Reproduction performances and conditions of group-housed non ...

- Paper I -
gonadotrophins (LH, FSH) (Hughes & Pearce, 1989). Both LH and FSH act on the ovaries
to stimulate the development of the pre-ovulatory follicle (Foxcroft & Hunter, 1985). LH is
furthermore essential for the maintenance of early pregnancy in the pig (Peltoniemi et al.,
1995) why lower pregnancy rate as a consequence of restricted feed intake may be due to a
reduction in LH pulse frequency (Peltoniemi et al., 2000). The negative effect of high feed
intake the first three days in pregnancy on embryo survival is believed to be caused by an
increase in hepatic blood flow and metabolic clearance rate of progesterone, as a consequence
of rapid weight gain, followed by a decrease in plasma level of progesterone
(Hughes & Pearce, 1989; Foxcroft, 1997). Progesterone is the primary director of uterine
development and secretion (Geisert & Yelich, 1997) and therefore a change in plasma level
of progesterone could imply detrimental consequences for embryo survival as indicated in a
study by Pharazyn et al. (1991). Progesterone is secreted from Corpus Luteum and because
the pig is polyovulatory, it is suggested that it is unlikely that the plasma progesterone concentration
may get below some essential threshold after the first three to four days of pregnancy
(Foxcroft, 1997).
The statement, that reproduction may be influenced by nutrition is further supported by
observations of wild sows. Matschke (1964 q.f. Graves, 1984) reported that European wild
sows were anoestrous during years of small quantity of mast and similar did Mauget (1981)
report that the timing of breeding season was influenced by the amount of mast available.
When looking upon the results from the studies reviewed, although it does not provide a
clear picture, it seems that pregnancy rate and litter size can be influenced by energy intake
in the non-lactative period. However, the question, which remains to be answered, is
whether variation in energy intake between female pigs in commercial group-housed systems
reaches magnitudes large enough to impair the litter size and pregnancy rate?
Andersen et al. (1999) observed that low ranking sows only spent about half as much time
at the trough at feeding compared to high ranking sows (40 vs. 90 % of observations) in a
group of pregnant sows. Although low ranking sows may be able to increase their eating
rate when competition for feed is high, as suggested by Brouns & Edwards (1994), it is
likely that the energy intake of the low ranking sows has been considerably lower than the
energy intake of the high ranking sows. Mendl et al. (1992) observed that low and none
success primaparous sows (the sows were divided into three groups according to their ability
to displace other sows in agonistic interactions) had significant lower weight gain than
high success primaparous sows (approximately 7, 9 and 19 kg, respectively in one month)
in their 7 th week of pregnancy. As Mendl et al. (1992) point out, the lower weight gain in
the low ranking sows is not necessarily a result of lower feed intake only but perhaps a
combination of lower feed intake and an elevated expenditure of energy for maintenance as
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