Feeding the pregnant gilt and sow — should season be considered?

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? Weight fluctuation over the production cycle of a sow is to be avoided. This is best achieved by feeding sows ad libitum during lactation and by restricting feeding during Ž . the following pregnancy Einarsson and Rojkittikhun, 1993 . Thus, a catabolic state of the sow at around time of ovulation may be avoided as much as possible by a liberal feeding during lactation and after weaning and, on the other hand, overconditioning and related problems upon farrowing are prevented by restricting feeding during pregnancy Ž . Foxcroft et al., 1996 . Furthermore, the most important aspect in optimizing embryonic survival is to generate an increased number of embryos by a high-plane feeding prior to mating, followed by feeding strategies during the post-mating period, which include Ž . restricted feeding, that further support developing embyos Ashworth, 1998 . These general nutritional considerations, however, may not account for seasonal fertility problems seen in the pig industry. This aspect seems important as seasonal Ž . infertility appears to affect the pig industry as a whole Peltoniemi et al., 1999b , Ž especially where pregnant sows are kept in group housing environments Love et al., . 1995 . This implies that season should be included as an aspect when planning nutritional programs for pigs. A controversial beneficial effect of a high feeding level for the first 4 weeks of pregnancy was reported on fertility in gilts and sows in two large Ž . Australian pig units during the seasonal infertility period of the year Love et al., 1995 . In the present review, endocrinological mechanisms mediating nutritional effects and considered of major significance are discussed, especially focusing on the period beyond Ž Day 12 of pregnancy, when CL become dependent on gonadotrophic support Peltoniemi . et al., 1995 . As information on nutrition–season interaction in the mated sow is very limited, we acknowledge the fact that the points discussed here may only be a part of the whole endocrinological mechanism mediating nutritional and seasonal effects on fertility of the sow. 4.1. Progesterone-mediated nutritional effects Nutritional effects on embryonic survival may be mediated via progesterone or direct effects on the conceptus and uterus. In the pig, to exert nutritional effects on embryonic survival, the progesterone-mediated mechanism is thought to be essentially effective Ž . only for 3–4 days after mating Foxcroft, 1997 . Furthermore, a threshold value of progesterone for establishment and maintenance of early pregnancy is considered Ž . unlikely for the polyovulatory pig Foxcroft, 1997 . However, unlike for progesterone, it is likely that such a threshold value exists for embryonic oestrogens. This argument stems from the fact that more than a certain number of viable embryos must be present on days 14 and 15 for successful continuation of the function of the corpus luteum Ž . Polge et al., 1966 , implying that there needs to be a quantitative threshold of the Ž . embryonic signal to maintain the corpus luteum Findlay, 1993 . Furthermore, it seems more than a coincidence that in seasonal comparisons, progesterone values fall during Ž . autumn Wrathall et al., 1986 and a seasonal disruption of an already established Ž . pregnancy is observed Love et al., 1993 , with sows often returning to oestrus between 24 and 30 days after mating. In autumn, it may be that after a period of 12 days of refractoriness, the CL function is somewhat impaired by a seasonal effect, resulting in lower progesterone production. This seasonal effect on CL function may be further suppressed by a commonly used feed restriction, which has been shown to reduce LH Ž . pulse frequency in the early pregnant gilt Peltoniemi et al., 1997a . Detrimental progesterone-mediated effects on embryonic survival related to a high plane of nutrition 3–4 days after mating may be reversed by exogenous progesterone Ž . Ashworth, 1991 , demonstrating the direct effector mechanism of progesterone in mediating the nutritional effects. We suggest, however, that nutritional effects on progesterone production are more complex than this, when season is accounted for in an applied feeding regime for the pregnant sow. After the short-term beneficial effect of Ž . feed restriction for 3–4 days after mating Jindal et al., 1997 , there may be an indirect counteractive effect of feed restriction on progesterone production, mediated via LH secretion as described above. Ž Mechanisms by which progesterone concentrations are reduced nutritionally Jindal et . al., 1997 may be different from that of a seasonal reduction in progesterone secretion Ž . Wrathall et al., 1986 . In the reduction of progesterone associated with a high plane of nutrition shortly after mating, the principle mechanisms that may be considered are the increased hepatic blood flow and the metabolic clearance rate of progesterone, and changes in splanchnic circulation diverting blood away from the ovarian circulation Ž . Foxcroft, 1997 . Hepatic blood flow and metabolic clearance rates are unlikely to be subject to seasonal photoperiodic changes, whereas LH secretion from the pituitary Ž . clearly follows a seasonal pattern of secretion Peacock, 1991; Peltoniemi et al., 1997a . 