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Seasonal effects on fertility in gilts and sows

The ancestral wild pig is a short day length seasonal breeder. The domestic pig appears to have retained some of this seasonality as evidenced by a reduction in fertility during the summer—autumn period. The most important aspect of this seasonality is a reduction in the number of mated sows that farrow. Many of these sows conceive and embryos develop normally for 20 25 days before pregnancy is terminated and the sow returns to oestrus (25 35 days after mating). In other species, transduction of photoperiodic information is achieved by release of melatonin during the dark period. In the pig, the pattern of melatonin secretion and the subsequent hypothalamo—pituitary—gonadal responses appear to be more complex. A relatively high light intensity is required for pigs to generate a distinct diurnal melatonin rhythm and they appear unable to respond appropriately to abrupt changes in photoperiod. Pigs on restricted feeding and maintained under long photoperiods (but not under short photoperiods) have higher concentrations of melatonin than do similarly maintained pigs fed ad libitum. Continuous release melatonin implants have a deleterious effect on farrowing rate, suggesting that the abnormally high melatonin concentrations observed in sows in summer—autumn play a role in the pathogenesis of seasonal infertility. Ad libitum feeding of sows during the first few weeks of pregnancy may prevent the increase in melatonin concentrations and so remove the seasonal influence on fertility. The pituitary response to different photoperiods is also somewhat confusing. Although there is some evidence of increased sensitivity to the negative feedback of ovarian steroids in the prepubertal gilts and weaned sows during summer—autumn, LH concentrations are increased in early pregnant sows. It is proposed that the failure of sows to maintain pregnancy in summer—autumn results from disruption of maternal recognition of pregnancy causing regression of the corpora lutea, loss of pregnancy and return of the sow to oestrus.

The ancestral wild pig is a short day length seasonal breeder. The domestic pig appears to have retained some of this seasonality as evidenced by a reduction in fertility during the summer-autumn period. The most important aspect of this seasonality is a reduction in the number of mated sows that farrow. Many of these sows conceive and embryos develop normally for 20 -25 days before pregnancy is terminated and the sow returns to oestrus (25 -35 days after mating). In other species, transduction of photoperiodic information is achieved by release of melatonin during the dark period. In the pig, the pattern of melatonin secretion and the subsequent hypothalamo-pituitary-gonadal responses appear to be more complex. A relatively high light intensity is required for pigs to generate a distinct diurnal melatonin rhythm and they appear unable to respond appropriately to abrupt changes in photoperiod.
Pigs on restricted feeding and maintained under long photoperiods (but not under short photoperiods) have higher concentrations of melatonin than do similarly maintained pigs fed ad libitum. Continuous release melatonin implants have a deleterious effect on farrowing rate, suggesting that the abnormally high melatonin concentrations observed in sows in summer-autumn play a role in the pathogenesis of seasonal infertility.

Ad libitum feeding of sows
during the first few weeks of pregnancy may prevent the increase in melatonin concentrations and so remove the seasonal influence on fertility.
The pituitary response to different photoperiods is also somewhat confusing. Although there is some evidence of increased sensitivity to the negative feedback of ovarian steroids in the prepubertal gilts and weaned sows during summer-autumn,

Introduction
The adverse seasonal effects on reproductive efficiency are of considerable economic significance to the pig industry worldwide.
Efficient production requires a constant flow through the facility of growing pigs, and seasonal infertility in the summer-autumn period can dramatically disrupt this flow. A 10% reduction in the number of sows farrowing over 4 months is estimated to reduce profitability by $Aus30 per sow per year (Cutler and Gardner, 1988). Decreases in productivity of this order or greater are not uncommon in Australia during the summer-autumn period.
It is now generally considered that photoperiod is the primary environmental cue to this seasonality and that the pig is essentially a short-day breeder. A considerable amount of research is being done to determine how pigs recognize and respond to changes in daylength.
Current understanding of seasonal breeding stems largely from work in sheep, pronounced short-day breeders. The decreasing daylengths of late summer-autumn alter the diumal profile of melatonin secretion and the duration of the period of high concentrations of melatonin associated with darkness. This changed profile reduces the negative feedback of oestradiol on the GnRH pulse generator allowing the development of an LH surge and ovulation. Use of the sheep as a model for pigs has revealed some similarities and many differences in the way pigs interpret and respond to changes in daylength and these will be discussed in this review.

