**3. Overview of the estrus cycle**

#### **3.1 Hormonal changes**

*Animal Reproduction in Veterinary Medicine*

sow is in estrus.

fully in estrus (proestrus).

Vocalization—"chatting" or "chanting" with the "boar"

*We use these and related definitions in our recent work.*

*Definitions of sow sexual behavior when she is in estrus.*

**Word Definition**

We present in **Table 1** our definitions of sow sexual behaviors that are observed

The boar is non-discriminating when deciding which animal or object to mount.

If the sow is not in full estrus, she will avoid the boar. If she experiences an adult boar while not in heat, then she will vocalize (squeal) and be aggressive towards him to express her objection. In the author's personal observations of feral pigs and outdoor pigs allowed to mate naturally, a sow not in estrus is very aggressive towards adult males. With the matriarchal social structure in nature, the sow is

In the period of proestrus, the sow is becoming interested in the boar, but she will still not be willing to stand still for mounting. In this period, she will seek a boar, interact with him and allow for mutual sensory exploration. When these behaviors are observed and the BPT is administered, the sow will not stand still. This period is referred to on the farm as a sow that is coming into estrus but not

Standing still Also called standing posture, the sow is motionless for intervals no greater

Standing reflex, locked up When a sow stands still for more than 10 consecutive seconds, usually by

When a sow is in estrus, latency is usually less than 30 s

and in some sow muscle shaking) Latency to lock up Time in seconds from application of back pressure until the sow locks up.

Pricked ears Lifting ears from resting position, usually while sniffing and exhibiting

Moving Before and after full estrus, sows will move when back pressure is applied Sniff When a boar is near or when they experience a pheromone, they move their rooting disk as they sniff (see videos) Chomping When the sow has nose-to-nose contact with a boar, or if they experience the

Vocalization—stress Sows express a high-pitch squeal when they are objecting to back pressure or

a boar. This vocalization is not expressed by all sows

apply the liquid to their VNO

resting

applying back pressure. This behavior is exhibited by sows when they see the boar, during the back-pressure test, or when being mounted. Sometimes displayed together with pricked ears and muscle contraction (rigid muscles

sexual interest. Ears either stand straight up or are obviously higher than

complete pheromone, they will open and close their mouth, and move their tongue in and out while they keep their head level or elevated so they can

The sow grunts in a low-pitch repeated manner when she sees, smells or hears

a boar. This is a stress vocalization sows make when are not in estrus

than 10 consecutive seconds

while a sow is in a breeding crate. Note that because her movements and boar interaction are restricted while in a breeding crate, she will not express the same number of behaviors as in the wild. When the sow expresses intense standing still, we say she is "locked up." A sow can stand still for a few seconds at any time and not be locked up or in estrus. Being locked up is the primary and only reliable sign that a

The boar will attempt to mate sows not in estrus and they will mate an object shaped roughly like a sow (e.g., a boar semen collection dummy)—anything shaped

like a cylinder that stands still will be mounted by an adult boar.

clearly dominant to the boar, except when she is in estrus.

**40**

**Table 1.**

The sow estrus cycle is of 18–24 days long [6], with the median and mode of 21 days. Sows are polyestrous animals. This means that with the appropriate nutrition, good health, and the proper environmental conditions, sows will cycle through the year. The sow estrus cycle is only stopped by pregnancy or lactation, and possibly old age. In sows, lactational anestrus is due to the inhibition of the GnRH pulse by the suckling stimulus [7]. Gilts reach puberty at 5 or 6 months old. For sows, the first estrus post-weaning takes place 4–7 days after weaning for most, but not all sows (some have a longer or shorter wean to estrus interval).

During the estrous cycle, sow's hormones change. The hormonal changes mark the different stages of the estrus cycle. The sow estrus cycle is divided in two main phases, follicular and luteal. These phases are further divided into four stages. The follicular phase is divided into proestrus and estrus, while the luteal phase is divided into metestrus and diestrus.

For our discussion, we will assume that the first day of estrus is day 0 of the cycle (**Figure 3**). The onset of estrus is mainly caused by an increase in estrogens. Sow estrus usually last 40–60 h (from day 0 to day 1–2 of the cycle) [6] but some sows can be on estrus for longer. Sow will only show sexual behaviors and accept the boar during the estrus stage of the cycle. During estrus, estrogen, Follicle stimulating hormone (FSH) and Luteinizing hormone (LH) secretion peaks. During estrus, ovulation is caused by the LH surge. Ovulation occurs 30–40 h after estrus onset [6].

Almost immediately after ovulation, sow will enter the luteal phase. During the metestrus stage of the luteal phase (days 2–5 of the cycle), the follicle tissue will start its development into a corpus luteum. This process is called luteinization [6]. The end of metestrus and the beginning of diestrus is marked by the end of the

#### **Figure 3.**

*Schematic of the timing of ovulation with associated hormonal and behavioral changes. Weaning is at time zero. From Pedersen [8].*

luteinization process. Diestrus is the longest stage of the estrus cycle. It starts once the corpus luteum is formed and last for 12–15 days after (days 6–17 of the cycle). The corpus luteum will secret progesterone to prepare the uterus for implantation and to maintain pregnancy in case of successful fertilization by the boar or artificial insemination. If the sow is not pregnant, the uterus will start secreting prostaglandin F2 alpha and luteolysis (degradation of the corpus luteus) will start after 15 days post ovulation (proestrus stage) [6]. Proestrus (day 17–21 of the cycle) is characterized by an increase in prostaglandin F2 alpha secretion and the completion of luteolysis. During this phase, progesterone secretion is reduced and estrogen, then LH, and FSH secretion increase. This increase will re-start estrus.

