Agriculture Reference
In-Depth Information
factors that influence it is first provided. Much attention is given to the first litter sow,
since effects of lactation on subsequent reproductive performance are particularly seen
in these animals (i.e. the so-called second litter syndrome). The impact of diseases on
reproductive performance will not be discussed. Optimal reproductive performance
of first and second litter sows is also affected by gilt development and management,
including the transition period to lactation, and is also affected by conditions following
fertilisation, during pregnancy. However, these aspects will be dealt with in other chapters
in this topic.
17.2
Reproductive physiology
The information regarding reproductive physiology presented below has largely been
taken from Soede
et al.
(2011).
17.2.1
Lactational anoestrus
During established lactation, peripheral concentrations of luteinising hormone (LH) and
LH pulsatility are suppressed due to the suckling-induced inhibition of the gonadotropin-
releasing hormone (GnRH) pulse generator (De Rensis
et al.
, 1993). The degree of LH
suppression is also related to the negative energy balance of the sow; in primiparous
sows on a low level of feeding, peripheral concentrations of LH were reduced compared
with sows on a high level of feeding (Quesnel and Prunier, 1998; Van den Brand
et al.
,
2000a). The effects of lactation on follicle-stimulating hormone (FSH) are less consistent
and are primarily due to negative feedbacks of inhibin (produced by follicles reaching
3 mm; Noguchi
et al.
(2010) rather than to suckling effects (reviewed by Prunier
et al.
,
2003). In the course of lactation, LH pulsatility is normally restored (Van den Brand
et al.
, 2000a), which may be related to a decrease in suckling frequency and/or to an
increase in pituitary LH responsiveness to GnRH (Bevers
et al.
, 1981; Rojanasthien
et al.
,
1987). Concomitantly with, but not solely due to, this increase in LH pulsatility, follicle
diameter increases during the course of lactation (reviewed by Britt
et al.
, 1985). hus,
with progressing lactation, the antral follicles in the follicle pool on the ovaries reach a
greater diameter, though most sows do not develop follicles beyond a diameter of 3-4
mm until after weaning (Lucy
et al.
, 2001). Occasionally, sows show early restoration of
LH-release, develop pre-ovulatory size diameter follicles (~8 mm) and ovulate during
lactation (Kemp
et al.
, 1998). This is not likely to occur during the first 2 weeks of lactation
since the ability of sows to mount a pre-ovulatory LH surge of sufficient magnitude to
induce ovulation develops during lactation (Sesti and Britt, 1993). These lactational
ovulations occur with more frequency in higher parity sows with a low litter size (low
suckling intensity) and/or low lactational weight loss (high feed intake).
17.2.2
Weaning-to-oestrus interval
The antral follicle pool at weaning may consist of approximately 100 follicles, varying in
diameter up to 6 mm (reviewed by Knox, 2005). Recruitment of follicles from the pool
takes place when pulsatile GnRH/LH release shifts from a low frequency/high amplitude
