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Home Grazing Management – Sheep How Long Do Larvae Take to Die?

How Long Do Larvae Take to Die?

A number of the strategies on this page rely on waiting for worm larvae to die (while no further contamination occurs) to achieve a low worm-risk pasture.

Worm eggs may only live a few weeks awaiting suitable conditions to develop to larvae, but once they do become larvae, they are much tougher and can live for many months.

Some will die immediately and others will live for more than a year, but it is temperature that determines how long, on average, they survive on the pasture waiting to be eaten by a sheep or goat.

The larvae are more active in warmer temperatures and they deplete their finite store of energy faster. Once their energy is consumed, they will die. In very hot conditions (over 40°C) larvae can rapidly dessicate (dry out) and die.

Well over 90% of worm larvae on an already well-contaminated paddock must die before it is considered to be a low worm-risk. Some months are then required for that population to build up to harmful levels again once livestock are present.

If only 80–90% or less reduction occurs, this leaves enough larvae (particularly barber’s pole worm) for a reasonably rapid population buildup once livestock are present.

Over 90% of larvae die in 2 to 6 months.

In warmer regions (e.g. coastal or far inland), 2 months is required during the hottest months and 5 months during the coldest months.

In colder regions (e.g. tablelands), 3 months is required during the hottest months and 6 months during the coldest months.

Alternating grazing of sheep with cattle

Alternating grazing between cattle and sheep is an effective means of reducing worm infection because most worms are host-specific. That is, most worms are only able to successfully infect either sheep or cattle, not both, and ingestion by the non-preferred host results in death of the larvae.

A notable exception is the ability of barber’s pole worm and stomach hairworm (Trichostrongylus axei) to successfully reproduce within sheep and young cattle (i.e. pre-weaning). Using dry cattle for alternate grazing is preferable, but large benefits still arise from using cows and calves and this should not prevent their use for this purpose.

Rotational grazing

The value of rotational grazing management as an aid for worm control is based on:

  • Graze periods in a paddock being sufficiently brief (less than 4–10 days depending on temperature) to avoid autoinfection (see life cycle page); and/or
  • Rest periods (i.e. the interval between consecutive grazing in a paddock by sheep) long enough to have allowed enough time for many of the larvae on the pasture to die. Typically these need to be 40–80 days depending on temperature (see life cycle page, and worms on pasture).

Grazing with cattle represents a period of rest from sheep grazing, during which the number of infective larvae on pasture declines because larvae continue to die. The same periods of cattle grazing, as described below for grazing management rest periods (40–80 days), are required to gain sufficient benefit for worm control in sheep.

Grazing paddocks with yearling cattle for 12 or 24 weeks (starting in December) in a summer rainfall region, before then grazing with Merino weaners, has been demonstrated to reduce burdens of barber’s pole worm and black scour worm by over 95% (see Southcott reference).

Grazing yearling cattle for 6 months prior to grazing with Merino weaners has been demonstrated to increase annual weight gain and greasy fleece weight by 65% (7kg) and 25% (0.8kg) respectively (see Barger reference).

There are a number of approaches to rotational grazing that differ in the time used for graze and rest periods. Faster growing pasture will require a shorter rest period between grazing events, while slower growing pastures require a longer rest period; the longer the rest period, the longer the graze period.

Choosing the period of grazing and rest is a balance between worm control and pasture utilization. Information on grazing management is available at:

Differences between worm species

The benefit of rotational grazing for worm control is greatest for barber’s pole worm because development from egg to infective larvae cannot be delayed and larvae do not remain in the dung pellet, but quickly emerge onto pasture. There is also benefit for control of scour worms, but the survival of these worms on pasture is prolonged because they can delay development and remain in the faecal pellet. In winter rainfall regions there can be a delay of several months between eggs being deposited in the dung and the appearance of scour worm larvae on the pasture.

Control of scour worms in winter rainfall regions is greatly assisted by the integration of effective drenching followed by brief periods of intensive grazing (see ’Smart Grazing’). This approach also has benefit in the summer rainfall regions (e.g. QLD/NSW Summer rainfall/slopes and plains and QLD/ NSW Summer rainfall / tablelands and slopes).

Benefits for worm control

Rotational grazing has been demonstrated to reduce barber’s pole worm infection in summer rainfall regions. Over two years of a trial, Merino ewes in a rotational system had worm egg count due to barber’s pole worm reduced by 57% (450 epg reduced to 195 epg) when compared to similar ewes being largely set stocked (see Colvin reference). The lower worm egg count was recorded despite the interval between drenching increasing from 78 days for set stocked to 144 days for rotationally grazed paddocks.

Application of ‘Smart Grazing’ in a winter rainfall region has demonstrated a reduction in the number of worm larvae on pasture of 50–95% (with the exception of thin-necked intestinal worm). In response, worm control of Merino weaners was significantly improved over the two years of the trial with body weight increased by 3.2 kg and clean wool growth by 250 g (see Niven reference).

Grazing with sheep when eggs cannot develop to larvae

In lengthy periods of cold, heat or dry, worm eggs cannot develop to infective larvae. In these periods, pastures to be prepared as low worm-risk paddocks, can be grazed by sheep regardless of their worm burden. The conditions and time required for this strategy will depend on the worm species (see ‘Factors contributing to paddock contamination with worms’).

This is a particularly successful strategy in the northern tablelands of NSW where a 4-month cold period is exploited. This only requires the paddocks to be actively managed (through spelling, grazing with cattle or ‘Smart Grazing’) in the 2 months prior to winter, so as to give a total of 6 months in which existing larvae are dying before being used as a low worm-risk spring lambing paddock.

In Mediterranean-climate areas with hot, dry summer months, the paddocks can be almost completely worm-free by the autumn break—particularly in areas of South Australia and Western Australia.

To find out how you can use grazing management to improve worm control on your property, follow this link to the WormBoss Worm Control Program for your region.


Barger I., Southcott W. (1978) Parasitism and production in weaner sheep grazing alternately with cattle. Australian Journal of Experimental Agriculture 18:340-346.

Colvin A., Walkden-Brown S., Knox M., Scott J. (2008) Intensive rotational grazing assists control of gastrointestinal nematodosis of sheep in a cool temperate environment with summer-dominant rainfall. Veterinary Parasitology 153:108-120.

Niven P., Anderson N., Vizard A.L. (2002) The integration of grazing management and summer treatments for the control of trichostrongylid infections in Merino weaners. Australian Veterinary Journal 80:559-566.

Southcott W.H., Barger I.A. (1975) Control of nematode parasites by grazing management—II. Decontamination of sheep and cattle pastures by varying periods of grazing with the alternate host. International Journal for Parasitology 5:45-48.

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