The oestrus cycle

Usually the reproductive strategy of large animals is to produce one offspring each pregnancy and to concentrate all the effort on its survival (monotocus species). The small animal tends to produce large number of offspring with the hope that one or two will survive (polytocus species).

Unlike the other large farm species such as the cow or the mare that have only one or occasionally two offspring, the pig is the only large farm animal that has a large number of offspring. During each oestrus cycle more then 15-20 eggs are usually ovulated. In sheep the situation is in the middle, some sheep breed have only one or two lamb, others 4 or 6.

The basic mechanism of the oestrus cycle in pigs are very similar to other farm species. However, there are some differences that are important to know for good management of reproduction in a pig farm.

The pig oestrus cycle (Fig 1,2 and 3) is characterised by 1) the development of the follicles up to ovulation and release the eggs at the best time to be fertilised, 2) the block of more ovulation and preparation of the uterus to accommodate the embryos and, 3) if there is not fertilisation, the development of other follicles for a new cycle and a new ovulation.

During the oestrus cycle can be distinguished a follicular phase and a lutal phase.

The follicular phase is the period between the end of luteal phase (day 16 of oestrus cycle) and the ovulation of follicles. This period in pigs is lasting about 5 days. However follicolar development starts as soon after the ovulation of the follicle of the previous oestrus cycle (day 0) when there is a surge of FSH that "recruit" a pool of follicles that start to develop. The further development of these follicle requires FSH and LH secretion. The growing follicles secrete oestradiol that stimulate LH secretion. When the follicles reach the pre-ovulatory size, if there is progesterone they will not be able to ovulate ad they will become atresic, conversely, if there is not progesterone, the large amount of oestradiol secreted from the dominant follicles induces a LH pre-ovulatory surge that will cause ovulation. Follicular development is present during all the oestrus cycle but this effects of the oestradiol are evident only during the last 4 or 5 days of the oestrus cycle when the progesterone levels are low. These high levels of oestradiol are required to induce oestrus behaviour and the LH surge for the ovulation. In the sow oestrus is very evident from the standing reflex.

During the oestrus cycle in the pig there does not appear to be distinct follicular waves as in cows; it has been proposed that the final growth of follicles to ovulatory size takes 19 days.

The luteal phase is characterised by the secretion of progesterone from of the corpus luteum. The corpus luteum is derived from the luteinization of the cells of the follicle after ovulation. In pig this phase is lasts about 16 day, when if there is not a pregnancy, the uterus secretes prostaglandin PGF2alfa that induces luteolysis and therefore a drop in progesterone secretion. In absence of progesterone a new cycle begins and a new set of follicles can ovulate.

The luteal phase lasts about 16 days because progesterone is required to prepare the endometrium for implantation and to produce"uterine milk" for the development of the embryos prior to placental attachment.

Since the factors controlling the oestrus cycle (i.e. GnRH, FSH, LH, oestradiol, progesterone inhibin, etc.) in pigs are the same as the other farm species like the cow

Fig. 1 The oestrus cycle in the sow



and since no consistent evidence of differences in circulating gonadotropin have been observed, the question arises: what are the mechanisms that allow 15-20 follicles to ovulate at each oestrus instead of one or two ?

One factor is that in the pig the more mature follicles assist the development of the less mature follicles. This is quite different from the cow were the most mature follicle (dominant follicle) is in competition with the less mature follicles and secretes paracrine and autocrine factors that blocks the development of the other follicles.

Furthermore in cow the dominant follicle secretes inhibin that block the FSH secretion; thus the plasma FSH levels are not sufficient to maintain the development of less mature follicles and they become atretic. In fact, the administration of FSH to a cow induces superovulation. In the pig the biological potency of porcine FSH is greater then the bovine FSH and it seems that porcine FSH secretory cells are less sensitive to inhibin.




Puberty is the time at which mating and conception can first occur naturally. This time in pigs is coincident with the onset of the first oestrus and ovulation. In the cow there are usually some short cycles that precede the first normal oestrus cycle.

In the pig there is an increase in ovulation rate over several cycles.

