Cleaner wool with fewer chemicals

Prof Schalk Cloete and Ansie Scholtz of the Animal Production Department, Elsenburg, tell Roelof Bezuidenhout how SA�s wool industry can get in line with international clean and green initiatives.
Issue date: 07 March 2008

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Prof Schalk Cloete and Ansie Scholtz of the Animal Production Department, Elsenburg, tell Roelof Bezuidenhout how SA’s wool industry can get in line with international clean and green initiatives.

There’s ever-increasing pressure on farmers to reduce chemical residues in agricultural products exported to Europe. This could soon force South Africa’s wool growers to carefully review the way they manage blowfly, sheep lice and sheep scab in their flocks. A ustralia, our biggest competitor in the Merino wool market, is already establishing its national clip as environmentally friendly. There, the incidence of important pollutants such as organophosphates (OP) and synthetic pyrethroids (SP) has declined and the Aussies have the figures to prove it. Meanwhile, the Australian wool industry is trying to replace these chemicals with more acceptable ones. But trade agreements and competition aren’t the only threats to the industry. External parasites are developing resistance to chemical treatment, limiting control options.

The logical way out of this dilemma, say Prof Schalk Cloete and Ansie Scholtz of the Animal Production Department at Elsenburg, is to try management strategies that will reduce the need for chemicals, while making more compounds available that are less harmful to the environment. “These include insect growth regulators such as Cyromazin and Diflubenzuron,” says Prof Cloete. “Spinosad, registered overseas for use in long wool sheep is a relatively new one but not an option for South African wool farmers, since it isn’t yet registered for local use. However, it may be the perfect pesticide to treat long-wool sheep in the face of an unexpected fly wave.” Researchers believe that in the short term there should be more focus on developing fly traps linked to grazing management, and on strategic decisions about shearing (versus sheep lice and blowfly) and crutching (versus blowfly).

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“Over the long term, breeding sheep genotypes that are resistant to parasites seems to be the only real option, even though this could influence other economically important traits in the animals,” says Scholtz. “At present, there isn’t a great deal of information on possible undesirable side effects that may stem from such a selection policy. There’s also scope for breeding sterile insect males. This is already being done in South Africa’s fruit fly, codling moth and false codling moth populations.” She adds that because sheep lice, sheep scab and blowfly cause the biggest problems in wool flocks – in terms of production losses as well as chemical residues – it would make sense to concentrate on alternative ways of managing these pests. Prof Cloete and Scholtz give farmers a rundown on current methods of handling sheep lice, sheep scab and blowfly.

Sheep lice and sheep scab

It’s always been quite easy to control, or even eradicate, sheep lice (Bovicola ovis) and sheep scab (Psoroptes communis ovis) by dipping. They only survive on sheep, so the whole population is exposed when a flock is dipped. But sheep lice’s resistance to SPs in particular has been noted since the 1980s. lready 90% of Queensland’s lice population are resistant to pour-on remedies containing SPs. OPs still work well but dips containing them could soon be taken off the market. S heep lice are also becoming a problem because of changing management practices. When dipping sheep against scab was still mandatory, sheep lice were probably kept on the edge of survival, but since dipping has been replaced by injections such as Ivermectin – which control scab, but not sheep lice – lice populations are booming. The process is helped by the fact that farmers are increasingly using insect growth regulators such as Cyromazine, which don’t kill lice either, against blowfly attacks.

In moving away from chemical treatments for sheep lice or sheep scab it’s critical to prevent re-infestations. The first step is to reduce close contact with other sheep, particularly untreated ones. One heavily infested sheep has been shown to re-infect 80 treated sheep within five months. Interestingly, breeds and individuals show significantly different levels of resistance and build-ups of lice populations. Sheep have to be regularly inspected for lice but often the signs associated with heavy infestation are quite clear. Animals scratch or rub against poles and look untidy. Nevertheless, Australia has been testing a chemical method for detecting lice in flocks.

How to limit chemical use

While injections aren’t effective, these lice have reasonably narrow comfort zones. They die at high temperatures or when a fleece stays wet during long periods of rainy weather. Sheep lice don’t even survive well in bits of wool that get hooked on fences, so don’t worry too much about infecting the flock in that way. But the lice can live up to 10 days in the woolly shoes worn by Australian shearers if the shoes aren’t placed in a microwave oven or fridge. Good dipping procedures are essential. For example, the Greg Richards dipping cage allows sheep with any length of wool to be dipped without exceeding accepted levels of residue in the fleece. Shearing can remove up to 66% of lice, and many of the survivors die from exposure to unfavourable conditions in the short wool. Simply drenching the fleece with clean water through a high pressure spray can reduce infestations. Adding an insect-repellant soap can kill up to 67% of the lice. Shearing in combination with thorough dipping in clean water can be nearly 90% successful.

The sheep blowfly Blowflies (Lucilia cuprina) do great damage to the wool as well as the animal and are hard to control, even with chemicals. However, DNA studies have shown that blowfly populations don’t migrate far, opening the possibility for strategic, localised control techniques focused on geographic areas even as small as 10km². Control measures include insect traps such as the Lucitrap which have already helped reduce blowfly populations on farms in the Western and southern Cape. The traps also help to monitor insect populations, allowing farmers to take preventative action before severe outbreaks begin. In the case of blowfly, the problem of chemical residue is worsened because it’s usually sheep with long wool that need treatment. And, as is the case with lice, blowflies are becoming resistant to chemicals. However, there are no signs yet that populations are resistant to Cyromazine, which is a relatively environmentally friendly compound. Spinosad can be used for spot treatments, but it’s expensive and not yet readily available. Shearing and crutching help to prevent attack. Shearing at the right time – when blowfly strike is more likely – helps even more. Don’t cut tails too short. Destroy blowfly maggots in shorn wool either by spraying them with dip, burning them or burying them.

Strategic management

Use strategic grazing management for better control of susceptible flocks. Another possibility is to plant pastures such as bird’s foot that are high in condensed tannins. The tannins help limit a build-up of lox in the wool, thus reducing blowfly strike. In Australia sheep bloodlines that are more resistant to blowfly than others are already available. Locally, sheep that tend to be prone to blowfly strike on their bodies have been found to have poorer scores for wool quality, uniformity of crimp, and the colour of the wool oil. However, fleece weight can suffer as a result of selection for blowfly resistance. Finding a genetic marker for resistance would help solve the problem. Hampering selection against fly strike is the large difference in the number of attacks from year to year. Droughts reduce attacks considerably, making it hard to identify susceptible sheep. Some Tasmanian farmers believe young sheep shouldn’t be treated at all, but placed in a high-risk area so that those most prone to attack can be culled. Once the most resistant animals have been selected, they can be managed as normal. This strategy could work but will probably be criticised from an animal ethics perspective. The introduction of sterile male parasites, which limit the growth of populations, holds some promise for the future. Contact Prof Schalk Cloete on (021) 808 5230, or e-mail [email protected] Contact Ansie Scholtz on (021) 808 5231 or e-mail [email protected]