Agriculture Reference
In-Depth Information
these behavioural changes, vulvar changes are also seen (increased redness, swelling
and mucus production). Although these signs of oestrus are orchestrated/caused by
oestrogens produced by pre-ovulatory follicles, there appears to be no direct relationship
between the peripheral concentrations of oestrogens and the intensity and/or duration of
the symptoms of oestrus in intact animals (Soede
et al.
, 1994). The olfactory and tactile
stimuli from the boar are most important to evoke the standing response (Signoret, 1970)
and oestrous detection is often performed by mimicking these tactile stimuli using the
so-called 'back pressure test, in the presence of a boar. In the presence of a boar, oestrus
lasts on average 40 to 60 h, but this varies from less than 24 h to more than 96 h for
individual sows (reviewed by Soede and Kemp, 1997). The duration of oestrus also varies
considerably between farms (Steverink
et al.
, 1999) and is affected by the intensity of boar
contact during oestrous detection, stress levels of the sows, parity (often shorter in gilts)
and weaning-to-oestrus interval (shorter when weaning to oestrus intervals are more
than 4 days). The exact mechanisms causing these effects are not clear. Since ovulation
takes place at approximately two-thirds of the oestrous period (reviewed by Soede and
Kemp, 1997), the timing of ovulation from onset of oestrus may vary considerably.
17.3
Lactation: optimize follicle development at weaning
Suppressed follicle development during lactation may cause reduced subsequent fertility.
To overcome this decrease in fertility, several management strategies can be applied.
These strategies aim to improve the energy balance of sows and/or reduce the intensity
of suckling in the last part of lactation, either by reducing litter size (split weaning)
or by limiting the period of suckling (intermittent suckling). Since lactation length in
itself is a major determinant of post-weaning ovarian activity, it is also discussed in the
present chapter.
17.3.1
Importance of weight loss
During lactation, most sows loose a substantial amount of their body protein and fat
reserves. When weight and protein losses exceed approximately 12%, reproductive
problems may occur, such as extended weaning-to-oestrus intervals, lower pregnancy
rates after insemination and lower subsequent litter sizes (Thaker and Bilkei, 2005).
Primiparous sows are especially at risk since they still have significant nutrient needs for
growth to maturity, a lower feed intake capacity and lower metabolizable fat and protein
stores (Hoving
et al.
, 2011). Studies evaluating influences of weight loss on reproductive
performance have therefore often used feed restriction in primiparous sows.
In the seventies and early eighties, lactational feed restriction was shown to have quite
dramatic effects on the weaning-to-oestrus interval (an increase of about 10 days) and to
have hardly any effects on ovulation rate and embryo survival. In more recent data, effects
of feed restriction on weaning-to-oestrus interval were quite small (less than a day) while
effects on ovulation rate and embryo survival were more pronounced (Table 17.1). Feed
restriction may decrease ovulation rate by about 2 to 4 and embryo survival by about 10
to even 20%. This change in responsiveness of sows to feed restriction during lactation is
