Producers often over emphasise the importance of drenching within a worm management program.

The need to treat can be markedly reduced by developing and implementing non-chemical worm management strategies.

This has a number of benefits:

• Reduced labor and drench costs
• Reduced onset of drench resistance
• Increased animal productivity

A well thought out non-chemical worm management program incorporating a number of non-chemical strategies can, over time, reduce the need to drench at all.
A professional adviser will help you to develop the best strategies for your property.

• Smart Grazing for winter rainfall districts
• Cross grazing with other species
• Spelling
• Nutrition
• Fodder crops and crop stubbles
• Rotational and cell grazing
• Genetics-breeding worm resistant sheep

 Smart Grazing for winter rainfall districts

Smart grazing is an improved, yet simple and reliable, strategy for the control of worms in weaner sheep during their first winter. It can counter the negative effects of summer rainfall that reduce the effectiveness of the 'two-summer treatment strategy' in one in three summers in the winter rainfall areas southern Australia.

The strategy hasn't been evaluated in the summer rainfall areas.

The why and how of 'Smart grazing'
Merino weaners are very susceptible to worms in their first winter. Consequently, they need to graze pastures that have as few worm larvae as is practicable. 'Smart grazing' combines intensive grazing for 30 days with each of the two 'summer' drenches to ensure that virtually no worm eggs are deposited on a chosen pasture from the first summer drench (November) until after the autumn break (March-April), when the weaners are put into these pastures.

Intensive grazing means using 2.5-3 times the normal stocking rate, for no longer than 30 days after each of the summer drenches is given. Not exceeding 30 days is critical because it takes three weeks for a worm larvae (eaten off pasture) to develop into an egg-laying adult in the sheep. After the intensive grazing period, the paddocks are de-stocked to allow the pastures to re-grow. This means that the total stocking pressure for the 'Smart grazed' paddock will be the same as that for a paddock continuously stocked at the farms normal stocking rate (Figure 1).

The intensive grazing will reduce pasture residues to around 800-1000 kg DM/ha after the first summer drench, and around 600 kg DM/ ha after the second. If there is insufficient feed, the periods of intensive grazing can be reduced. Alternatively, if there is excess feed the summer drenches can be 'staggered' for different mobs so as to provide a longer intensive grazing period.

Finally, the weaners must be drenched with an effective drench before they start grazing the 'Smart grazed' paddock after the autumn break.

Smart grazing on a typical farm 
A typical self-replacing flock of 5,000 DSEs in southern Australia is made up of 1,500 ewes, 1,500 wethers and 1,000 weaners, running at a winter stocking rate of 15 DSE/ Ha.

70 Ha of 'Smart grazed' paddocks must be prepared for the weaners. Thus, 2600 DSE (70 x 15 x 2.5) are needed to stock the 70 Ha at 2.5 times the normal stocking rate for each of the two intensive grazing periods - this is all of the wethers and 70% of the ewes on the farm.

A typical timetable for 'Smart grazing' 

Why does smart grazing work?
The intensive grazing periods:

• Reduce the amount of pasture dry matter, making the pasture less suitable for the survival of worm larvae.
• Ensure that there is no deposition of worm eggs on the pasture from the time of the first summer drench until the autumn break.
• Probably allow the drenched sheep to 'vacuum' up infective larvae, in much the same way as cattle do when they are used in alternate grazing programs with sheep.
• Have the same cumulative stocking pressure from November to March as set-stocked paddocks grazed continuously by wethers.
• Are quite flexible. What must not be changed is the need (i) not to exceed 30 days grazing after each summer drench, and (ii) for a fully effective compound at the summer drenches.

What are the benefits?
Results from a controlled experiment over 2 years in western Victoria show that, compared to weaners grazing paddocks prepared the usual way (grazed by wethers over the summer/ early autumn), weaners grazing 'Smart grazing' plots:

• Grew 13% more clean wool (2.29 vs. 2.03 kg) which was 3.5% broader (17.1µ vs. 16.5µ).
• Were 3 kg heavier in October (46.5 vs. 43.2 kg).

