Estrus in Sows and Gilts
Domesticated sows and gilts are classified as nonseasonal and polyestrous, with the estrous cycle lasting 18–24 (average 21) days. However, pigs do show seasonal declines in fertility associated with their ancestral relation to the wild pig. Sows are behaviorally anestrous during pregnancy. Ovulatory estrus usually does not occur during the first 3 weeks of lactation, except under conditions of group rearing, high feed levels, split or partial weaning, in combination with boar contact. Partial weaning or gonadotropin treatment can induce estrus during lactation; however, results in the first 33 weeks are inconsistent and not economical. Normal uterine physiology is reestablished by 20–25 days postpartum. Most sows exhibit estrus 3–7 days after weaning.
Estrus in gilts and postweaning anestrous sows can be initiated with exogenous hormones, and when they are inseminated at estrus, can result in good fertility, provided that the gilts are of proper age and weight and weaned sows are in good body condition. This hormone combination can be used to prevent the incidence of anestrus or delayed estrus in less fertile sows, most notably in parity 1 sows and sows weaned in summer and fall. However, these hormones do circumvent natural selection for reproductive efficiency, and this should be kept in mind when they are used in breeding management programs. Exogenous hormones should not be used as a longterm solution to address underlying reproductive problems in a herd.
Estrus lasts ~36–48 hours (1–2 days) in gilts and ≥48–72 hours (2–3 days) in sows. The interval to estrus after weaning and duration of estrus in sows can be influenced by length of lactation, nutrition, body condition, genetics, housing, boar exposure, and other management practices ( see Table: Factors Affecting Ovarian Activity of Pigs Factors Affecting Ovarian Activity of Pigs ). In loose-housed females, estrus is characterized by behavioral (eg, mounting, fence walking, vocalizing, tilted ears, kyphosis) and sometimes physical (eg, vulvar swelling, vaginal discharge) changes. In females confined to stalls, immobility and erect ears in response to boar exposure and the back-pressure test are the most obvious signs.
Ovulation generally occurs in mid to late estrus. During ovulation, ~15–24 ova are released over a 1- to 4-hour period. Ovulation rate increases over the first four parities, so that the fourth to sixth litters tend to be the largest in number. Ovulation rate can decrease when gilts or sows are undernourished. Most gilts are on full feed, thereby averting the adverse affects of undernourishment on early reproductive performance. In countries in which gilts are not routinely provided full feed, increasing energy intake for 10 days before estrus (ie, “flushing”) is performed. This has optimized ovulation rate under these circumstances. To prevent undernourishment in recently weaned sows, an energy-dense diet should be fed until after estrus and breeding.
Behavioral changes for estrus are most pronounced when the sow or gilt is exposed directly to the sight, sound, odor, and attention (nuzzling and grunting) of a mature boar. A sow or gilt in standing heat normally assumes a rigid, immobile, receptive stance when exposed to a boar. Physical changes such as vulvar swelling and discharge are often unreliable; they do, however, appear to be more marked in gilts than sows and commonly develop 2–3 days before estrus. The ultimate criterion of estrus is either standing to the boar or a positive response to the "back-pressure" test (an attendant applies pressure with the hands in the loin area, then gently sits on the pig’s back to elicit the standing reaction); this test is best conducted in the presence of a mature, active boar (in physical contact or in an adjacent pen) or, as a less-effective alternative, after exposing the sow to a synthetic boar-odor aerosol or taint rag.
Anestrus is a common problem. Failure to detect estrus must be distinguished from true cases of ovarian inactivity. First-litter and early-weaned sows are particularly vulnerable to postweaning anestrus. The primiparous sow must support her own growth as well as maintenance and lactation demands while her feed intake capacity is not yet fully developed. This problem can be avoided by breeding only gilts in good condition; not overfeeding during the first gestation; and encouraging energy intake during the first lactation by frequent feeding of high-density diets, wet feeding, and avoiding high temperatures in the farrowing rooms. Management practices that were once used to improve disease transmission between the mother and litter, including segregated early weaning, modified medicated early weaning, and medicated early weaning, are not commonly used now because of the detrimental effects on sow fertility. Partial weaning (split weaning) is still used; however, it is important to allow enough pigs to continue nursing so that sows do not immediately return to estrus and ovulate while still lactating.
