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Organics, sheep husbandry and fly strike

Organic NZ Magazine: 
March/April 2005
Dr Michael Morris

On conventional New Zealand farms fly strike is kept under control through a variety of means, including mulesing, tail docking, crutching and dagging sheep, and dipping and jetting with insecticides. The latter is not possible on organic farms, and other common techniques don’t fit well with Organics. This can make fly strike management a challenge. Animal welfare commentator Dr Michael Morris looks at the issues and the challenges.

While some organic farmers have reduced or eliminated fly strike simply through good hygiene and husbandry,1 in other cases such reduction comes at a high price in terms of labour. One Merino farmer for example told me that they have reduced fly strike to manageable levels only by continual monitoring of sheep, to the extent that they were “unable to take a Christmas break like other people”.2


The problem

Fly strike in sheep is a major economic concern to New Zealand farmers, with approximately three–five percent of sheep suffering from the disorder each year. Fly strike is caused by rapidly growing maggots that eat the living flesh of the sheep. Flies are attracted by matted faeces (dags) and urine on sheep, so they often lay their eggs on the breech area (breech strike).3

Sheep show signs of irritation during the first two days after eggs have been laid. Once fly strike has been initiated, further flies are attracted to the site. Maggots secrete ammonia, and the sheep can die from ammonia poisoning three–six days from the onset of the first strike.4

Fly strike is therefore an animal welfare issue as well as an economic one, and any improvement in animal husbandry that can reduce or eliminate this disorder needs to be encouraged. Unfortunately, research, prevention and control by the sheep farming community often exacerbate, rather than reduce, the welfare problems associated with fly strike.


Mulesing – putting sheep under the knife

Australian and New Zealand flocks both include a significant proportion of Merino sheep, valued for their fine quality wool. This breed is particularly susceptible to breech strike, because of the folds in the skin that trap dags, urine and sweat.

To prevent fly strike, Merinos and Merino crosses are subjected to an operation known as mulesing, whereby large areas of the skin are cut, to smooth out the skin surface. This operation is performed without anaesthetics or analgesia. One set of instructions for this operation recommends making six cuts, to produce two wounds with a maximum width of five–seven cm from the buttocks to the hock.5

The mulesing operation is far from painless. Mulesed lambs have demonstrated abnormal behaviour indicative of extreme pain 24 hours after mulesing and some were still in pain after two days. Levels of endorphins (natural pain killers) and cortisol (the “stress” hormone) were still high 24 hours after the operation. Combined with the behavioural data, this confirms a high degree of pain or stress. Mulesed lambs remembered the procedure and avoided the handler for 36 days afterwards.6

In spite of admitting that the operation is painful, the official New Zealand government code for animal welfare supports mulesing in Merino sheep.



New Zealand farmers dock the tails of sheep with the sole purpose of preventing dags from forming around the tail. Commonly used methods are to cut the tail with a sharp knife or to place a rubber ring around the tail. The latter operation causes the tail to shrivel and drop off due to lack of blood supply.

Sheep that have had their tails docked surgically or with rubber rings have shown behaviour indicating pain for up to 30 minutes after the operation. Sheep with tails surgically removed also had elevated cortisol levels 24 hours after surgery.7 In addition, bundles of peripheral nerves (neuromata), indicative of chronic long-term post amputation pain, have been found in docked lambs’ tails.8

Tail docking without an anaesthetic is permitted in lambs under 12 weeks of age in New Zealand. The effectiveness of tail docking in preventing fly strike is questionable,9 and organic Merino farmers have shown that mulesing is not necessary on organic farms.10


Gruesome experiments

In a previous article I mentioned intrusive cloning experiments performed on cattle in New Zealand, aimed at improving productivity.11 Unnecessarily intrusive experiments are also performed on sheep, in an attempt to understand or prevent fly strike. Experiments in the 1980s in Australia and New Zealand involved artificially inducing fly strike to advanced stages for the questionable objectives of quantifying sheep weight loss,12 and closely monitoring their death throes.13

Even within the last 10 years, New Zealand experimenters have allowed flies to develop to stages where welfare has been compromised.14 For example, one insecticide trial involved placing larvae onto the skin surface of sheep, and monitoring the effectiveness of insecticides in killing the larvae. The authors admit that their studies have “implications for animal welfare”.15 Another trial involved smearing liver or dung on to the skin of sheep, and exposing them to flies for nine days, until late instar maggots were present.16


Necessity demands another way

Fortunately, conventional farmers are being forced to place less reliance on chemical controls, under pressure from customers such as wool exporters who are concerned with the health risks connected with chemical insecticides and are demanding withholding periods of up to six weeks before shearing.17 During this period, the sheep can become particularly susceptible to fly strike, so finding nonchemical control methods has become vital. This has led to improvements in the quality and quantity of research into non-chemical alternatives for fly strike management.

Overseas markets are also becoming increasingly sensitive, not only to chemical residues, but to the animal welfare implications of farming. Tail docking is permitted under New Zealand Bio-Gro certification, and mulesing can also be allowed under restricted circumstances.18 However, allowing mutilations does not appear in keeping with the ethical image organic producers want to portray and may leave organic farmers open to a consumer backlash.