During winter, the egg counts from the 'Smart grazed' weaners didn't go higher than 250 epg, a trigger for drenching weaners used by many farmers and their advisers. In contrast, the weaners on the paddocks prepared by set-stocked wethers exceeded 400 epg in both years.

The numbers of worm larvae on the 'Smart grazed' pastures in winter were from one-half to a one-third of those on pastures in paddocks prepared by grazing with set-stocked wethers.

Preparing winter weaner paddocks by 'Smart grazing'

Figure 1

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 Cross grazing with other species

Cross grazing with cattle

Alternating grazing between cattle and sheep has been demonstrated to be an effective means of reducing worm infection.

This process relies on the fact that 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 barbers pole worm worm and stomach hairworm (T axei) to successfully reproduce within sheep and young cattle (ie. pre-weaning). Using adult cattle for cross grazing is preferable.

Scientists have demonstrated the benefits of alternating sheep grazing with cattle.

Grazing with cattle for the six months prior to sheep grazing reduced numbers of barbers pole worm and black scour worm recovered from animals at post mortem by 94% and 98% respectively. The benefits of alternating sheep and cattle grazing on weight gain and greasy fleece were significant.

Grazing sheep on pastures that had been grazed by cattle for the previous 6 months increased annual weight gain and greasy fleece weight by 65% (7kg) and 25% (0.7kg) respectively.

Similar results highlighting the benefits of alternating grazing have been demonstrated with periods of alternation between sheep and cattle as brief as 6 weeks. Prior grazing with cattle for the six week period January – mid February has been shown to reduce numbers of most of the important worms in sheep over the following month with the exception of brown stomach (Ostertagia) worm.

The total number of worms recovered from the sheep which grazed pastures that had prior cattle grazing was 2158 compared to 14,842 worms from sheep that grazed on pastures grazed by sheep for the previous six weeks. Note that the six weeks during January – February also took advantage of the time of year when conditions may have been least conducive for survival of larvae on pasture. In addition to the use of different ruminant species for alternating grazing, differences in parasite immunity between classes of animal within a species may be exploited to assist with the control of worms.

With this system, grazing management is planned so that pastures prepared for grazing by susceptible classes (e.g. recently weaned sheep) are previously grazed by more resistant classes (e.g. adult dry sheep). The length and timing of grazing by resistant sheep is critical to the success of this strategy.

Based on the results from sheep – cattle alternations, it is reasonable to assume that in the summer rainfall region, best results from grazing with resistant sheep would occur if grazing were timed to occur for at least the three months prior to January.

However, alternate grazing with cattle has been shown to be superior.

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 Spelling

Short term spelling is of little use in worm control. Some time ago it was believed that short term spelling for three weeks or so would reduce the numbers of infective worm larvae on a pasture.

These are the spelling periods typically seen in rotational or cell grazing strategies, or when leaving pastures unstocked after the autumn break in winter rainfall areas ('autumn saving').

However, studies in the 1960s clearly showed that this was not the case. Following these studies, it was shown that unless pastures were spelled for more than 10 weeks in autumn/winter there was no useful reduction in numbers of worm larvae.

The two influencing factors are:

• First, there is a time lag of several weeks, or even months, between eggs being deposited in the dung and the appearance of larvae on the pasture.
• Second, larvae can survive for many months during cool conditions.

In both winter and summer rainfall areas we now know that larvae from eggs deposited in the late summer and autumn can remain on the pasture in quite high numbers and infect sheep right up until mid-spring (when rising temperatures and increased sunlight finally kill them).

Knowing this, the 'Smart Grazing' strategy was developed for winter rainfall areas to give more reliable worm control in Merino weaner sheep during their first winter grazing period.

In summer rainfall areas, intermittent storms may occur in October and November. The intervals between storms are often hot and dry. Larvae released from dung pellets by the storms have a very short life expectancy under these conditions. Spelling paddocks during these periods will significantly reduce available larvae later on.

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 Nutrition

Nutrition has a major impact on how well sheep cope with worms. Good nutrition is a vital component of good non-chemical worm management.

