Australia is famous for her sheep, but Australian sheep are not famous for their fecundity! Fecundity relates to the power of a species to increase in numbers, and high fecundity is an important characteristic of domestic animals of use to man. The fecundity of a flock governs the number of surplus animals available for sale as lambs, as young culls, or as cast for age ewes, and as surplus wethers. It also influences the rate at which flocks are rebuilt after drought losses and the speed with which sufficient sheep can be bred to stock the ever-increasing areas sown to improved pastures in Australia's high rainfall zone and the wheat/sheep zone.
The national average of ewes marked to lambs mated is below 70 per cent1. Certainly, seasonal conditions sometimes contribute to the unsatisfactory results but many other factors have to be considered. During the last 45 years, diseases causing infertility have held the interest of veterinarians in most parts of the world. In Australia, work completed since the end of World War II has shed considerable light on the contribution of disease to the low reproduction rates recorded in the national flock, but the Australian approach has been slightly different from that followed in many other English speaking countries. Work in England and the United States has been concerned mainly with infectious diseases; work in Australia has included investigations into the nutritional and metabolic diseases, as well as the infections.
INFECTIOUS DISEASES AS A CAUSE OF OVINE INFERTILITY
Many of the infectious diseases causing infertility and/or abortion are present in Australia. Features which must be taken into consideration in assessing their economic importance include geographical distribution, incidence from year to year and level of loss the disease causes. This last point must include information about the number of properties affected and the losses recorded on each.
The diseases which cause infertility and/or abortion reported6 so far in Australia are Brucellosis, Listeriosis, Pasteurellosis, Toxoplasmosis, and the P.L.G. Virus abortion. Of these, Brucellosis is the only disease known to produce infertility, although some suspicion currently surrounds Toxoplasmosis.
Brucellosis appears to occur in many districts in Australia and it apparently occurs more frequently in British breeds than in Merinos. This breed difference may be due partly to the higher stocking rates experienced by British breeds. Should this be so, the disease may become more prevalent as stocking rates of Merinos increase on improved pastures. Brucellosis may occasionally cause quite spectacular "storms" of ovine abortion, but these appear to be infrequent and can hardly be regarded nationally as a serious cause of loss. This disease is spread from rams to ewes at mating; there seems to be little carryover of the infection in the ewe and no spread of infection from ewe to ewe. Affected rams may have palpable lesions, though some animals without lesions excrete the organism in their semen. The wastage among rams caused by this disease varies; in British breeds it may be high7 but it is usually low in Merinos8. The severity of the infertility caused by infected rams has not been investigated adequately. In mating trials in Queensland, infected rams sired fewer lambs than healthy rams, but this work needs to be repeated under conditions associated with normal flock management.
Vibriosis is most commonly manifest by abortion or a high incidence of neonatal deaths. This disease may have a wide geographical distribution2, but its seasonal incidence is extremely variable. When it occurs, losses may be heavy but its sporadic and apparently infrequent occurrence suggests that, at present, it is not a serious problem in the Australian sheep industry. However, judging from experience in the United States, Vibriosis may well be the disease so far reported in Australia with the greatest potential for causing serious economic loss from abortion.
Listeriosis, Pasteurellosis, Toxoplasmosis and virus abortion have all been recognised in New South Wales, as well as in other Australian States. However, there is no evidence to suggest at present that these diseases are prevalent, or that they have a wide geographical distribution, a continuous incidence, or that they cause serious losses. All these diseases. with the exception of Toxoplasmosis, produce oedema of the membranes, necrosis and ulceration, and thickening of the intercotyledonary chorioallantois and necrosis of the cotyledons. Toxoplasmosis produces multiple small necrotic foci in the cotyledons. In the foetus, Listeriosis produces small multiple necrotic foci in the liver and lung, and in Pasteurellosis, these may occur in the liver and kidney.
