Sheep genetics
Selection of Merino sheep resistant to body strike
Warren Hunt, formerly DPI&F
Dr Roger Lewer, DPI&F
Introduction
The Australian sheep blowfly (Lucilia cuprina) is a major parasite of Merino sheep in Australia that mostly effects animal health and productivity. Management relies heavily on pesticides to prevent and treat blowfly strike, however other tools can minimise its use - and residues on wool - as part of an integrated parasite management (IPM) program. One option is to breed sheep for resistance to body strike.
Resistance to body strike is a heritable trait in Merino sheep that can be selected against. Heritable traits are those characteristics in which the superiority of the parents is passed to their progeny. Wool producers can increase resistance to body strike in their flock by:
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choosing a stud that actively selects against body strike;
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selecting replacement rams with plain body types;
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culling hoggets (ewes and wethers) at classing that have excess wrinkle, devil's grip, lumpy wool, fleece rot, discoloured and yellow wool;
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retaining hoggets with white, waxy wool;
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culling any sheep that have to be treated for flystrike (except for poll struck rams).
Mulesing and breech jetting reduce the capacity to select against fly strike in these areas because the potential for challenge is removed. However, there is opportunity to improve resistance to body strike because it has a close association with fleece rot, one of the major predisposing causes of body strike in sheep in the eastern states of Australia. Research has shown that selection against fleece rot, in combination with selection against body strike, will deliver faster progress. This is due to fleece rot being present at a higher incidence than actual body strike.
Predisposing factors to body strike
Fleece rot
Fleece rot can be present at varying intensities, ranging from little more than a simple leaching of the fleece with colour bands (water stain), to a matted band of fibres with extensive gummy exudate (wool rot). Pseudomonas bacteria are the principal precursors for this condition.
Dermo/lumpy wool
This is a dermatitis infection arising from chronic infection with Dermotophilus sp. bacteria.
These two conditions lead to the development of body strike via:
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attracting female blowflies and encouraging deposition of eggs;
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providing moisture for the eggs to hatch;
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conditions in the infected fleece providing soluble protein for the young larvae.
There is evidence (Atkins and Mc Guirk, 1979 and Raadsma, unpublished) of a high genetic correlation (0.9 or 90 per cent) between fleece rot incidence and/or severity and incidence of body strike.
Genetic variation in susceptibility to fly strike
Genetic variation has been observed between, and within, different Merino strains and bloodlines in their resistance to fly strike. This is brought about by variation in the fleece and skin characteristics and in the immune system. These factors influence the effect of moisture and pathogens in the development of fleece rot and body strike.
Evidence of the resistance of different strains of Merino sheep to fly strike can be observed in long running trial flocks at Trangie Research Station in New South Wales and Mt. Barker in Western Australia (Table 1). In these cases sheep were selected and bred on their respective resistance or susceptibility to fleece rot and body strike over a number of generations.
Table 1. Differences in fleece rot, body strike and dermo between flocks selected for or against fleece rot susceptibility. Incidences are based on the last three drops of progeny in each line. (Sourced from unpublished data from Raadsma & Beetson via Raadsma & Rogan 1987).
| Experiment | Trangie NSW | Mount Barker WA | |||
| |
|
tible |
Resistant | Random |
Suscep- tible |
| No. of animals scored |
326 |
234 |
121 |
293 |
140 |
| Fleece rot incidence % | |||||
| Weaners (natural) | - | - | 30 | 49.5 | 79.7 |
| Hoggets (natural) | 5.4 | 38.5 | - | - | - |
| Hoggets (induced) | 56.4 | 82.9 | - | - | - |
| Body strike incidence % | 2.3 | 8.2 | - | - | - |
| Dermo incidence | - | - | 0.0 | 1.4 | 3.4 |
Selection for resistance
Sustained progress in breeding sheep more resistant to body strike can be achieved through gradual introduction of a resistant bloodline (ie continued purchase of rams from a stud actively selecting against fleece rot and body strike).
In a self-replacing commercial ewe flock, the first drop of F1 progeny would have a liability of half of the difference in the average body strike susceptibility between the two parental lines assuming the environments were similar.
Producers also have the option to improve resistance to body strike via within flock selection. This can be done through:
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direct selection, through exclusion or culling of body strike affected animals from the breeding flock;
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indirect selection on the basis of a correlated trait or series of traits (eg susceptibility to fleece rot); or
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a combination of both approaches.
This approach, however, has a different effect to buying rams from a flock in which resistance is a selection objective. These rams will bring with them genetic improvement that is cumulative and permanent. Ewe selection in commercial flocks will improve the resistance of the current ewes, but not of their progeny and is a temporary effect.
Other means of selection
When the incidence of fleece rot or body strike is low, there are other indirect means of selection for resistance. Selecting against yellow colour in greasy wool should lead to a reduction in the incidence of fleece rot. The greasy colour score of wool (ie yellowness) can be easily assessed, it is heritable and its genetic correlation with fleece rot is around 0.4.
Soft wool handle, coefficient of variation (CV) in fibre diameter and correlations between fleece rot and fleece weight have also been investigated as influencing resistance and susceptibility to fleece rot and fly strike. High CVs also appeared to have a significant influence in making sheep more susceptible to fleece rot (Watts, 1981 and Raadsma, unpublished).
Further information
Breeding and selection of merino sheep resistant to body strike as part of an integrated parasite management (IPM) program is described in the book Blowflies and lice information manual: A practical approach to producing low residue wool (DPI Shop Online).
For further information contact DPI&F on Telephone 132523 (Queensland residents) or (07) 3404 6999 (non-Queensland residents) between 8 am and 6 pm weekdays, or e-mail callweb@dpi.qld.gov.au.
This DPI Note is also published on the DPI's PrimeNotes CD-ROM.
Information contained in this publication is provided as general advice only. For application to specific circumstances, professional advice should be sought. The Department of Primary Industries Queensland has taken all reasonable steps to ensure the information in this publication is accurate at the time of publication. Readers should ensure that they make appropriate inquiries to determine whether new information is available on the particular subject matter.
File No: SW0005 . Created: Jan 2000 . Revised: Dec 2001