Sound pasture management, strategic supplementary feeding and use of fodder crops all contribute to enhancing nutrition.

Also see:

• Introduction
• Nutrients
• Nutrition to enhance resistance and resilience to worms
• What you can do for young sheep
• What you can do for ewes
• Conclusion - summary
• Further information - reading list 

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 Fodder crops and crop stubbles

Cropped paddocks normally have extremely low levels of worm contamination. Ploughing, fallow, planting and the period until the crop can be grazed reduce worm larvae levels to very low levels.

By the time a fodder crop or stubble is ready to graze, there should be very few surviving larvae available to infest stock and are very useful for susceptible sheep such as weaners.

However, such 'worm free' paddocks can be a very important means of selection for drench resistance. If sheep are drenched onto the fodder crop, the only worms that remain are ones resistant to the drench used. Careful thought needs to be given about how to manage the paddock after the crop has been used.

Summer fodder crops, including lucerne, can also provide excellent nutrition with minimal worm contamination.

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 Rotational and cell grazing

The use of rotational and/or cell grazing to assist sheep worm control needs careful consideration of the time that non-grazed paddocks are left empty.

Traditional cell grazing systems, with frequent rotations between paddocks, generally do not leave enough time between grazings for significant reductions in pasture worm contamination levels.

During the cooler, wetter months of the year this could require several months stock-free.

It is critical to monitor worm egg counts to detect any increase in worm infestations before production losses and/or animal health occur.

When planning a rotational grazing system, it pays to get professional advice to make sure that the system will deliver the hoped for benefits.

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 Genetics-breeding worm resistant sheep

Resistance to worms in sheep is in part due to genes. As a result, improved worm control in sheep can be achieved by selecting and breeding with sheep that have greater genetic resistance to worms.

Breeding worm resistant sheep is possibly the most sustainable worm management tool that can be used.  Combined with other non-chemical management strategies, the need to drench can be very much reduced.

Commercial sheep breeders can use rams from studs that are selecting for worm resistance, and, over time, their sheep will become more and more resistant to worms and require less drenching.

Also see:

• The case for improving sheep resistance to worms
• Breeding for fewer worms
• What difference will it make and when?
• Sources of worm resistant sheep, especially rams
• How to identify worm resistant sheep
• Combining selection for worm resistance with other traits
• Other tests to identify worm resistant sheep
• More information on breeding sheep resistant to worms

 Why improve sheep resistance to worms

Sheep have some ability to control the numbers of worms they carry, through a process commonly termed host resistance.

Some benefits of sheep controlling their own worms include:

• They can be drenched less often
• By carrying fewer worms there will be lower pasture contamination
• They will be less affected by worms in bad 'worm seasons' than susceptible sheep

Sheep resistance to worms can be improved by stimulating acquired immunity, say through vaccination and/or nutritional means.

Another approach is to improve immunity by breeding from animals with superior worm resistance.

By breeding from sheep with high worm resistance it is possible to improve both

• Innate immunity (the inherent ability of some sheep and breeds to resist infection by worms) and
• Acquired immunity (which develops in sheep after they have been exposed to worms. Most adult sheep have good acquired immunity to worms, whereas lambs do not).

 Breeding for fewer worms

The process of choosing superior animals and using them as breeders is the basis of genetics-based stock improvement and is often called 'selective breeding' or 'selection'. For selective breeding to work the characteristic must be measurable and heritable (inherited) so the lambs will have some of the characteristics.

Resistance to worms is heritable and can be measured using individual worm egg counts.

Sheep with superior worm resistance carry fewer worms and pass fewer worm eggs in their droppings.

If weaner sheep are selected for worm resistance they remain resistant as adults and even during lambing and lactation, so the benefit is life-long.

Sheep selected for resistance to one species of worm (eg. barbers pole (Haemonchus contortus) or black scour worm (Trichostrongylus spp.) are generally also more resistant to other worm species.

 Breeding for resistance - What difference will it make and when?

Breeding for worm resistance takes time but it is a sustainable, long-term tool to assist sheep worm management.

