Heat Detection 101: A Beginner's Guide to Pig Breeding

If you are new to pig farming, one of the most confusing things is knowing when to breed your gilt or sow. "My sow just came on heat," a farmer wrote in a WhatsApp group this week. "What treatment do I give before crossing? I'm new to the business." The answer is simple in principle but requires careful observation: you breed her during the 3-day window of standing heat, which occurs every 21 days [1][2].

This guide is for absolute beginners. By the end, you will know: what heat is, how to detect it, when to breed, and what to do if you miss the window.

The pig reproductive cycle: the basics

A female pig (gilt if she has never farrowed, sow if she has) cycles every 18-24 days (average 21 days) once she reaches puberty at 5-7 months of age [3]. Each cycle has two phases:

• Follicular phase (5-7 days): The ovaries develop follicles that produce estrogen. Estrogen causes the visible signs of heat (swollen vulva, behavioral changes). This phase ends with ovulation.
• Luteal phase (13-15 days): After ovulation, the follicle becomes a corpus luteum that produces progesterone. Progesterone maintains pregnancy if the sow was bred; if not, the corpus luteum regresses after 13-15 days and the cycle starts over [3].

The "heat" or "estrus" that you observe is the follicular phase — specifically the last 1-3 days when estrogen is highest and the sow will accept the boar. This is called "standing heat" because the sow stands rigidly when pressure is applied to her back (the standing reflex).

The 3 stages of heat

Stage 1: Proestrus (1-3 days before standing heat)

The sow is coming into heat but is NOT yet ready to breed. Signs:

• Vulva swelling and redness: The vulva becomes enlarged, red/pink, and warm to the touch. This is often the first visible sign and can last 3-7 days before standing heat. In gilts, vulva swelling is more pronounced than in sows [1][4].
• Restlessness: The sow paces, fences-walks, sniffs other pigs, and is more vocal than usual.
• Decreased appetite: She eats less than normal.
• Vaginal discharge: A clear, sticky mucus may be visible. This is normal and indicates approaching ovulation.
• Will NOT stand for back pressure: If you press on her lower back, she walks away. She is not yet in standing heat [1].

Stage 2: Standing heat / estrus (1-3 days) — THE BREEDING WINDOW

The sow is ready to breed. This is the only stage during which breeding will result in pregnancy. Signs:

• Standing reflex (the definitive test): When you press firmly on her lower back (just above the tail), she stands rigidly, ears prick up, and she does not move. This is the "standing reflex" and it is the gold standard test for heat detection. If she stands, she is in standing heat [1][2][5].
• Ear twitching: When her back is pressed, she twitches her ears forward and upward. This is a reflexive response to the pressure.
• Locking of hind legs: Her hind legs may lock into a rigid stance.
• Seeking the boar: If a boar is nearby, she actively seeks him out, stands facing him, and allows mounting [5].
• Vulva less swollen but still pink: The vulva may be slightly less swollen than in proestrus but remains pink and may have a more watery discharge.
• Reduced feed intake: Many sows eat very little during standing heat.

Duration: Standing heat lasts 1-3 days (average 2 days) in sows, and is often shorter (1-2 days) and less intense in gilts [1][4]. Ovulation occurs approximately 30-36 hours after the onset of standing heat, typically late on day 2 of standing heat [2].

Stage 3: Metestrus (1-2 days after standing heat)

The sow is going out of heat. Signs:

• Vulva shrinking: The vulva returns to normal size and color.
• No longer stands for back pressure: She walks away when pressed.
• Resumes normal behavior: Eating, resting, social interaction return to normal.
• May have whitish discharge: This is normal — it is the mucus plug forming if she was bred.

When to breed: the golden rule

Breed the sow ONCE during standing heat, ideally 12-24 hours after you first observe the standing reflex. If using natural service (boar), breed her on day 1 of standing heat and again on day 2. If using artificial insemination (AI), the standard protocol is: first AI 12 hours after onset of standing heat, second AI 24 hours later [2][5].

Why timing matters: Sperm survive 24-36 hours in the sow's reproductive tract. The egg survives 6-12 hours after ovulation. If you breed too early, the sperm dies before the egg is released. If you breed too late, the egg is already dead. Breeding 12-24 hours after onset of standing heat ensures sperm is present when ovulation occurs (30-36 hours after standing heat onset) [2].

