies to your mob (or make up your own scenario).

Question and answer

For those who prefer a problem based approach to learning, answer the following questions.
Each of the questions below links further down the page to the answers.

Questions:

  1. What is the purpose of a strategic drench?
  2. Which classes of sheep receive a routine (strategic) drench, and when?
  3. When is a summer drench given and to what classes of sheep?
  4. The online Drench Decision Guide (DDG) for Western Australia assists you to decide whether a mob of sheep should be drenched now and when to test again. Open the DDG and answer the questions it offers based on the scenario (from below) that you are using. Try at least three of the following scenarios.
  • Weaners in May that received a combo drench into a prepared winter weaner paddock about a month ago, less than 5% are scouring, an egg count shows 80 epg.
  • Weaners in late January that were drenched with Monepantel at weaning in early December. There are no signs of scouring and they look healthy.
  • Ewes in November. No drench was required before lambing. About 15% are scouring.
  • Ewes received a first summer BZ/LEV/ML drench in early December, it is now mid January. Summer has been unusually wet. Sheep are not scouring but a couple of ewes were found dead in the paddock and others are lagging during mustering.
  • Late-lambing ewes in good condition just prior to being moved to their lambing paddock. A worm egg count shows 350 epg.
  • Lambs just about to be weaned, they look well-grown with no obvious signs of worms.

Answers:

You can also click on each question below to go to WormBoss pages with related information.

1. What is the purpose of a strategic drench?

Strategic drench: a drench given at a critical time to sheep that are susceptible to worm infection (e.g. weaners and pre-lambing ewes) and also given at times to reduce worm larval contamination of a pasture grazed by the drenched sheep over the following weeks or months. The sheep themselves may have had a low worm egg count at the time of this pre-emptive treatment.

2. Which classes of sheep receive a routine (strategic) drench, and when?

In WA, the routine ‘Summer-autumn drench’ program is recommended for the main sheep areas (the South-West Medium to High Rainfall Zone). In the Low Rainfall Cereal Zone, routine treatment is needed only for younger age classes. It is critical that all drenches given in summer and autumn are given with a fully-effective drench, as otherwise drench resistance is likely to increase more rapidly.

The South-West Medium to High Rainfall Zone

  • Lambs at weaning
  • Weaners in early summer, after the pasture has dried off (this can often be done as the lambs go into a crop stubble paddock)
    Note: If weaning occurs when the lambs are on dry pasture or are to be placed onto a crop stubble, the weaning and summer drench coincide and only a single drench is needed.
  • Hoggets (last year’s lambs, including maiden ewes) in early summer, as for the weaners.
  • Adult sheep in autumn, between the end of March and the end of April
    Worm burdens in sheep of this age rarely justify a summer drench, but this can be easily checked with aWormTest (especially maiden ewes) if you are concerned.
  • Late-lambing ewes (lambing later than mid-June) pre-lambing if a WormTest is not conducted (see Routine WormTest times, below). In early-lambing ewes, the March-April drench removes the need for a specific pre-lamb treatment.
  • In barber’s pole worm areas, a long-acting drench should be considered for ewes lambing in May or June.

Low Rainfall Cereal Zone

  • Lambs at weaning or as lambs are moved onto a crop stubble if this is within a few weeks of weaning.
  • Hoggets (last year’s lambs) in early summer, unless WormTests over some years indicates there is rarely a need.

3When is a summer drench given and to what classes of sheep?

The strongly seasonal nature of Mediterranean environments lends itself to a routine drenching program, as dry summer and autumn pastures are unfavourable for worms, and sheep drenched then do not pick up new worm burdens. This is the basis of the highly efficient ‘summer drenching’ program. However, summer drenching has been confirmed as the main factor causing the high levels of drench resistance in WA, as any worms surviving these drenches (i.e. resistant worms) are the source of future worm populations.

To reduce the risk of drench-resistance without affecting the effectiveness of worm control, ‘summer drenching’ should be replaced by ‘Summer-autumn drenching’. (See DAFWA FarmNote No. 348: ‘Sheep worms — a change to summer autumn worm control’ by clicking here, then Animal Health, then Parasites.)

Under this program, drenches for adult sheep are delayed until autumn, and only weaner and hogget-age sheep are drenched in summer. Most adult sheep have low worm egg counts in early summer and a drench at that time is not warranted; delaying a drench until late March or April allows some less-resistant worms to survive. Provided that the worm population on the property includes sufficient less-resistant worms to dilute resistant worms, the overall resistance level on the property will be reduced.

4. The online Drench Decision Guide (DDG) for Western Australia assists you to decide whether a mob of sheep should be drenched now and when to test again. Open the DDG and answer the questions it offers based on the scenario (from below) that you are using. Try at least three of the following scenarios.

  • Weaners in May that received a combo drench into a prepared winter weaner paddock about a month ago, less than 5% are scouring, an egg count shows 80 epg.
  • Weaners in late January that were drenched with Monepantel at weaning in early December. There are no signs of scouring and they look healthy.
  • Ewes in November. No drench was required before lambing. About 15% are scouring.
  • Ewes received a first summer BZ/LEV/ML drench in early December, it is now mid January. Summer has been unusually wet. Sheep are not scouring but a couple of ewes were found dead in the paddock and others are lagging during mustering.
  • Late-lambing ewes in good condition just prior to being moved to their lambing paddock. A worm egg count shows 350 epg.
  • Lambs just about to be weaned, they look well-grown with no obvious signs of worms.

 


Links to the learning topics for Western Australia

  1. Introduction
  2. Grazing management
  3. Breeding for worm resistance
  4. Worm testing
  5. Drenching (you are currently on this page)
  6. Drench resistance management
  7. Sheep worms