A sow's complete cycle is 147 days: 115 days pregnant, 28 nursing, 4 to 6 days back into heat. That is exactly 21 weeks and exactly seven oestrus cycles. Which means the calendar for a small herd is not a spreadsheet problem, it is a grid of seven three-week slots that you fill as you grow.
Most backyard herds have no calendar at all. Sows get served whenever somebody notices them standing, and weaners come out in ones and twos across the year. Fine way to keep pigs, bad way to sell them. Uniform lots fetch more, an empty pen can actually be disinfected, and a batch you can predict is a batch you can aim at a market.
The 21-week grid
Start from the four numbers you can actually control.
| Stage | Days | Notes |
|---|---|---|
| Gestation | 115 | Merck: 115–116 ± 1 from first service |
| Lactation | 28 | The Leyte field data has backyard herds weaning at a median of 45 days |
| Weaning to service | 4 | Merck: >85% in heat within 4–6 days of group weaning |
| Full cycle | 147 | 21 weeks. Seven 21-day cycles. |
That 147 is why every commercial batch farrowing system runs on multiples of three weeks. The Farm Advisory Service in Scotland lists four standard schemes: weekly (21 groups), two-week (11), three-week (7) and five-week (4). Only one is any use below fifteen sows.
The three-week scheme, with seven groups. Four groups pregnant, one being served, two nursing. A sow leaves a slot and returns to the same slot 147 days later, and if she misses she drops exactly one slot. Nothing else divides cleanly.
Here is a real grid, dated, starting the first Monday of January 2027.
| Slot | Serve | Farrow | Wean | Back to service |
|---|---|---|---|---|
| 1 | 4 Jan | 29 Apr | 27 May | 31 May |
| 2 | 25 Jan | 20 May | 17 Jun | 21 Jun |
| 3 | 15 Feb | 10 Jun | 8 Jul | 12 Jul |
| 4 | 8 Mar | 1 Jul | 29 Jul | 2 Aug |
| 5 | 29 Mar | 22 Jul | 19 Aug | 23 Aug |
| 6 | 19 Apr | 12 Aug | 9 Sep | 13 Sep |
| 7 | 10 May | 2 Sep | 30 Sep | 4 Oct |
Slot 1 comes back to service on 31 May, three weeks after slot 7. The grid closes on itself, and every three weeks the same week carries one service, one farrowing and one weaning.
Which scheme for your herd size
You do not need seven sows to use a seven-slot grid. You need seven slots and permission to leave some empty.
| Herd | Scheme | Sows per slot | Something happens every | Litters per event |
|---|---|---|---|---|
| 3 sows | One group | 3 | 21 weeks | 3 |
| 4–6 sows | One group, or two slots 9 weeks apart | 2–3 | 21 or 9 weeks | 2–3 |
| 7–10 sows | Full 7-slot grid, some slots empty | 1–2 | 3 weeks | 1–2 |
| 11–14 sows | Full 7-slot grid, all slots filled | 2 | 3 weeks | 2 |
This is the recommendation nobody makes, and it is the most useful thing on this page: draw the seven-slot grid now, at four sows, and leave three slots empty. A gilt you buy goes into an empty slot rather than being served the week she happens to stand. A sow you cull empties a slot that stays drawn. The herd grows into a structure instead of accumulating into a mess.
The alternative, which is what most people do, is to serve every animal as she comes and end up with nine sows farrowing on nine unrelated dates: no all-in all-out anywhere, no week the pen block is empty enough to disinfect, and weaner lots of eight that nobody drives out for. My biosecurity starter kit assumes a pen block you can empty, and an unbatched herd never gives you that week.
One group of three to six sows means the whole herd is out of production for the same 21 weeks. That is a cash-flow problem, not a biology problem. Split into two groups nine weeks apart once you hit five sows, or run a small fattening batch on bought weaners between your own farrowings. My Christmas and fiesta timing article covers where those bought batches should land.
Group weaning is your synchroniser, and it is free
The Merck Veterinary Manual states that group weaning on a single day triggers oestrus in more than 85% of sows within 4 to 6 days. That is not a hormone protocol. That is weaning everybody on Wednesday morning instead of one on Monday, one on Thursday and one whenever you get round to it.