Seasonal Breeding in European Wild Boars
In attempting to understand the seasonal reduction in reproductive efficiency, it is useful to refer to the studies of the clearly seasonal European wild boar, from which the domestic pig is largely derived.
European wild boars are distinctly seasonal breeders, usually producing only one litter per year farrowed in the spring such that conditions in subsequent months are most conducive to the survival of both the sow and her progeny (Mauget, 1982). The rut or breeding season producing this litter occurs in late autumn-early winter; the timing of commencement of the rut is very much influenced by food availability.
A plentiful supply of mast (nuts from forest trees such as the beech and oak) is associated with early breeding activity. Farrowing is followed by a long lactation of about 3 months during which the sow remains in anoestrus. Weaning is a gradual process and lactation normally ceases in mid-to late summer and the anoestrous state continues after weaning until the next rut. Wild sows can produce two litters per year if lactation is terminated by removal or death of the litters before midsummer. However, sows weaning a litter after the summer solstice are unlikely to come into oestrus and will remain in anoestrus until late autumn (Mauget, 1982). An interesting aspect of reproduction in wild boars is the synchrony of oestrous activity that occurs within any group of sows at the start of the breeding season.

Delayed puberty in gilts
There is some confusion in the literature about the effect of season on puberty. However, most reports indicate that puberty is delayed during the summer period (reviewed by Hughes 1982). The effect on puberty is much more evident in the absence of boars. The intimate and continuous association of the boar, as occurs with domestic gilts, masks the innate seasonal breeding activity (Paterson et al., 1991). In the many studies on the boar effect on puberty, the effect of season has been largely ignored but application of practices developed from these studies dramatically reduces the adverse effects of season on attainment of puberty.

Prolongedweaning to oestrus intervals
The literature on increased weaning to oestrus intervals during summer and autumn has been reviewed by Claus and Weiler (1985). During summer and autumn, most sows have a normal weaning to oestrus interval. A minority of sows (most evident in the parity one group) have a considerably prolonged weaning to oestrus interval (> 30 days) and this increases the mean figure (Hurtgen et al., 1980;Mattioli et al., 1987). The minority group of sows essentially behaves like its wild counterpart, in which weaning during the summer and autumn is followed by a prolonged period of anoestrus (Mauget, 1982).
In Australia, this is a relatively minor component of the seasonal infertility complex. The importance of this manifestation of seasonal infertility has declined as weaning age has been reduced (3-4 week weaning), boar influence on the weaned sow has been optimized and feeding levels of sows during lactation and in the weaning to oestrus period have increased. The condition of the sow at weaning, reflecting the level of feeding during the previous pregnancy and lactation, influences the interval from weaning to oestrus (reviewed by Aherne and Kirkwood, 1985) at any time of the year, but it becomes critical during the summer-autumn period.

Reducedfarrowing rate
The reduction in the proportion of mated sows that farrow is currently by far the most important aspect of the seasonal infertility. The annual farrowing rate pattern in a 3000 sow intensive piggery ( Fig.   1) shows that farrowing rate is reduced for 16 weeks after the summer solstice. The most common evidence of this problem is delayed (>24 days) return to oestrus (usually 25 to 35 days after mating) as illustrated in Fig. 2 (Love, 1981;Reilly and Roberts, 1992). Sows at this time may show normal oestrous behaviour or may have an undetected oestrus and go through further oestrous cycles of normal duration (21 days) before being remated (Stork 1979;Love, 1981;Mattioli et al., 1987;Reilly and Roberts, 1992).
Pregnancy losses also occur at later stages of pregnancy leading to the term 'autumn abortions' (Stork, 1979;Almond et al., 1985). There is evidence that some sows do not develop corpora lutea after mating, and remain in anoestrus (Williamson et al., 1980).