4.2. LH-mediated nutritional effects LH is an essential component for maintenance of the corpora lutea and early Ž pregnancy in the pig Anderson et al., 1967; Spies et al., 1967; Kraeling and Davis, . 1974; Kraeling et al., 1992; Peltoniemi et al., 1995 . Therefore, any factor, including nutrition, which affects LH secretion during early pregnancy, may have implications for Ž . embryonic survival and maintenance of early pregnancy Peltoniemi et al., 1999c . A lot of research work has emphasized the role of nutrition on gonadotrophin Ž secretion during lactation, after weaning, and before and shortly after ovulation for reviews, see Booth, 1990; Einarsson and Rojkittikhun, 1993; Cosgrove and Foxcroft, . 1996 . A great majority of the studies have concentrated on the oestrous cycle and gonadotrophin secretion related to the follicular phase and ovulation. Little attention has been paid to the role of nutrition as a modulator of gondotrophin secretion during early pregnancy when CLs have become dependent upon pituitary support. Insulin, glucose, amino acids and free fatty acids provide metabolic signals to the Ž brain that influence food intake, energy balance and body weight regulation Oomura, . 1976; Porte and Woods, 1981; Fernstrom, 1983 . Of these, insulin and glucose are considered to be the most directly and fundamentally involved in the nutritional Ž . modulation of the hypothalamic–pituitary–gonadal axis Booth, 1990 . In the prepubertal gilt, feed restriction to 50 of ad libitum intake clearly reduced Ž . LH pulse frequency in long-term Prunier et al., 1993 and short-term experiments Ž . Cosgrove et al., 1993 . This degree of feed restriction showed up in metabolic markers such as plasma free fatty acid concentrations, plasma insulin concentrations and plasma glucose concentrations. Gilts with restricted feeding had metabolic characteristics of Ž animals that had been denied feed low pre-prandial glucose, insulin and urea and high . free fatty acid concentrations , a metabolic state that was clearly related to reduced LH Ž . pulsatility Prunier et al., 1993 . When low-energy post-mating feeding strategies are Ž . applied according to latest recommendations Cosgrove and Foxcroft, 1996 , this should reduce LH pulsatility in the early pregnant gilt, since such a recommendation essentially is equivalent to a 40–50 feed restriction. As a favourable effect of high feeding level after mating on fertility in summer– Ž . autumn was found Love et al., 1995 , we hypothesised this favourable effect was mediated via LH secretion. We therefore conducted studies to determine the effect of Ž . commonly used level of feed restriction feed-restricted gilts received 1.8 kgrday on Ž LH secretion in pregnant gilts in summer–autumn and spring Peltoniemi et al., . Ž . 1997a,b . In spring, the feed-restricted gilts 23.4 MJrday had fewer LH pulses than Ž . Ž . control gilts on 3.6 kg 46.8 MJ of feed Peltoniemi et al., 1997a . This reduction in LH pulsatility related to feed restriction, however, was not significant in summer–autumn, Ž . when seasonal disruption of pregnancy is observed Peltoniemi et al., 1997b . These studies showed that moderate feed restriction, capable of altering LH secretion, also caused metabolic changes, as indicated by plasma insulin and free fatty acid concentra- tions in experimental feed-restricted gilts. The feed-restricted pregnant gilts tended to show higher preprandial FFA concentrations, lower preprandial insulin concentrations, and a greater pre- vs. post-prandial difference than in animals fed nearly ad libitum Ž . Peltoniemi et al., 1997a . These findings may be considered metabolic characteristics of Ž . pigs deprived of feed Prunier et al., 1993 . Alterations in energy intake correlated with Ž weight and back fat changes, and the metabolic changes recorded Peltoniemi et al., . Ž 1997b . Furthermore, it was observed that LH characteristics pulse amplitude, pulse . frequency and area under the LH curve showed a tendency towards higher values when the gilts had more energy intake and showed the metabolic characteristics of satiated Ž . animals Peltoniemi et al., 1997a,b . LH values were highest in animals, which were given a high-density diet to ad libitum, maximising nutrient intake. In conclusion, feed restriction of the usual order after mating may reduce LH pulse frequency in the early Ž . pregnant gilt Peltoniemi et al., 1997a . However, the effect of a 50 feed restriction and season as such on LH characteristics found was not considered strong enough to Ž . induce CL regression and disruption of pregnancy Peltoniemi et al., 1997b .

5. Seasonal disruption of pregnancy: the present view