So: wean the batch together, take the sows away and leave the piglets in the pen they know. Then watch hard from day 3 to day 7, twice a day, with the boar in front of her if you have one, and the back-pressure test on every sow every time.
Merck's timing, once she stands: with twice-daily checking, inseminate 8 to 12 hours after standing heat starts, then again 12 to 16 hours later. Oestrus lasts 36 to 48 hours in a gilt and 48 to 72 hours in a sow, so a sow gives you more room than a gilt does. If you are inseminating rather than running a boar, the practicalities are in my AI walk-through, and AI is what makes batching properly workable for a small herd because a boar cannot serve five sows in one week and a flask of semen can.
Where hormones actually earn their place
Altrenogest is registered in the Philippines. MSD sells it as Regumate Porcine, and the Philippine label gives 5 mL per head per day orally in the feed, 18 consecutive days for gilts and 3 to 9 days for sows, with heat following after withdrawal.
For weaned sows, save your money. Weaning day already does it.
For gilts it is different, and this is the one place I would spend. A gilt has no weaning day. If you are trying to slot four new gilts into slot 6 in April, the only ways to get them all standing in the same week are altrenogest or luck. Talk to your vet first: the 18-day dosing has to be uninterrupted, and skipping a day resets the whole thing.
Do not use prostaglandin to bring a sow into heat on a guess. PGF2α will terminate a pregnancy in a sow more than 15 days in, and a "not in pig" sow who is actually in pig is a mistake that costs you a whole litter. Any hormone protocol on a breeding female is a vet conversation, not a feed-store purchase.
The farrowing pen is the real constraint
Group size is set by how many farrowing pens you have, not how many sows you own. Two sows cannot farrow in one pen. Count the days that pen is tied up:
| Event | Day of cycle | Pen days |
|---|---|---|
| Sow moves in | 110 | (start) |
| Farrowing | 115 | 5 |
| Lactation | 115–143 | 28 |
| Wash, disinfect, dry | 143–147/148 | 4–5 |
| Total | 37–38 days |
Thirty-eight days is longer than a 21-day interval, so one sow per slot needs two farrowing pens, alternating. Two sows per slot needs four. Most people get this wrong because they count sows and not pens.
Day 110 rather than day 100 is deliberate. Five days is enough for her to settle and does not waste pen time you do not have. Give a first-litter gilt a few days more, because gilts savage litters more than sows do and an unfamiliar pen on farrowing night is one of the triggers. Pen layouts that work in a Philippine backyard has the dimensions.
The four or five days of downtime are not slack. That is the whole health argument for batching: an empty, washed, dried pen breaks the disease cycle between litters, and it is why first-week piglet mortality responds to management at all.
Free Tool
Farrowing Date Calculator
Put in the service date and get the due date, the day to move her to the farrowing pen, the weaning date and the earliest sensible re-service.
Working the calendar backwards from the market
Here is where a calendar stops being tidy-mindedness and becomes money.
December is a buyer window more than a price window. On PSA's backyard farmgate series, 2014 to 2025, December beats November every year, but by a median 1.8%, about ₱3 a kilo, and it was the year's top month in only 3 of the 12 years; the usual national high is May-June. What December does have is lechon operators and families buying whole pigs, who pay over the trader rate for the right pig on the right day. If you want to catch them, you cannot decide in October.
Service to a finished 90 to 100 kg pig is about 308 days. 115 gestation, 28 lactation, then 150 to 180 days of grow-out on backyard feeding, per my time-to-market breakdown.
| Target sale | What you are selling | Farrow | Serve |
|---|---|---|---|
| 15 December | Finished pigs, 90–100 kg | 5 June | 10 February |
| 1 July | Weaners, 25 kg, to fatteners aiming at December | 22 April | 28 December |
| Local fiesta, variable | Whatever the fiesta wants | minus 6.5 months | minus 10 months |
Two things fall out of that table.
If you sell weaners, your peak is not December, it is late June. Your buyers are the ones chasing December and they want stock in hand by the first week of July. A weaner producer planning for a December market has planned for the wrong month by half a year, and it is the commonest calendar mistake I see argued about in farming groups.
