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Western Australia worm update - December 2012

Brown Besier, DAFWA, Albany (

The unseasonal rains in many parts of WA in early December have implications for sheep worm control, especially for weaners and hoggets. We saw some major worm problems last year when late rains also occurred, and must be careful that we heed the lessons.

There are 2 major concerns:

  1. Delaying summer drenches because delayed harvesting means crop stubbles are not yet available. There is a risk that large worm burdens may cause worm disease if not removed in good time – young sheep typically have their highest worm burdens at this time of year. An early summer drench usually prevents these from becoming serious, but as we saw last year, if drenching is delayed until say after Christmas, there is a real chance that worms will get on top of the weaners.   The risk will be especially severe if further rain occurs. It is better to drench weaners and let them return to the present paddock than put it off for more than a couple of weeks. Once a crop stubble is available, there will be no need for another drench as the sheep are moved in.
  2. Where pastures have not yet dried off, the recent rains may kick them along. The basic rule is that is if some green pasture persists, there will be some worm larvae – even if only on part of a paddock. Most of these will die after couple of weeks of hot temperatures (even on green pasture), but it is not unusual to have larvae persisting well into December in such situations. Where perennial pastures survive through summer, there will always be some areas where worm larvae may develop and survive.

Wormier pastures mean a greater need to monitor for worms.  Summer drenches should be given as usual to lambs and usually to hoggets, but where the green pasture season is longer than normal, worm egg counts should be taken 5 -6 weeks later to check that a significant number of worms was not picked up after the treatment. This always applies where sheep graze perennial pastures – greater pasture availability means an increased worm risk.

Do hoggets need a summer drench?
For some time we have advised that adult sheep should be drenched in autumn, not summer, but lambs and hoggets (ie, the 2 youngest year-groups) should receive a routine summer drench.

However, in some situations hoggets have very low worm burdens in early summer and a drench can’t be justified. This indicates an early development of their worm immunity, but the chances of this vary with sheep management patterns. If the hoggets (last year’s lambs) were born early in the season, before say mid-June, they may have had sufficient worm contact as lambs, to add to their hogget-year worm experience, to develop a good worm immunity by around 18 months of age.

However, hoggets born later in winter, and now closer to 15 - 16 months of age, are less likely to have their full adult worm immunity, and a summer drench will be needed.

A worm egg count will easily tell whether summer drenches are needed in hoggets. There is usually no value in an egg count in lambs (which almost always need drenching), or adults (which usually don’t), although occasionally there are departures from the normal pattern and a drench can be dropped.

For December 2012 state outlooks, please follow the links below:
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