**3.3 Skeletal and smooth muscle contractions**

*Animal Reproduction in Veterinary Medicine*

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

*Schematic of the timing of ovulation with associated hormonal and behavioral changes. Weaning is at time* 

LH, and FSH secretion increase. This increase will re-start estrus.

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

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

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.

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

an increase in vulva size often refer as swelling.

**42**

vulva reddening.

**Figure 3.**

*zero. From Pedersen [8].*

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

#### **3.4 Gilt development**

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, or because they were not bred after weaning.


*The percentage figures refer to the % of sows that first show that feature on each day. \* n = 13.*

#### **Table 2.**

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

Replacement gilts are selected based on their growth rate, body composition, and their mother's reproductive success [17]. In general, gilts selected as replacement are moved from the growing facility to the gilt development unit (GDU) within the breeding farm when they are around 150 days old. To accelerate the onset of puberty, gilts in the GDU are often exposed to live boars. Gilts can have direct contact with a boar or indirect contact through pen fencing. Usually, groups of vasectomized boars are introduced into gilt's home pen for at least 20 min per day. Boar should not be housed in the GDU unit since gilts will be habituated the boar olfactory, visual, tactile, and auditory stimuli and this could decrease effective heat detection by farm workers [17].

When boars are introduced into the gilt pen, farm personnel will check gilts for estrus behavior and vulva changes described above. Daily boar exposure will induce estrus in most gilt within 10–20 days [17]. Gilts in heat are then moved to breeding stalls so they can habituate to the new environment and are breed in their second estrus. The term heat-no service (HNS) is commonly used to identify gilts that had their first estrus but were not breed. At the time of first service gilts should weight 135–150 kg and have a back fat of 12–18 mm [16, 17]. After 23 days of boar exposure, gilts that have shown no sign of estrus, can be hormonally treated to induce estrus by use of PG600. Gilts that did not come into heat after 28 days of boar exposure are usually culled from the breeding herd [16]. Gilts can be treated with Altrenogest to synchronize their estrus cycles.