Slot 3 of the grid above, served 15 February, misses the December finisher window by five days. Slot 2 lands early. That is what the grid is for: once it is drawn you can see which slot hits which market, and give that slot your best gilts.
When a sow does not settle
The honest weakness of batch farrowing is the repeat breeder, and every guide flags it. The Scottish advisory service warns that "higher culling rates can occur if returns fall out with a service group." A sow back at 21 days is one slot behind, which on a three-week grid is fine. In a one-group system she has nowhere to go for months.
The decision rule I would use in a small herd:
| Situation | Do this |
|---|---|
| Returns once, at 18–24 days | Serve her in the next slot. Normal. |
| Returns twice | Serve her once more, and check the boar or the semen source before you blame her |
| Returns three times | Cull. She has burned 60 to 75 non-productive days |
| Irregular return, 25–40 days | Suspect early embryo loss. Vet, not another service |
| No heat at all by day 10 post-weaning | Score her condition. A sow weaned below BCS 2.5 often will not cycle |
That last row is the link back to feeding. A sow stripped to body condition 2 in lactation is the commonest cause of a delayed return in a Philippine backyard herd: a feeding failure showing up three weeks later as a calendar failure. Read the lactation intake problem before you conclude a sow is infertile.
And the arithmetic that should make you ruthless: every non-productive day costs ₱53 to ₱81 in feed alone, so three failed services is ₱3,200 to ₱4,900 for nothing, a quarter of what a replacement gilt costs. The seven classic breeding mistakes covers what else drives returns, and half of them are boar problems dressed up as sow problems.
Free Tool
Sow Productivity Calculator (PSY)
Litters and weaners per sow per year for your herd, and what your non-productive days are actually costing.
Bisaya / Cebuano
Kalendaryo sa pagpaanak sa anay
- Ang tibuok siklo sa anay 147 ka adlaw. 115 ka adlaw mabdos, 28 ka adlaw nagpasuso, 4 hangtod 6 ka adlaw una siya mag-init pag-usab. Kana pito ka semana nga tulo, o 21 ka semana
- Ang gestation dili 114 ka adlaw kondili 115 ngadto 116. Ibalhin siya sa farrowing pen sa adlaw 110
- Lutasa ang tanan nga baktin sa usa ka buntag. Sobra sa 85 porsyento sa mga anay mag-init sulod sa 4 hangtod 6 ka adlaw human sa paglutas. Libre kini nga pamaagi, dili nimo kinahanglan ang tambal
- Kung tulo hangtod unom ka anay ang imong nabuhi, usa ra ka grupo. Kung pito ngadto napulo, buhata ang pito ka slot nga grid ug pasagdi nga bakante ang uban
- Duha ka farrowing pen kada anay kada slot. Ang pen gamiton ug 37 hangtod 38 ka adlaw, mas taas pa kay sa tulo ka semana
- Kung gusto nimo makabaligya og humok nga baboy sa Disyembre, papaliwaton siya sa Pebrero. Kung baktin ang imong gibaligya, ang imong panahon Hunyo ug Hulyo, dili Disyembre
Isulat ang petsa sa dingding sa tangkal. Petsa sa pagpaliwat, petsa sa pagpanganak, petsa sa paglutas. Tulo ka petsa kada anay. Kana ra ang kinahanglan aron matukod ang tibuok kalendaryo.
Where to start on Monday
You do not rewrite the herd. You draw the grid and let the herd walk into it over one cycle.
Take a piece of plywood. Seven columns, four rows: serve, farrow, wean, re-serve. Fill slot 1 from whichever sow is closest to weaning right now, then add 21 days across the row. Ten minutes, and that is the calendar.
Then for the next 21 weeks, every sow goes into the nearest slot rather than the day she happens to stand. Four days early, serve her in that slot. Eight days late, hold her to the next one, which costs 13 non-productive days and buys you a herd with a shape. Worth doing to a given sow once, not twice.
The herd is on the grid inside one cycle. Then the interesting part starts: choosing which slot lands on which market.
