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This article was published in 1981
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Summer Infertility in Sows
Dr. R.J. Love, M.V.Sc., Ph.D., M.A.C.V.Sc. , Senior Lecturer, Dept. of Veterinary Clinical Studies, University of Sydney NSW
Summer infertility is a major problem in the Australian pig industry which causes substantial economic loss every year. The problem appears to be worldwide and has been reported from the U.S.A., U.K., France and Yugoslavia.
TIME OF THE YEAR
The problem is generally referred to as a summer infertility but unfortunately it does not coincide very well with the summer period (i.e. the hottest period of the year). Most studies have found that the reduction in fertility commences with sows that are mated from mid-summer and continues until mid-autumn (January, February, March, April, or in piggery management terms, Week 1-16). However, there is some variation in onset, duration and severity of the problem in different piggeries, and in the same piggery from one year to the next (Hurtgen et al. 1980 (a); Love 1978; Paterson et al. 1978).
MANIFESTATIONS
Anoestrus
A very common problem seen during this period of the year is that of sows failing to express oestrus within 10 days of weaning their previous litter. In field situations this problem is very apparent but only one detailed study of this problem has been made (Hurtgen et al. 1980 (b)). This study showed that the post-weaning anoestrus problem was most severe in first parity Sows. It also indicated that individual housing of sows and cooling during the weaning to mating interval had no beneficial effect. Another aspect is the failure of gilts to commence cycling.
Return to oestrus after mating
Most studies have shown simply an increase in returns to oestrus after mating during the period of infertility. Some studies have looked at the interval from mating to the return oestrus and these have shown a normal proportion of sows returning after a normal cycle length (18 - 24 days) but an increase in delayed returns to oestrus (24 days). Most of the mating to return intervals were in the range of 25 to 37 days (Love 1978; Paterson et al. 1978).
Abortions
The incidence of observed abortions has been shown to be higher in sows mated during late summer and early autumn (Stork 1979). There is always a problem observing early abortions particularly where sows are group housed. There is no evidence indicating at what stage of gestation these abortions are occurring.
Not-in-Pigs
Sows found not-in-pig (N.I.P.) when due to farrow are a very important component of the seasonal infertility problem. These are again highest in those sows mated during the summer-autumn period (Love 1978; Stork 1979).
Litter size
There is general agreement amongst the studies that have been done that there is no significant seasonal effect on numbers of piglets born. Those sows that conceive and maintain a pregnancy produce a normal litter of piglets (Love 1978; Paterson et al. 1978).
There are two components which should be considered separately:
(i) anoestrus and
(ii) having expressed oestrus and been mated, the failure of the sow to produce a litter.
Anoestrus
In the piggeries with which I am involved anoestrus is not a serious problem and I consider the following factors may be important in preventing this problem.
(1) ad lib. feeding of sows during lactation
(2) ad lib. feeding of sows from weaning to oestrus. Sows not showing oestrus after 12 days go on to a reduced level of feeding
(3) the sows are penned in pairs in close contact with the boars so the pheromones from the boar stimulate normal oestrus behaviour
(4) the sheds where the sows are kept from weaning to mating are reasonably cool.
In attempting to explain this seasonal anoestrus problem it is worth considering the reproductive behaviour of wild pigs.
Most of the detailed studies have been done in the northern hemisphere (Signoret 1979). In the wild the pig has a well defined seasonal breeding cycle producing one litter per year. Wild pigs maintained under intensive conditions immediately loose this seasonal breeding pattern.
Summary in Southern Hemisphere terms:
Mating - May
Farrowing - September
Weaning - December
The boar influence is probably one of the major factors which determine seasonal breeding pattern. Normally the boar remains solitary and is only loosely associated with the herd except during the rutting season. He then enters the herd and the sows come into oestrus and are mated often within a week providing highly synchronised farrowings. Such synchronisation favours survival of the progeny, as it allows for cross-suckling.
The boar influence is all important in stimulating commencement of oestrus activity in the sows and gilts in the herd. Experimentally it has been shown that odours produced by the boar are most important but sight and sound also contribute.
Contrast this one litter per year with the modern piggery where we expect the sow to breed the year round regularly as clockwork. To me it is surprising that the sow performs as well as she does.
The reason for this success is, at least in part, due to forcing the boar and sow into a close association which does not normally exist in the wild. The boar effect is present all the time or at least at the times when we want the sow to express oestrus.
Obviously boer influence is not sufficient to completely overcome the seasonal anoestrus as this continues to occur in intensive piggeries. Minimising this problem requires maximising the boar effects on the sows, as well as attention to nutrition as mentioned earlier.