Decreasedlitter size
Reports on the effect on litter size resulting from matings during the summer and autumn vary from no effect (Love, 1978;Paterson et at, 1978;Mattioli et at, 1987)  A number of factors have now been recognized that help to explain these variable results.

Light intensity
In humans there is an inverse linear relationship between light intensity and melatonin plasma concentrations during the photophase. The greater the suppression of melatonin in the light phase the higher the concentrations of melatonin during the dark phase (Lewy et al., 1980). Adequate light intensity is therefore critical to the generation of a distinct circadian rhythm of melatonin.

Rale of change in photophase
Most studies involving artificial lighting have used abrupt and extreme changes in photoperiod (from 8 to 16 h of light in one change). Paterson et al. (1992a) increased or decreased the photoperiod incrementally from 12 h light:12 h dark mimicking the natural rate of change in daylength and introduced the pigs into the controlled lighting conditions at the spring equinox. It is under these conditions that nocturnal increases in melatonin have been observed most consistently. As suggested by these authors, pigs may be unable to respond appropriately to sudden changes in photoperiod. In contrast, sheep show immediate and appropriate changes in melatonin when exposed to an abrupt change in photoperiod (Cutler, 1988).

Season
Exposure to natural light with the gradual decrease and increase in intensity at dusk and at dawn could provide quite a different stimulus to the on-offlighting situation that occurs in controlled environments used in many experiments. When early pregnant sows were exposed to natural light in an open-sided building, demonstrable nocturnal rises in melatonin during the night were seen only in spring and summer (Fig. 3). In this experiment the spring and autumn light periods would approximate 12 h light:12 h dark, the summer 15 h light: 9 h dark and winter 9 h light:15 h dark at this latitude. The other aspect of the melatonin profiles in this experiment was that overall concentrations in plasma were lowest in winter and highest in summer and autumn. Similar seasonal differences in melatonin concentrations were found by Peacock et al. (1991). The low winter concentrations of melatonin found were similar to those produced by pigs in continuous darkness (Griffith and Minton, 1991), and may reflect the lower light intensity during winter. However, the midday light intensity to which these sows were exposed was greater than 300 lux in mid-winter and was as high as could be achieved in housed pigs without exposure to direct sunlight. The high concentrations of melatonin in plasma in summer-autumn may also be similar to the higher concentrations observed under continuous illumination (Griffith and Minton, 1991).

Physiological state
Most studies investigating changes in melatonin concentrations have used young boars (Minton and Cash, 1990), castrates (Griffith and Minton, 1992) or gilts (Diekman el al., 1992; Paterson et at, I992a). Others have used lactating sows (De Boer and Hacker, 1986), cyclic sows (McConnell and Ellendorif, 1987) or pregnant sows (Peacock et al. 1991). Nocturnal increases in melatonin have been demonstrated in at least one study in all these classes of pig except for the lactating sow. The lactating sow is remarkable in that daytime increases in melatonin were observed in sows and these were related to feeding (De Boer and Hacker, 1986).

Age
The amplitude of the nocturnal melatonin rise declines with age in rats (Pang et al., 1984) and humans (Waldhauser et al., 1988). Recent work in our laboratory has shown that distinct nocturnal increases in melatonin were more obvious in young pigs (7 weeks of age) than in prepubertal gilts or multiparous sows (C. Klupiec, R. Love and G. Evans, unpublished).

