The following applies to a traditional intensive pig farming system:
Swine move within their production lifecycle in fairly discrete life stages in a sequentially linear fashion. A newborn pig is a piglet and is born weighing approximately 3 lbs. After about three weeks of lactation, where a single litter of piglets spends all of the time with their mother, the piglets are weaned. At this point each piglet weighs on average around 15 lbs. After weaning the weaner pigs are moved to a nursery where they remain until 11 weeks of age. A 55 lbs weaner is then considered a grower pig, spending the next 5 weeks growing to approximately 130 lbs, at which point it is moved once more to a finisher facility where it remains until it is ready for slaughter at 25 weeks – or about 270 lbs.
In order to replace the animals moving is such a linear fashion, replacement animals are required. To reduce the possibility of disease outbreaks some swine herds function on an all-in-all-out principle, where no animals are brought to the heard from outside sources. Therefore, replacement animals have to come from within the heard itself. For this purpose some of the animals are kept for breeding purposes. Such animals are called gilts – if they have never had a littler previously, or farrowed less than twice, after which point they are called sows. Females can start breeding after 8 – 18 months, and will go into esterus every 21 days if not bred.
Gilts and open (unbred) sows are usually kept in group housing until successfully bred, when they are moved into individual gestation area. Females remain here for the duration of gestation, when they produce 9 – 12 piglets. After weaning, females are sometimes moved to group housing and bred again during the next esterus as early as 5 – 7 days after weaning.