It is intended here to mention only the relation of sheep conformation to fly susceptibility and methods by which sheep may be rendered less susceptible to fly strike.
Crutch strike in ewes is by far the most common type of fly attack, but a btief mention to head strike in rams and body strike will be made, as these three types of strike are all associated with the conformation of the sheep.
Body strike in normal seasons is fortunately, not very common, but in severe "fly waves" and in very wet seasons it may become serious. Probably the most common sites are extensions from the withers and back down the sides and down the shoulders. Weak-backed sheep, those with "devil's grip" and those where the wool is less dense and often quite open along the back, allow the entrance and retention of rain and encourage the development of water rot. Such a condition is very favourable to the fly, and among such sheep body strike is common. By proper selection it is possible to cull out those sheep that are not densely covered along the back, and so reduce the incidence of water rot and body strike.
Head strike always occurs to some extent in Merino rams, and although injuries received in fighting undoubtedly encourage the flies, it can readily be seen that many of these head strikes occur in rams in which the horns are set close to the side of the head and in the more wrinkly-headed rams. Normal selection eliminates this type of ram from the flock as such characters are undesirable.
Crutch strike, assuming its major importance in ewes, is by far the commonest type of strike and is very intimately connected with conformation of the breech. Wrinkly crutched ewes—that is, those classed as C. type—are highly susceptible to fly-strike when compared with the less wrinkled B. type and the plain crutched A. type sheep.
These breech wrinkles may be classified according to their site as medial folds, lateral folds, lower crutch folds and tail folds. The lateral folds may continue down towards the hock in very wrinkly sheep, while the medial folds often continue to the crutch folds. The folds include large flaps on the side of the tail, together with folds on butt of the tail extending down to the tip.
If sheep are not watched closely during severe fly season, they may not be found until the strike has extended over a considerable area involving most of the above folds. If, however, recently struck sheep are examined, the majority of original strikes will be found to occur on the medial folds and the tail flaps or tail tip, and that the maggots spread from these to involve other areas. It appears rather strange that, of recorded strikes, more occur on the left side of the breech than on the right while up to the present time the reasonfor this is unknown.
Reducing Susceptibility of Ewes to Crutch Strike.
Owing to the continued failure to discover an ideal fly dressing which will, apart from killing maggots and healing wounds of an original strike, repel the flies for some considerable time after application, it has been necessary to consider other means of controlling or reducing the fly problem. That which has been receiving considerable attention lately has been the means by which sheep may be rendered less susceptible to crutch strike, by eliminating crutch wrinkles.
Three methods by which this object may be achieved are:—
1. Surgical removal of crutch folds.
2. Application of caustic for removal of crutch folds.
3. Breeding out crutch wrinkles.
Removal of folds by application of caustic has been done on at least one occasion in Queensland, but as treated sheep subsequently failed to grow wool on the breech area, no encouragement can be given to this method of fold removal.
The surgical removal of crutch wrinkles, known as the Mules Operation, entails the entire removal of wrinkles from the region of the breech, preferably when sheep are young, as during the lamb marking operation. Experiments with the Mules Operation has shown that it definitely renders sheep less susceptible to crutch strike. It, however, is only temporary in that each succeeding drop of lambs must be operated upon in order to reduce their susceptibility. The operation is definitely painful to the lambs, but as against this objection, treated sheep would suffer considerably less from the ravages of the fly. Should this operative method for fold removal become wide-spread, it will be very necessary to adopt some permanent mark for the identification of nil treated sheep.
The selection and use of plain breeched rams and ewes for breeding appears a much more permanent method of reducing susceptibility to fly by eliminating crutch wrinkles. Progeny of such parents are definitely more plain in the breech and consequently less susceptible to fly than sheep bred from more wrinkly-breeched parents. Many Merino breeders, however, state that the A. type plain-breeched ewes are not as profitable as the more wrinkly ewes in wool production, and that by eliminating the C. type ewe, considerable loss in fleece weight and wool quality would result. Recent experimental knowledge, however, has not supported these statements, and the conclusion drawn by Belichner, Carter and Turner on experiments conducted by them, summarises the position: "Under the conditions ofthe experiment no marked difference was evident in the wool production of A. and C. type merino ewes. Though the A. type may produce a somewhat lower weight of greasy fleece, the yield is higher, so that no difference is apparent in the weight of clean scoured wool produced. In addition to this, A. type fleeces are inclined to be ofgreater commercial value."