Level of feeding
The level of feeding affects melatonin secretion in species other than pigs. Three weeks of 50% feed restriction in rats caused increased scotophase melatonin concentrations (Chik et al., 1987) and complete deprivation in post-menopausal women caused a dramatic and immediate increase in daytime melatonin (Beitins el al., 1985). Diurnal profiles were not determined in the latter experiment. Melatonin concentrations are also known to be considerably increased in anorexia nervosa and abnormal in other eating disorders in humans (Ferrari el al., 1989). Most of the experiments studying the melatonin concentrations in pigs have used various degrees of feed restriction (estimated as 60 -80% of ad libitum intake). We    Other studies have looked for differences between sows that have a normal post-weaning oestrus (less than 7 days after weaning) and sows that have a prolonged postweaning anoestrus. These seasonally anoestrous sows respond normally to pulsatile administration of GnRI-1 (Armstrong and Britt, 1985), exogenous gonadotrophins (Dial et al., 1984;Britt et al., 1986) and exogenous oestrogen (Cox et al., 1983;Dial et at, 1984;Almond and Dial, 1990 (1991) found that daily melatonin supplementation increased the number of gilts reaching puberty in both autumn and spring. Inducing a marked nocturnal rise in concentration of melatonin in plasma appears to advance puberty in any season. The sustained increased concentration of melatonin following oral administration ( > 8 h) was similar to that reported for sheep and goats but was in contrast to work in hurrians in which only a transitory rise in melatonin plasma concentrations occurred (Wetterberg et at, 1977): The difference between the ruminants and humans was explained previously on the basis of retention of the oral dose in the rumen and gradual release to the lower intestinal tract for absorption (Kennaway and Seamark, 1980). Evidently the absorption of melatonin is different in the two aforementioned monogastrics.
Continuous release melatonin implants have no effect on the age at which gilts reach puberty during the summer-autumn period (Peacock et at, 1991;Paterson et al., 19926). These implants have also been used in sows in an attempt to improve farrowing rate during the summer-autumn period. In this trial, half of the gilts were implanted 6 weeks before entry into the boar shed for mating; half the parity 1 sows were implanted at weaning approximately 4 weeks before mating (this parity group was mated at the second post-weaning oestrus); and half the older sows were implanted during the second week of lactation (approximately 3 weeks before the next mating). This implant regimen had no effect on the interval from weaning to oestrus or litter size but did have a significant deleterious effect on the farrowing rate in the more numerous older sow group (Table 1).
Sows during the summer-autumn period have been shown to have unusually high plasma melatonin throughout the 24 h period. These high concentrations may be involved in the pathogenesis of seasonal

. Other Factors Interacting with Photoperiod
If change in daylength per se was the cause of seasonal infertility, then the problem would be much more predictable thoughout the summer-autumn period. A feature of seasonal infertility is the extreme variability from piggery to piggery, from week to week within the same piggery (Love, 1981) and even from pen to pen of sows mated in the same week. It would appear that long daylengths are fundamental to inducing seasonal infertility and increase the sensitivity of the sow to other aspects of her environment.
A number of factors may be responsible for this variability.

Nutrition
In wild boars, it appears that cessation of breeding activity is independent of the plane of nutrition but commencement of the breeding season is very dependent of food availability (Mauget, 1982(Mauget, , 1987 Appetite is adversely affected by high temperatures and possibly also by season independent of temperature (Mauget, 1987), resulting in greater loss of body condition and extending the interval from weaning to oestrus.
Autumn abortions have been shown to be prevented by either increasing the temperature in the sow house or increasing feeding levels during autumn. The occurrence of abortions appeared to be unrelated to body condition and was considered to be associated with the increased energy demands of the cooler autumn months (Almond et al., 1985).

Social interactions
The pig has an extremely well developed olfactory and vomeronasal system that can detect signalling and priming pheromones. It seems likely that there is a spectrum of pheromonal communication in pigs similar to that described in rodent species. In rodents there are both male-female and female-female pheromonal interactions (reviewed by Vandenbergh, 1983 Such apparently nebulous influences may explain the dramatic variation in the adverse effects of seasonal infertility on groups of sows. It is not unusual for a pen of 20 sows to achieve a farrowing rate of less than 50%, while a second pen of sows mated in the same week and maintained in the same building have a normal farrowing rate. It appears that there are some group associated influences that determine reproductive success.

