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Paddocks are considered "low worm-risk" when over 90% of the worms on them have died, and further contamination with worm eggs has not occurred.  This generally takes 3–6 months (depending on the temperatures at the time). However, in some environments, most worm larvae will die once temperatures have been extremely hot for a few weeks

Surveys show many people are in favour of preparing low worm-risk paddocks but most of them are unaware how long is needed.

More information on preparing low worm-risk paddocks can be found here: Grazing management

or the specific recommendations for your region are found in your regional Worm Control Program.


What conditions kill worm larvae?

A common misconception, based on having fewer worm issues in winter, is that frosts kill worm larvae on the paddock. This is a myth. 

Worm larvae are quite tough and can easily survive cold, frost and snow in Australia.

However, they are able to be killed by extremely hot, dry conditions. The graph above shows that daily maximums consistently at 35ºC will greatly shorten lifespan, with most larvae dying within 2 months. But for larvae to be killed rapidly requires temperatures to be above the 40ºC mark for a few days to a few weeks.

Why are there fewer worms in winter?

Worm EGGS need specific conditions to hatch into infective larvae. Daily maximum temperatures need to be in the 12–18ºC range (depends on worm species), combined with sufficient moisture (usually about 10–15 mm rain over a few days). If these conditions are not met within 1–3 weeks (again depends on worm species), the eggs will die. >> More information on conditions for development of worms.

Sheep and goats can and will be affected by worms in winter

While it may be too cold in some areas for eggs to develop to larvae, the pasture may still be heavily contaminated with larvae that developed in the previous 6 months during warmer times. These infective larvae will be ingested by sheep and goats grazing the affected pastures during winter. Depending on the contamination levels, it is quite possible for the stock to acquire large worm burdens in winter resulting in illness and death.

Using this knowledge to control worms

When preparing low worm-risk paddocks, you rely on the existing worm larvae dying, while no further contamination of the paddock occurs. 

In the colder districts of Australia, a longer period is required for preparation through winter (6 months) than in summer (3 months) for most larvae to die and a low worm-risk paddock to be created.

In warmer districts, a month can be taken of these periods. In very hot districts, a further month can be taken off;  extreme temperatures over 40°C for a couple of weeks will kill most larvae.