Hormonal Control of the Estrous Cycle in Pigs
Estrous synchronization may be achieved by group weaning of lactating sows on a single day, with estrus typically occurring 4–6 days later. Administration of a commercially available combination of 400 IU of equine chorionic gonadotropin (eCG) and 200 IU of human chorionic gonadotropin (hCG) per 5 mL dose given as a single IM injection at or within 12 hours after weaning can improve estrus within 7 days of weaning and further tighten the synchronization of estrus. This eCG and hCG combination also can induce estrus in gilts with delayed puberty and prevent postweaning anestrus in primiparous sows.
Fixed-time insemination protocols continue to gain interest in the swine industry. Current recommendations call for feeding progestin for 14 days to synchronize the start of the follicle phase in the mature, cycling gilt, and then administering the eCG-hCG combination after last feeding of the progestin, followed by a GnRH analogue. With weaned sows, a GnRH analogue is administered 83–96 hours after weaning. Breeding of both gilts and sows is then performed using a single timed artificial insemination (AI ) 20–33 hours after GnRH administration (depending on product and route of administration).
Exogenous prostaglandin induces luteolysis of the corpus luteum only after day 12 of the estrous cycle and, therefore, is not a practical agent for estrous cycle control. However, estrus may be synchronized by induction of abortion in sows pregnant >15 days by administration of prostaglandin F2alpha (15 mg, IM; then 10 mg, IM, 12 hours later) or an equivalent analogue. Estrus may also be synchronized by feeding altrenogest (15–20 mg, PO, daily for 14–18 days), with estrus being observed 4–9 days after the last dose with appropriate boar exposure. Combination eCG and hCG may be given on the day of progestagen withdrawal to better synchronize estrus.
Breeding in Pigs
The three methods of breeding are pen mating (boar run with females), hand mating (supervised natural mating), and AI. Pen mating is generally found on smaller operations and works best in a pen of pigs in various stages of the estrous cycle. Pen mating with a group of recently weaned sows is less desirable, because their estrous cycles may occur close together and lead to overuse of the boar. In hand mating, the female is usually mated two or three times during estrus, with the first service on the first day of standing estrus, and subsequent matings at 24-hour intervals; confirmed matings should be recorded. Many commercial producers breed the sow or gilt once daily as long as she will accept the boar. The use of two different boars may increase the number of pigs per litter; however, infertility in one of the boars may be masked.
In AI programs, heat detection is performed either twice or once per day. If heat detection is performed twice per day, gilts should be inseminated twice, 8–12 hours after the onset of standing heat and again 12–16 hours later. Sows should be inseminated 24 hours after onset of standing heat and again 18–24 hours later. If heat detection is performed once per day, gilts should be inseminated within 4 hours and sows within 12–16 hours from when they were first observed in standing heat. A second insemination should be performed as described above for those animals that remain in standing heat.
Timing of AI may need to be modified based on a particular farm’s availability of labor, building design, or herd genetics. Some experienced users of AI obtain satisfactory results in gilts with a timed, single insemination; however, performing two inseminations is more common and more successful at improving outcomes. Inseminations can be performed using either single-sire (sourced from one boar) or pooled (sourced from multiple [three to six] boar ejaculates) extended semen. In general, single-sire matings are performed when particular genetic (ie, breeding or show animals) offspring are desired, whereas matings with pooled semen are used as a means to produce market hog offspring. Minimum suggested values for extended semen used within 72 hours after collection are provided in (Suggested Values and Protocols for Use of Extended Porcine Semen in an Artificial Insemination (AI) Program Suggested Values and Protocols for Use of Extended Porcine Semen in an Artificial Insemination (AI) Program ). Total sperm numbers in a dose of semen depend on quality and storage time of the semen.