Temperature
Heat stress has been proposed by many as the cause of seasonal infertility (reviewed by Wettemann and Bazer, 1985). The strongest argument against heat stress playing a direct role in the pathogenesis of seasonal infertility is the timing of this problem. The period when seasonal infertility operates commences in midsummer and extends well into the autumn period when temperatures are very mild and certainly less severe than in the early summer period (Fig. 1). Provision of cooling in commercial piggeries has also failed to reduce the severity of seasonal infertility (Hurtgen and Leman, 1980). Rather than high environmental temperatures being involved, there is evidence that low temperatures or cold stress without an adequate compensatory increase in feeding level is responsible for the 'autumn abortion syndrome' (Almond et al., 1985). Sows can maintain pregnancy in midwinter on these relatively low feeding levels. It therefore appears that sows in this period are more sensitive to energy requirements than at other times.

Housing
Provision of individual stalls for mated sows significantly improves the farrowing rate during the summer-autumn period (Hurtgen and Leman, 1980). One large piggery in Australia has half its sows in stalls for the first few weeks of pregnancy and the other sows are maintained in groups of about 20; only the grouped sows show any evidence of seasonal infertility. There is also some evidence that group size is important, groups of five or six sows appear to be less affected by seasonal infertility than do large groups. It is not known how use of individual stalls reduces seasonal infertility: it may be a reduction in stress; it may ensure each sow receives an adequate level of nutrition; or it may be that use of individual stalls disrupts the group social interactions that have negative effects on fertility.

The Effector Mechanism Disrupting Pregnancy
Delayed puberty in gilts and anoestrus in sows have counterparts in other seasonal breeding species.
However, the failure of sows to farrow that apparently ovulate normally and develop normal embryos into the fourth week of gestation is not a common reproductive strategy and appears to be a metabolically wasteful process. A similar phenomenon has been described in a tropical rodent (cloud forest mice) in coping with precarious food and water supplies. Mild restriction of either commodity in this species causes pregnancy failure and return to oestrus (Heideman and Bronson, 1992).
It has been suggested that pregnancy failure in sows is the result of inadequate pituitary support (Wrathall et al., 1986). Once ovulation has occurred in pigs, the corpora lutea function autonomously  , 1987). Together with the evidence for biphasic production of oestrogens by embryonic tissues (Geisert et al., 1990), this finding indicates that the second signal is provided by oestrogens, but its mode of action has yet to be determined.
It seems more than coincidence that disruption of the second phase of the embryonic signal (required for the pregnant state to be maintained) results in return to oestrus at the same time as frequently occurs in seasonal infertility. Failure of the second embryonic signal could provide the basis for pregnancy failure in seasonal infertility.
However, the presence of the viable embryos producing apparently normal concentrations of oestrone sulfate (Y. S. Pan, personal communication;Mattioli et at, 1987) suggests that, rather than a failure of production of this signal, there is a failure to respond to it. Such a hypothesis suggests that the function of the second signal is different from the first and may be, as suggested by Garverick et al. (1982), to increase LH receptors in the corpora lutea. Failure of maintenance of the corpora lutea could then result from inadequacy of the signal (removal of the embryos) or failure of effective occupancy of receptors (seasonal infertility), both situations resulting in delayed return to oestrus. These animals exhibit greater testicular regression in response to food restriction when maintained on short photoperiods than on long photoperiods (Nelson et at, 1992).
Sow diets are formulated to satisfy the nutrient requirements of the animal rather than appetite. Thus it has been suggested that sows spend much of each day hungry (Hutson, 1989) because they perceive that their food supply is inadequate. If the appetite of the sow is satisfied by feeding the same nutrients in a bulkier diet, there may be benefits to both sow welfare and productivity (Brouns and Edwards, 1992