A number of organisms may infect lambs after they are born. These include Clostridia, Corynebacterium, Pasteurella haemolytica, Salmonellae, Staphylococcus and E. coli. Surveys completed so far suggest that this wide assortment of organisms cause comparatively few deaths among lambs. However, because infected lambs may linger for a few days and because the symptoms and post-mortem findings may be spectacular, woolgrowers may become alarmed at the appearance of what they regard to be a "new disease".
Infections of the ram may cause infertility. Conditions specific to the scrotal contents include abscesses due to Corynebacterium, Brucella ovis, and Actinobacillus seminis5. Of these, Brucellosis is the only condition that has received much attention and adequate control measures now appear available. Other infections causing pyrexia in rams may also cause seminal degeneration which may impair fertility4.
PARASITISM AND FERTILITY
Cutaneous myiasis may reduce reproduction rates through loss of breeding ewes, unmothering (sic) and neglect of lambs and perhaps from other causes which have not been explored. The overall position is summarised in the following table3.
Effect of multiple strikes from before mating to end of lambing on the proportion of wet ewes.
|Number of strikes||% wet ewes|
|Protected by Mules Operation|
|Not protected by Mules Operation|
|Struck three times||58|
|Struck four times||47|
Parasitic mange on the scrotum of the ram may also lead to increased testicular temperatures and hence to seminal degeneration.
NUTRITIONAL DISEASES AND REPRODUCTION
Considerations of nutritional diseases and fertility raise interesting philosophical points. Some workers contend that animals which are not producing at their maximum potential are diseased. This concept raises questions about the relationships between body condition and reproductive performance. Both New Zealand and Australian workers have found that during the breeding season a relationship exists between body weight and ovulation rate. Thus the expression of a propensity for twinning is influenced by the condition of the ewes at mating time. Similarly the birth weight of lambs is influenced greatly by the nutritional welfare of the mothers during late pregnancy11. The consequences of inadequate nutrition include small light lambs, delayed onset of lactation, severe neonatal mortality and sometimes serious losses of ewes from pregnancy toxaemia9.
Of these conditions neonatal mortality is probably the most important contributor to low reproduction9, but disease appears to be comparatively unimportant6.
Australia is currently witnessing the effects of a pasture revolution. Over 30 million acres in the high rainfall zone of temperate Australia have been sown to improved pastures, based largely on subterranean clover Over 80 years have elapsed since subterranean clover was first recognised as a valuable pasture plant in Australia.
A little over 20 years ago, it was realised that, in certain circumstances, subterranean clover contained isoflavones, which induced symptoms of hyper-oestrogenicity in grazing sheep10. The classical syndrome included dystocia, neonatal mortality, eversion of the uterus, lactation in wethers and maiden ewes. Spectacular decreases occurred in reproduction rates on affected farms but the gravity of the problem has not been defined adequately. More recently, attention has been drawn to the possibility that ewes grazing oestrogenic pastures at mating time may suffer from temporary infertility and a depression of ovulation rates. This finding raises an important and challenging question; do improved pastures contribute to the unsatisfactory reproduction rates of flocks in some of the more favoured pastoral areas? Goitre may also occur quite commonly among lambs born to ewes grazing improved pasture. This condition is known to be a manifestation of nutritional disturbances but the hypothesis that there may be a genetic predisposition to the disease is challenging and raises many opportunities for further investigation.
SOME PHYSIOLOGICAL CONSIDERATIONS
In most circumstances, the recognition of disease depends on the presence of symptoms, clinical signs and/or lesions. Abortion, still-births, sanguineous discharges, eversions, abnormalities of the male reproductive organs and of the semen usually lead to more detailed investigation. However, the loss of fertilised eggs, of embryos before implantation, or low fecundity of the ram flock usually escape detection. The loss of one or two ova while the other implants escapes notice and debars the ewe from subsequent opportunities to bear twins during that breeding season. Thus the fertility of a flock may appear to be normal, although its fecundity is depressed. Current investigations (Braden, pers. com.) suggest that gene x environment interactions may contribute to such a situation. Here again, strain x environment interactions may be important.
Such findings re-emphasise the philosophical question already posed. Should animals that are not producing at their maximum potential because of inapparent dysfunction be regarded as being diseased?