Before puberty, there is some ovarian activity and the administration of GnRH is able to induce follicular development and ovulation. Thus, what are the factors inhibiting the ovarian activity before puberty in pigs?

The answer is more than one:

-Changes in sensitivity to negative feedback effect of oestogen at the hyphotalamic level (the gonadostat theory)

-age-related increases in the metabolic clearance of oestrogens

-maturation of the hypothalamic-pituitary axis due to environmental factors such as nutrition (a certain amount of body fat and body mass is required before puberty can be reached e.g. insulin and other metabolic hormones increase the sensitivity of the hypothalamus to the positive oestradiol feed-back), photoperiod (not important in pig) and pherormones (pheromones receptors are present in gilts a little before puberty).

The reproductive management of the gilts

The inefficient management of gilts contributes to the 30% or more of the non-productive days in many herds. Therefore it is important to ensure that gilts reach pubertal oestrus at the earliest possible age and weight.

Which reproductive techniques that can be utilise to ensure this outcome?

The induction and synchronisation of pubertal oestrus in gilts

In the prepubertal gilt the osmic receptors of the bulberonasal organs are already responsive to male pheromones. Therefore one of the most efficient method to induce puberty in gilt is the "boar effect". The direct contact of gilts with a mature boar by visual, olfactory and tactile stimuli induces puberty. Therefore the contact of a group of 6 to 10 gilts with a mature boar for 10-15 minutes each day will induce puberty. If the gilts are not yet pubertal, an increasing percentage of them become in oestrus each week and therefore it is possible to have a better utilisation of the facilities of the farm. It is also possible to identify the more precocious gilts. It has been shown that the more precocious gilts will have better lifetame reproductive performances.

The use of the boar effect in gilts that have reach the weight and the age to for the puberty results in a large percentage of gilts becoming in oestrus in few days and can be more difficult to have a good utilisation of the farm facilities and distinguish the more precocious gilts.

With the use of exogenous hormones like PG600 (PMSG+HCG) is possible to induce fertile oestrus in a high proportion of late pre-pubertal gilts. However, the resulting litter size can be variable and the predictability with which gilts continue to cycle is low. Finally, it is possible to have absence of behavioural oestrus in some gilts that have ovulated (silent oestrus).

The synchronisation of oestrus in cycling gilts

The synchronisation of oestrus in cyclic gilts can achieved by the administration of oral progestagen like allyl trembolone (Regumate) for 14-18 days. The gilts will come into oestrus about 5-6 days after the end of the treatment.

Another possibility is the use of luteolytic agents like prostaglandin PGF2alfa. The problem with the PGF2alfa in pigs is that the corpus luteum is sensitive to PGF2alfa only after day 12 -13 of oestrus cycle .

When should the gilt be inseminated?

At oestrus the gilts should be inseminated twice on am/pm or pm/am basis with the first insemination done immediately if the gilts are checked for oestrus only once at day or by back pressure, or approximately 12h after the first detection of standing heat if the gilts are checked twice a day.

The pregnancy and parturition

In pig the pregnancy is quite short, 114 days (3 month, 3 weeks and 3 days) and the day of farrowing highly predictable.

To avoid the luteolisis at day 16 of oestrus cycle, the embryo at around day 11-12 of the oestrus cycle secretes estradiol that re-directs uterine prostaglandins from the vascular system of the uterus to the lumen of the uterus thus blocking the luteolitic effect.

Different system can be utilised for pregnancy diagnosis: rectal palpation (at approximately at day 28), determination of progesterone plasma levels (around day 21) or oestrone sulphate (day 21-28). However at the moment, the most practical systems is ultrasound. It has been determineted that with ultrasound high accuracy (95%) can be achieved from day 18 of pregnancy. Pregnancy diagnosis as early as possible is important to be able to identify the non pregnant animals to include them in the breeding program. In pig farms failure to identify non pregnant animals causes a decrease in reproductive pherformances and will increase reproductive herd problems.

The mechanism of parturition is similar in pigs as in other farm species. from practical point of view it could be beneficial to induce farrowing during the working hours to save some piglets. Different systems have been utilised based on the hormones involved in the parturition; prostaglandin injections (single or double), relaxin injection (alone or in combination with PGF2alfa), oxytocin (alone or in combination with PGF2alfa) and beta blokers (carazololo) (Tab 1).