Modelling has shown that by using the current method of selecting sheep on the basis of worm egg counts, it can take up to 10-12 years of consistent selection to significantly reduce the worm egg count to the point where fewer annual drenches are required.

This might seem like a long time but selection for worm resistant sheep is just like other breeding programs (eg. improvements in fibre diameter where reduction by one micron can take up to ten years of selective breeding).

Also improvements in technology (eg. the identification of a gene or genes for sheep resistance to worms) may speed up the selection process in the future.

Over this time the worm egg count in the flock may fall to half its original value.

A lower worm egg count also means less pasture contamination with infective larvae, thereby stalling the cycle of continual re-infection. Grazing paddocks with resistant sheep have fewer larvae and this in turn reduces worm burden of sheep that graze the pasture subsequently.

Lower worm egg count (and worm burden) means a reduced need for drenching.

In 'wormy' seasons resistant sheep survive better than animals not selected for resistance to worms.

 Sources of worm resistant sheep, especially rams

There is very little difference in resistance to worms between sheep from studs or commercial flocks that have not been selecting for resistance.

The fastest and cheapest approach to breeding worm resistant sheep is to identify rams that carry fewer worms through a low worm egg count, and breed from those animals.

Rams may be bred on-farm or bought from a stud that is selecting for host resistance.

Some of the Central Test Sire Evaluation Schemes are also testing rams for worm resistance and semen from these animals can be purchased.

A professional adviser can help you find a stud that is breeding worm resistant sheep that meet your other criteria for flock sires.

 How to identify worm resistant sheep

Breeders can identify resistant animals on their property, by taking individual worm egg counts and calculating a worm egg count Estimated Breeding Value (EBV) for each animal.

A ram's worm egg count EBV indicates how readily the ram's worm resistance will be passed on to its lambs.

Desirable, worm resistant rams that are predicted to sire lambs with lower worm egg counts have a negative worm egg count EBV. A very worm resistant ram will have a worm egg count EBV of -1.

On the other hand susceptible rams that are predicted to sire offspring with higher worm egg counts will have a positive worm egg count EBV. A very susceptible ram will have a worm egg count EBV of +1.

 Combining worm resistance with other traits

There is no marked genetic relationship between worm egg count and production traits such as fibre diameter, fleece weight and body weight. This means that if you picked out the best 20 rams on the production traits, 10 would be above average for worm resistance and 10 would be below average.

Therefore, it is possible to find rams that are both resistant to worms and have good production traits.

Some sheep selected for host resistance to worms tend to scour more than normal (hypersensitivity scouring) in some environments. Research is being conducted to find the reason for this.

 Other tests to identify worm resistant sheep

A worm egg count is currently the most direct measure of worm burden. However other tests, including tests for genetic markers, are being researched.

A laboratory-based blood test that detects levels of antibodies against worms has also been used.

Researchers are also investigating other methods of measuring worm infestations as an alternative to worm egg counts.  Results to date are promising in terms of speed of test and price.  Accuracy is being confirmed.

 More information on breeding worm resistant sheep

For more information on breeding sheep for worm resistance and the Nemesis Program please use the following links:

1. www.csiro.au/nemesis or Dr Sandra Eady, CSIRO Livestock Industries, Locked Bag 1, Armidale DC NSW 2350. Ph 02 6776 1394, Email sandra.eady@csiro.au
   
2. WA Department of Agriculture – Karlsson J, Greeff J & Woodgate R, (2003), Sheep worms - breeding worm resistant sheep

Contacts

Queensland: Maxine Lyndal-Murphy, Department of Primary Industries and Fisheries, Animal Research Institute, Fairfield Road, Yeerongpilly.  Tel: 07 3362 9447,  Email Maxine.Lyndal-Murphy@dpi.qld.gov.au

New South Wales: 
CSIRO – McMaster Laboratory, Armidale NSW 2350.
Amy Bell
Tel: 02 6776 1399,
Email: amy.bell@csiro.au

Victoria: No Contact

South Australia: No Contact

Western Australia:
John Karlsson
Department of Agriculture Western Australia, Katanning
Tel: (08) 9821 3221
Email: jkarlsson@agric.wa.gov.au

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