#### **3.2 Anatomical changes (vulva size, color, and temperature)**

The introduction of assisted reproductive techniques, such as Artificial Insemination, has shifted the responsibility of estrus detection to humans with the assistance of a live boar. The traditional way to identify if a sow has come into heat and is ready to be breed, is based on the occurrence of sexual behaviors before, during or after the backpressure test (BPT) and by detecting physical changes in sow vulva. Physical changes associated with the onset of estrus are reported to include vulva reddening, increase in vulvar temperature, the presence of sticky mucus, and an increase in vulva size often refer as swelling.

Langendijk et al. [6] found that out of 130 sows only 87% showed an increase in internal vulva redness. In this study, they also found a significant variation on the time vulva reddening occurred. Even when reddening onset varied between individual sows, it always occurred before ovulation. Thus, they suggest that insemination of sows that shows vulva reddening should be delayed until the end of vulva reddening.

The increase in vulva size and infrared temperature observed during estrus have been correlated to the high estrogen levels during estrus. The elevated levels of estrogen increase the vaginal and vulvar blood flow resulting in both an increase in vulva temperature as well as vulva swelling [9]. The literature is contradictory related to vulva features. Sykes et al. [9] and Scolari et al. [10] found that the infrared vulva temperature increased by 1 C° the day of estrus whereas Simoes et al. [11] found that the temperature increase was during the proestrus period.

**43**

*\* n = 13.*

**Table 2.**

*Understanding Sow Sexual Behavior and the Application of the Boar Pheromone to Stimulate…*

Un-published studies recently conducted by the authors showed that not all sows will show these physical changes. We carefully measured color, size, surface temperature and vaginal temperature in sows before and during estrus (**Table 2**). Sows might show these physical changes at a different stage of the estrus cycle. Because not all sows will show changes in vulva color, temperature, or size during estrus, the onset of these changes varied among individual animals. Vulva physical and thermal changes are not reliable indicators for heat checking for all sows of use on the farm.

Myometrial activity before and after estrus is either absent or of low amplitude

and frequency [12, 13]. During estrus, sow myometrial electrical activity and contraction frequency and amplitude increase [12]. Myometrial contractions are regulated by progesterone, oxytocin, and estrogen concentrations [14]. High progesterone reduces uterine contractions whereas high oxytocin and estrogen levels increase them [14]. Oxytocin and estrogen secretion increase during estrus and sexual stimulation and arousal, although direct neuromuscular activation could be via the brain and spinal nerves. In vitro studies found that after an estrogen perfusion, there was a significant increase in peristalsis going from the isthmus uteri towards the corpus uteri [15]. Similar results were found with an oxytocin perfusion [15]. Uterine contractions are necessary for the movement of sperm from the uterus to the fallopian tubes. This could explain why seminal oxytocin and estrogen increase uterine contractions [12]. Boar presence induces sow oxytocin release and increases sow's myometrial activity [12]. The effects of each individual boar stimu-

lus (olfactory, tactile, and visual) on oxytocin release are still not clear [12]. Some sows will also show skeletal muscle contraction that one would call shaking during the standing reflex. From our behavioral studies, we estimated that fewer than one in ten sows will show this behavior. Skeletal muscle movement can

To continue the production cycle and swine sustainability, sows need to be replaced by gilts. Breeding farms should target to have an annual replacement rate lower than 50% [16]. In 2012, the average annual replacement rate was 45% [17]. This mean that around 900 gilts are needed per year to replace culled sows in a 2000 sow unit. Usually sows are culled due to low reproductive performance, lameness,

> **First day of estrus**

\* 0.00% 15.38% 0.00% 84.61%

Vulva reddening 31.82% 18.18% 13.64% 36.36% Vulva IR temperature 4.55% 59.09% 9.09% 27.27%

Vulva swelling 31.82% 13.64% 0.00% 54.55% Presence of sticky mucus 40.91% 36.36% 22.73% 0.00%

*Proportion of sows (*N *= 22) that showed vulva changes before, during, or after the first day of estrus.*

**Day after first day of estrus**

**No change**

**Measurement Day when the change was visible**

**Day before estrus**

*The percentage figures refer to the % of sows that first show that feature on each day.*

be easily perceived on sow shoulders, flank, neck and ears.

or because they were not bred after weaning.

**3.4 Gilt development**

Vaginal temperature (C˚)

*DOI: http://dx.doi.org/10.5772/intechopen.90774*

**3.3 Skeletal and smooth muscle contractions**

*Understanding Sow Sexual Behavior and the Application of the Boar Pheromone to Stimulate… DOI: http://dx.doi.org/10.5772/intechopen.90774*

Un-published studies recently conducted by the authors showed that not all sows will show these physical changes. We carefully measured color, size, surface temperature and vaginal temperature in sows before and during estrus (**Table 2**). Sows might show these physical changes at a different stage of the estrus cycle. Because not all sows will show changes in vulva color, temperature, or size during estrus, the onset of these changes varied among individual animals. Vulva physical and thermal changes are not reliable indicators for heat checking for all sows of use on the farm.
