Oestrous Cycle

The domestic sow is generally polyoestrous, but the wild pig is a seasonal breeder with the main breeding season occuring in autumn with another peak in April.
There is some influence of photoperiod on reproduction in the domestic sow.
- Anoestrus occurs more commonly in summer and less commonly in February and March.
- Ovulation rate is lower in summer.
- Artificially reducing the day length from May-August will decrease the interval from weaning to oestrus from 23.6 days on average to 5.7 days.
The oestrous cycle normally lasts 21 days, interrupted only by pregnancy and lactation.
- During lactation, the physical stimuli of suckling will suppress cyclic activity.
Oestrus normally lasts ~53 hours.
Ovulation occurs 38-42 hours after the onset of oestrus.
Many sows show an anovulatory oestrus 2 days after farrowing.
When weaning occurs at 5-6 weeks, oestrus occurs within 4-6 days.
Earlier weaning results in a slightly longer interval before subsequent oestrus.

Cyclic Changes in the Ovaries

The ovaries of a mature, cyclic sow are large and look like mullberries.
- Surface lobulations are due to elevations of large follicles and corpora lutea.
When mature, follicles reach a diameter of 0.8-1cm.
When mature, corpora lutea reach a diameter of 1-1.3 cm.
It is difficult to study follicular dynamics in a sow because it is hard to identify a single follicle due to the large number present.
Except during the follicular phase, there is continuous proliferation and atresia of follicles.
There is normally a pool of about 50 follicles 2-5mm in diameter at any one time.
Between day 14-16 of the cycle,there is follicular recruitment under the influence of gonadotrophins.
- This is possible due to a decline in progesterone, and subsequent withdrawal of negative feedback.
A large number of these recruited follicles are destined for ovuation.
At day 21-22 it is possible to identify the pre-ovulatory population.
The growth of pre-ovulatory follicles during this follicular phase is associated with rapid atresia of small follicles. There is also a block to replacement of atretic follicles within the proliferating pool, so there must be some ovarian control mechanism. What exactly this mechanism is is currently unknown, but various substances have been proposed, including:
- Steroids
- Growth factors
- Growth factor binding proteins
- 'Follicle regulatory protein'
The ripe follicle is a 'se-shell pink' colour. It has a fine network of surface blood vessels and one point which is very transparent. This transparent point indicates the site of imminent ovulation.
Haemorrhagic follicles are common.
After ovulation, a considerable number of follicles ~ 0.4cm remain on the ovary. Some of these enlarge gradually to 0.9cm by day 18.
Immediately after ovulation, the ruptured follicle is seen as a congested depression on the ovary.
Accumulation of blood clot soon gives the ruptured follicle a conical shape.
By day 3 after ovulation, its cavity is filled with a dark red blood clot.
By day 6, the blood clot is replaced by a connective tissue plug, or slightly yellow fliud.
- Clots may persist up to day 12.
- Fluid may persist up to day 18.
Corpora lutea attain maximum size at 12-15 days post-ovulation.
After this, they gradually regress to the next oestrus.
They are dark red up to day 3.
They then change colour and remain 'wine red' up to day 15.
As the corpus luteum regresses between days 15-18, the colour rapidly changes to yellow, creamy yellow or buff.
The mechanism of luteolysis is not fully understood in the sow.
- Corpora lutea are unresponsive to PGF2α until 12-23 days after ovulation.
- During luteolysis, corpora lutea are invaded by macrophages.
- Macrophages produce Tumour Necrosis Factor (TNF).
- TNF and PGF2α probably work together to cause luteolysis.
- TNF also inhibits oestradiol production, thus eliminating the luteotrophic source.
There is rapid regression of corpora lutea at the next oestrus, but throughout the next dioestrus the corpora lutea remain as distinct entities.
After this they rapidly regress to grey pin-head foci on the ovary.
During the luteal phase of the cycle, oestrogens are luteotrophic. This may prolong the lifespan of the corpora lutea for several weeks. The result of this is pseudopregnancy.

Endocrine Changes during the Oestrous Cycle

Oestrogens start to rise at the time of corpora lutea regression.
Oestrogens reach a peak about 48 hours before the onset of oestrus.
The ovulatory LH surge occurs at the start of oestrus, 8-15 hours after the peak in oestrogen.
LH concentrations remain low and fluctuate throughout the rest of the cycle.
FSH value vary considerably, but there are two surges.
- One surge is concurrent with the LH peak.
- A larger surge occurs on day 23 of the cycle.
Progesterone concentrations closely follow changes of the corpora lutea.
- In the first 8 days post-ovulation, there is good correlation between progesterone concentrations and the number of corpora lutea present.
- By day 12 post-ovulation, the correlation is less obvious.
There are two surges of prolactin.
- One surge occurs at the same time as the LH and oestrogen surge.
- A second surge occurs during oestrus.