Boars should not be overused ( see Table: Suggested Guidelines for Boar Usage Based on Breeding Program a Suggested Guidelines for Boar Usage Based on Breeding Program a ). If sows are weaned in groups, a boar-to-sow ratio of 1:4 for mature boars and 1:2 for young boars is recommended. In hand mating, a mature boar should be used for ≤2 breedings/day. When using natural service, a boar-to-sow ratio of 1:15–1:25 (average 1:17 or 18) is usually needed. When using AI, the boar-to-sow ratio can be increased to 1:150–1:400. Currently, however, most commercial breeding operations use AI and purchase semen from outside genetic suppliers, and most boars on the farm are used for detection of estrus and not for breeding.
Pregnancy in Pigs
Sperm cells reach the oviducts within 30 minutes of breeding and continue to establish the sperm reservoir over the next 2 hours. Fertilization occurs within 2–6 hours of ovulation, provided sperm are already present. Fertilization rates approach 100% in sows; however, embryo mortality up to 30%–40% accounts for the usual litter size of 12–16 total born pigs. Embryos enter the uterus ~48–60 hours after ovulation. Embryos hatch from the zona pellucida and form blastocysts 144 hours after ovulation. Physiologic recognition of pregnancy in the dam (embryos secreting estradiol) occurs by day 12–14 of gestation, with intrauterine migration and distribution of embryos. Embryo attachment begins by day 13–14, with implantation complete by day 40; a minimum of four embryos must be present at this time for pregnancy to continue. Skeletal mineralization develops by day 35, with fetuses immunocompetent by day 70–75. Fetal deaths that occur after day 40 can result in expulsion or retention of recognizable piglets. Retained dead fetuses in this sterile environment become mummified and are usually expelled at the time of farrowing. The average gestation length is 115 ± 2 days and is somewhat shortened in sows with large litters.
Embryos are at greatest risk of dying during the first 30 days, and efforts should be directed toward avoiding stresses to the sow (eg, overfeeding, heat, handling or moving, immunization) during this critical period. Pregnancies of < 16 days are especially sensitive to heat stress. Avoiding exposure to outside animals reduces disease risk. Reports of embryo losses before day 30 of pregnancy are controversial for both full-feeding (>4–5 lb or >2 kg) and limit-feeding sows and gilts after breeding. However, perhaps more critical is that nutritional allotment is increased to meet the metabolic demands of the pregnant gilt that is still growing and of the weaned sow that is recovering her body condition from lactation. Farrowing less than five piglets can be indicative of breeding failures or early embryo death after the time of attachment.
To increase colostral antibodies, the gilt or sow should be immunized during the last 6 weeks of gestation. An immunization program may include vaccination against Escherichia coli, atrophic rhinitis, and erysipelas, and provision of any other vaccines appropriate for the disease situation on the individual farm.
Pregnancy Determination in Pigs
Several techniques are available for pregnancy determination ( see Table: Common Tests for Detection of Pregnancy In Pigs Common Tests for Detection of Pregnancy In Pigs ). In most larger commercial breeding herds, pregnancy is typically determined using real-time ultrasound. On smaller farms, pregnancy is most commonly determined by noting that the female does not return to estrus in 18–25 days;this method can be 75%–85% accurate.
There are three types of ultrasound machines: pulse echo (A-mode), Doppler, and real-time B-mode. Pulse echo or amplitude depth involves emitting ultrasonic waves from a hand-held transducer placed on the skin in the flank area. Reflected waves from a fluid-filled area (ie, developing conceptus or fetus) are picked up by the transducer and converted into either an audible or visual signal. Doppler ultrasonography detects changes in sound frequency (fluid movement) using an audible signal; movements indicative of pregnancy include blood flow in middle uterine or umbilical arteries, fetal heartbeat, and fetal movements. Real-time B-mode ultrasonography involves visualization of a 2-dimensional image of scanned tissues directly under the transducer.
Ultrasonographic techniques are generally used at 22–75 days to determine pregnancy, with real-time ultrasonography being used most often at 24–30 days after breeding. Although uncommonly used for this purpose, rectal palpation can be performed to confirm pregnancy at >30 days gestation. When performing rectal palpation, the examiner palpates for fremitus, size, and position of the middle (medial) uterine artery in relation to the external iliac artery. The tone and tension of the cervix and weight and contents of the uterus can also be used to help confirm pregnancy. Other possible techniques, such as hormonal assays (eg, estrone glucuronide, progesterone, prostaglandin) and vaginal biopsy, are not economically feasible.
