Ž . Ž
. Ž
. Fig. 1. Pattern of light intensity lx s and mean SEM serum melatonin concentrations pgrml in
European wild boars s l and in domestic gilts s B over a 48-h period.
In the pig, many researchers have been unable to demonstrate the nocturnal rise in Ž
melatonin concentration seen in other species Brandt et al, 1986; Peacock, 1991; .
Ž .
Diekman et al., 1992; Bollinger et al., 1997 . However, Paterson et al. 1992 reported a consistent nocturnal rise in melatonin concentration in pigs under varying photoperiodic
Ž .
circumstances and work by Klupiec et al. 1997 implied that the previous confusion had Ž
. been due to methodological problems. We carried out an experiment Tast et al., 2000b ,
Ž where individual European wild boars were cannulated by intra-arterial catheters Tast et
. al., 2000a for frequent blood sample collection. In the same experimental design, we
used domestic YorkshirerLandrace crossbred gilts fitted non-surgically with intravenous Ž
. jugular catheters Peacock, 1991 . Blood sampling was carried out at a 2-h interval for
48 h. The sampling was repeated in each season in Finland to establish seasonal variation in melatonin secretion for these experimental animals. As illustrated in Fig. 1
Ž .
summer profiles , a consistent and repeatable melatonin profile was found in all of the experimental animals. Therefore, it is proposed that this aspect of the neuroendocrino-
logical basis of seasonal reproduction in the pig is not different from that of other seasonal breeders.
3. Environmental factors involved in seasonal infertility
As photoperiod functions as the primary cue to seasonal infertility, other environmen- Ž
tal factors then determine the level of reproductive activity in summer–autumn Love et .
al., 1993 . Of these environmental facors, housing, ambient temperature and social interactions are considered to be of significance. One of the main environmental cues
Ž affecting onset of sexual activity in the European wild boar is availablity of feed Pepin
. and Mauget, 1989 . Likewise, nutrition is considered an important environmental factor
in the physiology of seasonal infertility. As a focus of the present review, the role of nutrition in seasonal infertility is dealt with on its own.
3.1. Housing and social stress Because of welfare considerations and related legislation in Europe, group housing of
pregnant sows has become a common practice, and in the long run, it will be the only
acceptable way of housing pigs intensively. Available information on effects of the type of housing on fertility and seasonal fluctuation in fertility of the sow seems very limited.
Ž .
Ž Work carried out in Australia Love et al., 1995 and the USA Hurtgen and Leman,
. 1980; Leman, 1992 suggests that individual housing may in fact protect sows from the
seasonal reduction in farrowing rate. Farrowing rates for group-housed sows were clearly reduced during the seasonal infertility period in large production unit, whereas
Ž individually housed sows showed no seasonal fluctuation in farrowing rates Love et al.,
. Ž
. 1995 . This study Love et al., 1995 provided strong evidence for the benefit of
individual housing as the experimental animals were maintained within the same buildings and received similar management. However, epidemiological findings in
Finland suggest that other manifestations of seasonal infertility, such as a prolonged weaning-to-oestrus interval, may be more common a finding in sows kept in individual
Ž .
stalls Peltoniemi et al., 1999a . On the other hand, the seasonal reduction in farrowing Ž
rate related to group housing may partly be prevented by feeding measures Peltoniemi .
et al., 1999b . There are clear indications that ‘‘social stress’’ or grouping of sows may elevate
Ž cortisol concentrations, especially in low-ranked sows shortly after grouping Tsuma et
. al., 1996 . A stress hypothesis for seasonal infertility was proposed by Wan et al.
Ž .
1994 , who suggested that those pigs with a greater adrenal responsiveness to ACTH administration were more likely to suffer from seasonal infertility. This correlation was
found to be particularly significant for gilts, which are known to be more susceptible to seasonal infertility than older sows. This hypothesis implies that the environmental
determinants of seasonal infertility that are related to stress are likely to be important in the pathophysiology of seasonal infertility. Therefore, effects of grouping density, feed
restriction, and heat on gonadotrophins may, at least in part, be mediated via cortisol. To
Ž .
support this, Love et al. 1995 clearly showed that seasonal reduction in farrowing rates was worsened by increased stocking density.
3.2. Temperature High ambient temperature leading to heat stress has, in many instances, been
Ž .
associated with seasonal infertility Love, 1978; Stork, 1979; Reilly and Roberts, 1992 . However, attempts to alleviate the seasonal infertility of sows by cooling systems have
Ž .
generally failed Stork, 1979; Hurtgen and Leman, 1980 . Furthermore, seasonal infertil- ity has been described across all the continents under a diversity of climatic conditions,
yet the severity of the condition does not correlate with any of these variations in temperatures. The strongest argument against temperature being of great significance in
the physiology of seasonal infertility is the fact that the duration of seasonal infertility period exceeds long beyond August, when temperatures have fallen and the climatic
conditions cannot be described by anything but cool in countries like Finland. Therefore, it can be suggested that temperature may not play a major role in the pathogenesis of
seasonal infertility. However, high ambient temperatures reduce the feed intake of sows during lactation, thereby somewhat increasing the weaning-to-oestrus interval in sum-
Ž .
mer–autumn Lynch, 1989; Prunier et al., 1996, 1997 , which is one of the manifesta- tions of seasonal infertility.
3.3. Boar exposure Ž
Apart from season affecting fertility of the boar Claus and Weiler, 1985; Andersson .
et al., 1998 , the male role in the seasonal infertility of gilts and sows is essential in terms of prevention of seasonal infertility. The adverse effect of season on attainment of
Ž puberty in gilts was largely prevented by proper use of boar stimulation Paterson et al.,
. 1991 . In addition, more educated use of the boar with weaned sows has decreased the
Ž .
weaning-to-oestrus interval in the summer–autumn Love et al., 1993 .
4. Feeding the pregnant gilt and sow — should season be considered?
