Preparing low worm-risk paddocks

Problem: Continuous recontamination of the paddocks with worm eggs that develop to larvae is a major cause of ongoing worm problems for sheep or goats.

Solution: Preparing low worm-risk paddocks to prevent animals from becoming heavily infected with worms is a key strategy in effective and profitable worm control.

Benefit: Low worm-risk paddocks for key classes of stock at particular times of the year reduce both production loss and the need for chemical (drench) intervention. In turn, fewer drenches result in

• Less cost
• Less labour
• Slower development of drench resistance

Strategic preparation of low worm-risk ("clean") paddocks can also benefit other classes of stock due to a lower overall level of worms on the property.

Which stock class and when to prepare will vary according to region.

Regional recommendations

In summer rainfall areas, where barber's pole worm predominates (QLD and NSW north of a line through Sydney), prepare low worm-risk paddocks for:

• Weaners
• Lambing ewes

In winter and non-seasonal rainfall areas and Mediterranean climates (WA, SA, TAS, VIC and NSW south of a line through Sydney)

• Weaners during their first winter

What are low worm-risk paddocks

To merit the description, “low worm-risk” these paddocks need to have such a low level of infective worm larvae on them, that when sheep or goats—just treated with an effective drench—are introduced, it takes a few months before worm numbers build up to levels that cause illness in the stock.

Preparation of low worm-risk paddocks

This involves:

• Allowing time for most of the existing worm eggs and larvae to die
• Preventing more worms for contaminating the pasture

Allowing time for most of the existing worm eggs and larvae to die

A very large proportion of the worm larvae on a paddock must die before being considered low worm-risk. This is generally about 95% from when a pasture is moderately to heavily contaminated. While an 85–90% reduction may sound significant, it is usually not enough.

In Australia, a 6-month period is sufficient for 95% larvae to die, and this can sometimes be reduced to a 2-month period in very hot, dry locations.

The following graph shows the rate at which barber’s pole worm larvae die, but is quite similar for scour worms. Choose the temperature line that fits your location in the few months prior to when the low worm-risk pasture is required and find where larval survival (left side of graph) drops to 5% to show the number of days required for 95% larvae to die.

Source: Modeled from death rate of the L3 population in 'Simulation of pasture larval populations of Haemonchus contortus' by IA Barger, PR Benyon & WH Southcott. Proceedings of the Australian Society of Animal Production (1972) 9: 38

Your program will describe the times required in your region to prepare a paddock.

Preventing more worms from contaminating the pasture

While the larvae on the paddock are dying, further contamination must be prevented. The simplest and surest way is to exclude sheep or goats (and alpacas) from the pasture during the preparation period.

However, this may not be the best use of the paddock.

Where possible, utilise feed by grazing with cattle or horses (not sheep, goats or alpacas).

Or use the paddock for growing a crop or making hay or simply leave the paddock empty while the pasture is regrowing.

You can graze the paddock with sheep, goats or alpacas under very specific conditions.

After they receive a drench proven to be effective on your property, graze in the 2–4 weeks after the drench, and then remove the stock.

• 2 weeks for goats,
• 3 weeks for sheep in barber’s pole worm areas
• 4 weeks for sheep in scour worm areas

If a long-acting product is used, grazing can be longer in line with the actual amount of persistence the product provides on your property. You should monitor worm egg counts to identify what length of protection you actually achieve.

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.