Tab.1 Some treatments to induce farrowing in sow


farrowing (hours from treatment))

PGF2alfa (day 112)

30.4± 9.4

Relaxin (day 111) + PGF2alfa (day 112)

25.9± 3.7

PGF2alfa (day 111) + Oxytocin (day 112)

25.2± 7.5

Carazolol (after milk let down)


The post-partum period in sow

During lactation in sows there is lack of follicular development and therefore absence of oestrus and ovulation. These effects seems mainly due to a lack of LH secretion due to the inhibitory effect of suckling. With time from farrowing there is a gradual increase of LH secretion and, after 30-35 days from farrowing, because of a decline in suckling frequency and milk production, it is possible to have follicular development, ovulation and oestrus. More detailed study has demonstrated that there is some LH secretion during the first 56-72 h after farrowing , then LH secretion decrease with a nadir around day 7-14 post-partum and then increases again at day 21-28 of lactation.

Traditionally the farmer removes the piglets from the mother (weaning) around day 21-28 of lactation. Immediately after weaning there is an increase LH secretion and, after 5-6 days from weaning, most of sow show oestrus and can be inseminated.

On some farms the weaning time has been shortned to 13-14 days.

There is a relationship between lactational and the interval weaning to oestrus (WEI). Weaning at day 10-13 the WEI is about 7 days and the average litter size is about 10 piglets while weaning at day 26 the WEI is significantly shorter (about 5-6 days) and the litter size larger (11-12 piglets).

However the estimated piglets per sow per year are higher in early weaned compared late weaned sows (31.2 versus 28.3). One problem that can arise with very early weaning is that in some sows there is not enough LH for the LH surge and therefore the follicles cannot ovulate. These sows therefore will have a long period of anestrous.

In Fig 4 are showed some treatments to induce ovulation also during the lactation period in sow. The result are encouraging.

In conclusion in sows during the post-partum there is an unavoidable period of infertility due to the inhibitory effect of suckling, the recovery of the brain and pituitary from the effects of high steroid hormones levels during pregnancy and the time required for complete uterine involution (about 21 days).

The management of the weaning to oestrus interval

It has been observed that there are higher LH plasma levels before weaning in sows with a shorter WEI. Therefore it is possible that a decrease in the suckling stimulus immediately before weaning will increase LH secretion and therefore advance reproductive activity during the WEI. Two technique have been developed to do this . The first is based on removing all the piglets from the mother for some hours on 3 or 4 consecutive days, the second, that is much more practical, consist in removing half of the litter (the heavier piglets), a few days before final weaning. In this way it should Tab 2 effect of SW in primiparous ad pluriparous sow (CONT=control sow; SW=split-weaned sow)













n. of sows





day of weaning

22.3± 4.14

25.83± 2.98

22.4± 23.61

25.5± 2.72


6.4± 3.4.7

6.4± 5.74

5.92± 4.01

5.56± 4.14

total piglets born

9.78± 2.73a

10.46± 2.66 b

10.27± 2.73 a

10.5± 2.69 b

n. in oestrus < 16 d





n. return in oestrus ³ 16 d





a, b p<0.001







be possible to decrease the suckling stimulus and milk production with a beneficial effect on WEI.


Timing of the A.I. after weaning in sow.

Sows coming into heat soon after weaning (day 3-4) will show oestrus over 3 days, so insemination must be begun on day 2-3 of heat. Sows with an average interval from weaning to onset of heat (5 days) present a heat period of approximately 2 days and should be inseminated 24h after heat onset, with a second insemination to be done 12h later. Sows showing delayed heat after weaning (day 6 and beyond) tend to show heat only for a short period (1 day) indicating that insemination should be performed within 24h of heat detection.

A protocol for oestrus synchronisation after weaning has been developed in sows. It consist in the injection of PMSG 24h after weaning and GnRH injection 48h later to stimulate ovulation. The insemination can be done at a fixed times 24 and 36h after GnRH administration. With this protocol oestrus detection is not required.