Parturition in Pigs
The preparturient period involves restlessness and nest building in the last 24 hours. Mammary glands become turgid, and the secretion changes from serous to milk as parturition approaches. Parturition is initiated by increased cortisol levels, which also stimulate release of PGF2alpha from the uterus. PG F2-alpha causes luteolysis of the corpora lutea and release of relaxin, which causes relaxation of the birth canal and cervix. Oxytocin is released from the pituitary gland, which causes uterine contractions and onset of labor. Piglets are usually delivered at frequent intervals (average 10–15 minutes; range 5–45 minutes). Uterine horn evacuation is random.
The stillbirth rate usually is 5%–10%; intra-uterine deaths are due to infection, incorrect position in the uterine horn during delivery, or anoxia. Anoxia occurs when the umbilical cord ruptures or becomes constricted because of the extreme length of the uterine horn or when there is a delay in transit along the birth canal. Stillborn and weak piglets also may result from low temperatures in the farrowing house or low Hgb levels (< 9 g/dL) in the sow. Any increase in the time interval between pigs born (eg, because of exhaustion, atony of the uterus, or dystocia) increases the chance of injury or death to the piglets still in the uterus.
Piglets are born in both cranial (60%) and caudal (40%) presentations. Assistance can be provided in the form of oxytocin injections (10–30 IU) and manual removal of piglets. Walking the sow for a few moments also can be helpful. The number of pigs born alive can be increased by approximately one per sow if an attendant is present to assist delivery ( see Preweaning Mortality Preweaning Mortality read more ). Passage of the fetal membranes should occur within 4 hours of delivering the last piglet.
Farrowing can be induced by IM injection of 10–15 mg of natural PG F2-alpha or equivalent dose of synthetic analogues. Farrowing generally occurs 18–36 hours later (most within 22–32 hours) in 80%–90% of sows when PG F2-alpha is given at or after 113–114 days gestation. Some farms use induction to prevent late farrowings only for those that have not started by day 115. Induction can be used so that most farrowings occur during normal working hours, avoiding evenings, weekends, and holidays. Good records are essential, and average days of gestation for the sow herd and individual breeding dates for each sow must be known. PG F2-alpha must be used within 72 hours of the expected farrowing date to prevent an increase in stillbirths. The slightly premature piglets require good environmental conditions, particularly in cold weather. Farrowings may be concentrated into an even shorter period by injecting 20 IU of oxytocin, IM, 15–24 hours after the PG F2-alpha injection. In this situation, oxytocin shortens the interval to parturition but can increase the likelihood of dystocia. One recommendation is to give oxytocin 15–24 hours after prostaglandin only if milk can be stripped from the teats. Successful farrowing can also be induced by giving a single vulvomucosal injection of 5–10 IU of oxytocin.
Incidence of dystocia is low (1%–2%) in sows. As with all polytocous species, uterine inertia accounts for most dystocia in swine. Other causes include fetal malposition, obstruction of the birth canal, deviation of the uterus, fetopelvic disproportion, and maternal excitement. A thorough digital examination of the birth canal is prerequisite to therapeutic intervention. Medical treatment for unobstructive dystocia may include use of an ecbolic agent (oxytocin at 20–30 IU, every 30 minutes, up to 3 times). Administration of injectable calcium may be warranted if uterine inertia is suspected.
Lactation peaks at 3–4 weeks postpartum, and most sows are weaned at 21–24 days. Sows that have been on an 8-week lactation produce 400–700 lb of milk. Poor lactation is an important cause of impaired productivity in pigs.
Preweaning Mortality in Pigs
Supervised farrowing alone can help to reduce piglet mortality, because it minimizes stillbirths, allows for drying or warming of piglets and observation of nursing activity, and prevents crushing and cannibalism. Other management techniques available to reduce piglet mortality include cross-fostering, split-suckling, well-designed farrowing crates and pens, prepartum vaccination of sows, appropriate feeding programs for lactating sows, and cleanliness.