It has been found that gilts isolated from boars and then suddenly exposed at the time when oestrus is required will induce oestrus activity very effectively. This approach is used routinely in sone piggeries. The novelty of this stimulus is important as sudden exposure is more effective than rearing in continual contact. This essentially mimics what happens in the wild - the young gilt approximately 8 months old suddenly being exposed to the boar influence.
Failure to produce a litter
It has been shown experimentally that injection of sows with ACTH during oestrus induces formation of cystic follicles which lead to return to oestrus after a normal cycle length, delayed return to oestrus or prolonged anoestrus. As these sows were unmated the possible effects of these cystic follicles on pregnancy are unknown. From this work it has been proposed that stress at, just prior to, and/or during oestrus, is the cause of the seasonal infertility problem.
At least the majority, if not all, the sows which are affected by the seasonal infertility problem establish a pregnancy. This has been shown by detection of oestrone sulphate, produced by the foetuses, in the sow's blood during the fourth week of pregnancy (Y.S. Pan, personal communication). The majority of these sows also have an oestrus 22-37 days after mating. This oestrus may be a normal behavioural oestrus with normal fertility or the sow may not express oestrus behaviour and so go undetected. The latter class of sows has been identified by vaginal biopsy. Subsequent periods of oestrous may also be 'silent' or the sow may revert to normal oestrous behaviour.
The logical and simple explanation of the manifestations of seasonal infertility is that the sow conceives and establishes a pregnancy, aborts during the third or fourth week of gestation, and a few days later returns to oestrus. This and subsequent oestrous periods may be silent or active leading to considerable delay in return to oestrus (over 40 days) and N.I.P.'s at farrowing time.
Whether or not these abortions result from stress at mating or stress at a later stage of pregnancy or whether or not stress is involved at all remains to be determined. Stress, if involved, is not just heat stress but a summation of the stresses imposed on the pigs.
An alternative explanation is that the seasonal infertility, as we see it, is really a remnant of the natural seasonal breeding activity of pigs. Wild pig populations in Europe have been shown to have a distinct seasonal breeding cycle which disappears immediately the pigs are domesticated. Day length, particularly decreasing day length, may be responsible for the seasonal infertility problem. However, a trial conducted by us found no benefit associated with maintaining photoperiod at a constant 16 hours.
Prevention
Until further evidence is available, the main preventive measures are those which minimise stresses on the pigs prior to mating and during the first five weeks of pregnancy.
Treatment of the problem
It is essential that an accurate method of pregnancy detection be used to identify those sows which are not pregnant. The Dopla type ultrasound instruments are much more satisfactory than the instruments that simply detect fluid accumulation, as many false positives occur with this type of instrument under farm conditions. All sows should be pregnancy tested at 30 to 35 days of gestation. If this is done then virtually all the problem sows can be identified. Sows found non-pregnant should be moved into close proximity to boars and watched closely for evidence of oestrus.
The fertility of these sows if re-mated is the same as for the rest of the herd.
REFERENCES
Benjaminsen, E. and Karlber, K. (1980). Plasma progesterone in the sow after weaning. Proc. of the Internl. Pig Vet. Soc., p. 22
Christenson, R.K. and Ford, J.J. (1980). Behavioural anoestrus in confinement-reared gilts. Proc. of the Internl. Pig Vet. Soc., p. 43
Close, R.W. and Liptrap, R.M. (1975). Plasma progesterone levels in sows with induced cystic ovarian follicles. Res. in Vet. Sc. 19:28-34
Hurtgen, J.P., Leman, A.D. and Crabo, B. (1980:(a)). Seasonal influence on oestrous activity in sows and gilts. Jl. of the Amer. Vet. Med. Assn. 176:119-123
Hurtgen, J.P., Leman, A.D, and Crabo, B., (1980-(b)). Effect of season, parity and housing factors on oestrus and fertility in swine. Proc. of the Internl. Pig Vet. Soc., p. 20
Love, R.J. (1978). Definition of a seasonal infertility problem in pigs. Vet. Rec. 103:443-446
Paterson, A.M., Barkex, I. and Linsay, D.R. (1978). Summer infertility in pigs; its incidence and characteristics in an Australian commercial piggery. Aust. Jl. of Exp. Agric. and Anim. Husb. 18:689-701
Signoret, J P. (1980). The effects of the male on female physiology in behaviour. Reviews in Rural Science IV., University of New England. p. 23-27
Stork, M.G. (1979). Seasonal reproductive inefficiency in large pig breeding units in Britain. Vet. Rec. 104:4952
Webster, W.R. (1978). Evaluation of a PMS and HCG mixture for alleviating summer infertility in sows. Aust. Vet. J. 54:26-29