The animal rights group People for the Ethical Treatment of Animals (PETA) recently launched an international campaign to boycott Australian wool because of welfare concerns with the wool and sheep meat industry. Their main concern was with the mulesing operation, although they also mention live sheep shipments, tail docking and hypothermia in sheep caused by premature shearing. The PETA campaign has so far proved effective in forcing the Australian wool industry to consider a phase out of the mulesing operation by 2010.19


Opportunites for Organics

With increasing concern over welfare issues in wool production, stimulated by the PETA campaign, organic sheep farmers in New Zealand have a perfect opportunity to promote an ethical alternative. This would have to include a ban on mulesing and tail docking, but researchers have discovered that sheep fed on pasture plants with more roughage are less susceptible to dag build-up, and therefore to fly strike, than sheep fed on grass.20 Other approaches being investigated include selective breeding of “ethically improved sheep”, with shorter tails and less wool around the breech to collect dags and urine.21

Other projects that show promise include biological control of strike flies with parasitic wasps,22 and a synthetic bait that can trap blowflies or lure them away from sheep.23



  1. Dymock, J.J. and Forgie, S.A. (1995) Large scale trapping of sheep blowflies in the northern North Island of New Zealand using insecticide-free traps. Australian Journal of Experimental Agriculture 35, 699-704. Morris, M.C. (2000) Ethical issues associated with sheep fly strike research, prevention and control. Journal of Agricultural and Environmental Ethics 13, 205-217
  2. Morris 2000 (ibid.).
  3. Heath, A.C.G. and Bishop, D. (1995) Flystrike in New Zealand. Surveillance 22, 11-13.
  4. Sandeman, R.M., Collins, B. and Carnegie, P. (1987) A scanning electron microsopy study of L. cuprina larvae and the development of blowfly strike in sheep. International Journal for Parasitology 16, 69-75.Guerrini, V.H. (1988) Ammonia toxicity and alkalosis in sheep infested by Lucilia cuprina larvae. International Journal for Parasitology 18, 79-81.
  5. Gherardi, G.S. and Seymour, M. (1996) Mulesing for flystrike control. Agriculture Western Australia, farmnote.
  6. Fell. L. and Shutt, D. (1989) Behavioural and hormonal responses to acute surgical stress. Applied Animal Behaviour Science 22, 283-294.
  7. Shutt D.A., Fell, L.R., Connell, R. and Bell, A.K. (1988) Stress responses in lambs docked and castrated surgically or by the application of rubber rings. Australian Veterinary Journal 65, 5-7.
  8. French N.P & Morgan K.L. (1992) Neuromata in docked lambs' tails. Research in Veterinary Science 52, 389-390.
  9. French, N., Wall, R., Cripps, P.J. and Morgan, K.L. (1994) Blowfly strike in England and Wales: the relationship between prevalence and farm and management factors. Medical and Veterinary Entomology 8, 51-56.
  10. op cit., note 1.
  11. Morris, M.C. Animal welfare and the beef and dairy industry. Organic New Zealand, May 2003, p.46-47.
  12. Heath, A.C.G., Bishop, D.M. and Tenquist, J.D. (1987) The effects of artificially induced fly-strike on food intake and live weight gain in sheep. New Zealand Veterinary Journal 35, 50-52.
  13. Guerrini, 1988 (op cit., note 2).
  14. Morris, M.C. (2003) Issues associated with research on sheep parasite control in New Zealand – a descriptive ethic. Journal of Agricultural and Environmental Ethics 16, 187-207.
  15. Wilson, J. A., Heath, A.C.G., Stringfellow, L., Haack, N.A. and Clark, A.G. (1999) Relative efficacy of organophosphorus insecticides against susceptible and resistant strains of the strike blowfly Lucilia cuprina (Calliphoridae) in New Zealand sheep. New Zealand Veterinary Journal 44, 185-187.
  16. Blackwell, G.L., Potter, M.A., Cottam, Y.H. and Blair, H.T. (1997) Susceptibility of Romney and Perendale sheep to flystrike by the Australian Green Fly, Lucilia cuprina (Weid.), and fly attractant trials. Proceedings of the New Zealand Society for Animal Production 57, 37-40.
  17. Anon (1997) Farmers and scientists tackle Aussie blowfly. AgResearch Science 13, 10 ( Anon (1999) Flystrike awareness, management and control. AgResearch, AgFact no. 29. (
  18. BioGro standards, Module 4.3, Livestock Production, version 1, April 2001.
  19. PETA Press Release 9 November 2004, “Group denounces industry’s transparent attempt to forestall retailers’ action”.
  20. Leathwick, D.M. and Atkinson, D.S. (1995) Dagginess and flystrike in lambs grazed on Lotus corniculatus or ryegrass. Proceedings of the New Zealand Society of Animal Production 55, 196-198.
  21. Scobie, D.R., Bray, A.R. and O'Connell, D. (1997) The ethically improved sheep concept. Proceedings of the New Zealand Society of Animal Production 57, 84-87. Scobie, D.R., Bray, A.R. and O’Connell, D. (1999) A breeding goal to improve the welfare of sheep. Animal Welfare 8, 391-406.
  22. Bishop, D.M., Heath, A.C.G. and Haack, N.A. (1996). Distribution, prevalence and host associations of Hymenoptera parasitic on Calliphoridae occurring in flystrike in New Zealand. Medical and Veterinary Entomology 10, 365-370.
  23. Farrell, R., Ward, M. and Horton, B. (2001). Use of LuciTrap by groups of wool producers to reduce pesticide applications 1998-2001. Final Report DAQ 146, Department of Primary Industries, Queensland. Morris, M.C. (2005) Tests on a new bait for strike flies (Diptera: Calliphoridae). New Zealand Journal of Agricultural Research, in press.