Heat detection protocol: what to do every day

Check every non-pregnant female twice daily (morning and evening) for heat. Use this protocol:

1. Visual check (5 seconds): Look at the vulva — is it swollen, red, or discharging? Look at her behavior — is she restless, pacing, or seeking other pigs?
2. Back pressure test (10 seconds): Press firmly on her lower back, just above the tail. If she stands rigidly with ears pricked, she is in standing heat. If she walks away, she is not.
3. Boar exposure (if available): Walk a boar past the sows' pens. Sows in heat will stand at the fence, vocalize, and seek the boar. This is the most reliable heat detection method and can detect heat 6-12 hours earlier than back pressure alone [5].
4. Record observations: Note the date, signs observed, and whether she stood for back pressure. This helps you predict the next heat (21 days from this one) and track the duration of heat for each individual female.

When to expect heat

Gilts (first heat): Puberty at 5-7 months of age, or 80-100kg body weight. Heat cycles are often irregular for the first 2-3 cycles. Breed gilts at their second or third heat (not the first), when they are 7-8 months old and 130-150kg — breeding too young results in small litters and stunted growth of the gilt herself [4].

Weaned sows: Most sows show heat 3-7 days after weaning (average 4-5 days) [3]. If she does not show heat by day 7 post-weaning, something is wrong — usually excessive weight loss during lactation, or uterine infection. Check her body condition and consult a vet if heat is delayed beyond 10 days.

Post-mating heat check: Check all bred sows at 18-24 days after mating. If she shows heat again, she did not conceive ("returns to heat"). If she does not show heat, she is presumed pregnant. Confirm with ultrasound at day 28-35 [3].

Common heat detection mistakes

• Checking only once a day: Heat can start and end between morning and evening checks. Check twice daily.
• Relying on vulva swelling alone: The vulva swells during proestrus (before the sow is ready to breed) and during estrus. Vulva swelling alone does NOT mean she is ready. Always confirm with the back pressure test [1].
• Not using a boar for detection: A boar detects heat 6-12 hours earlier than humans. If you have a boar, use him for heat detection even if you breed by AI [5].
• Breeding at the wrong time: Breeding on the first day of standing heat is often too early. Wait 12-24 hours after the standing reflex first appears.
• Missing the window entirely: If you check heat only every 2-3 days, you will miss the 1-3 day standing heat window. Daily checking is essential.
• Not recording dates: Without records, you cannot predict the next heat. Every heat event must be logged with date and duration.

What to do if you miss the heat

If you miss the standing heat window, the sow will return to heat in 18-24 days (average 21 days). Mark the calendar and check her twice daily starting on day 18. The next heat will follow the same pattern as the one you missed.

If the sow is not showing heat at all (no vulva swelling, no behavioral changes) for more than 6 weeks, possible causes: (a) she is pregnant (check with ultrasound or wait for delivery), (b) she has a silent heat (estrous signs not visible but she is still cycling — use a boar to detect), (c) she has cystic ovaries (veterinary diagnosis + hormone treatment), (d) she is in poor body condition (improve feed and wait), or (e) she is genetically subfertile (consider culling).

FarmWise integration

FarmWise tracks each female's heat cycle based on the dates you log. When you record a heat event, the app calculates the expected next heat (21 days later), sends a reminder 3 days before, and prompts you to check twice daily during the expected window. When you record a breeding event, it calculates expected farrowing date (breeding + 114 days) and starts the farrowing preparation countdown. No more missed heats or forgotten farrowing dates.

Sources & References

• [1] Pork Gateway (Ohio State University): Estrus or Heat Detection — vulva reddening, swelling, mucous discharge prior to estrus; standing estrus shorter and less intense in gilts than sows. Source: porkgateway.org
• [2] The Pig Site: Effective Heat Detection — estrus period lasts 1-3 days with ovulation often occurring late on the second day of standing heat. Source: thepigsite.com
• [3] Merck Veterinary Manual: Breeding Management of Pigs — normal uterine physiology reestablished by 21-28 days postpartum; most sows exhibit estrus 3-7 days after weaning. Source: merckvetmanual.com
• [4] AHDB (UK): Heat detection in pigs — signs of coming into heat include swollen, red vulva, watery discharge, clitoris changes. Source: ahdb.org.uk
• [5] Utah State University: Swine Artificial Insemination for Beginners: Heat Detection — swelling of the vulva is the most easily recognized physical sign; boar exposure improves detection. Source: digitalcommons.usu.edu