1. Introduction
The European wild boar is a true seasonal breeder with a breeding season occurring Ž
. in early winter with piglets born once a year in late spring Mauget, 1982 . Despite
being capable of producing piglets throughout the year, the domestic sow shows a Ž
. reduction in fertility in late summer and early autumn Love, 1981 , at a time that the
Ž European wild boar is in seasonal reproductive arrest Mauget, 1982; Claus and Weiler,
. 1985 . The reduction in fertility commonly reported all around the world justifies the use
of the term ‘‘seasonal breeder’’ for the domestic pig as well as for the European wild Ž
. boar Love et al., 1993 . Manifestations of this seasonal infertility include a reduced
Ž .
farrowing rate Xue et al., 1994; Love et al., 1995; Peltoniemi et al., 1999b , delayed Ž
puberty of gilts Ehnvall et al., 1981; Paterson and Pearce, 1989; Peltoniemi et al., .
Ž 1999a,b , a prolonged weaning to oestrus interval Hurtgen and Leman, 1980; Benjamin-
sen and Karlberg, 1981; Sterning et al., 1990; Prunier et al., 1996; Peltoniemi et al., .
Ž 1999a and, possibly, a reduced litter size during late summer and early autumn Claus
. and Weiler, 1985 . Although photoperiod is considered the primary environmental cue
Ž .
to seasonal infertility Love et al., 1993 , a whole variety of other environmental factors seem to interact with season either to exacerbate or to alleviate this infertility. These
Ž environmental factors include housing group vs. individual; Hurtgen and Leman, 1980;
. Ž
. Ž
Love et al., 1995 , feeding level Love et al., 1995 , light conditions Andersson et al., .
Ž .
Ž .
1998 , boar exposure Paterson et al., 1991 , group size Love et al., 1993 , ambient Ž
. temperature Love, 1978; Stork, 1979; Hurtgen and Leman, 1980; Prunier et al., 1997
Ž .
and interactions between females Wilson and Love, 1990 . The significance of seasonal infertility has increased in recent years as group housing of dry sows has become more
Ž .
common Peltoniemi et al., 1999b . In the present paper, recent advances in seasonal infertility-related research are reviewed. The focus of the present review is on the
interaction of nutrition and season in the feed-restricted, early pregnant gilt and sow. A review on the effects of season, housing and nutrition on gonadotrophins is given
Ž .
elsewhere Peltoniemi et al., 1999c .
2. Effector mechanism mediating seasonal effects on the pituitary–gonadal axis
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