Fostering piglets

Fostering piglets has been part of basic husbandry for so long that few people have questioned it. Over time things can change so that the original reasons are remote from current practice. On some farms too many pigs are being moved too often and on other farms fostering is getting in the way of effective disease control. Another view, by animal behaviour researchers, is that fostering doesn't always look right - it seems to hinder as much as help the fostered pigs and their foster sows.


There are as many different ways of fostering as there are pig producers. It's hard to fault the logic of various approaches, but in most cases there are no data to back them up. The data shows that in an attempt to make things better for very small pigs we are seriously disadvantaging the bigger pigs, which are also moved in fostering management.

While the evidence is clear that survival rates are better for small pigs, which are in litters with pigs of a similar size, data from a range of experiments show clearly that fostering does not help big pigs; it leads to more fighting and skin wounds and makes sows restless and aggressive.

Big pigs do not easily fit in when they are moved to smaller or younger litters. It takes about six hours for half of them to get a drink and they have a battle to establish teat order, even if they are a little bigger. Their growth performance suffers and they never catch up to their unfostered siblings. Their lifetime performance suffers and it's doubtful that the survival of the smaller pigs makes up for the loss. These observations have led to the recommendation to 'leave them alone and let them grow'.

We know from a raft of studies that when litter weight variation is reduced, survival, especially of the smallest pigs, increases. The challenge is to make sure the bigger pigs do not suffer. The littlest pigs seem able to grow close to their potential, but the big pigs fall behind fairly quickly.

While producers perceive a sense of evenness about fostered pigs, the growth-check suffered means they do not achieve reasonable performance at weaning. Moving the bigger pigs back just retards the growth of the fastest growing pigs and so, while the group looks more even, performance is not helped.

Weight objectives

The farm production objective is to get the maximum weight of pigs out of the facility. If there is too much focus on evenness, the good pigs are held back and they never catch up. The experiments supporting this work involved a daily inventory of 400 pigs and matching numbered pigs to numbered teats and twice-weekly weighing. The performance of 40 litters in 4 batches across 2 treatments was compared. One treatment involved continuous fostering to maintain evenness of litters. In the other, treatment fostering was limited to the first two days of life.

The limited fostering group grew faster and weighed nearly a kilogram more at weaning (19 days). While there was much more variation in the size of the pigs in the weaned litters, there was no difference in mortality rate. Both groups recorded mortality rates of less than 9%.

Continuous fostering reduced growth rate by 20%. It did, however, reduce the variation in weight at weaning by 50%, but reduced variation in weight is not desirable if it is associated with a reduction in growth rate.

In a second study, researchers looked at methods of fostering that involved moving the smallest pigs in a litter to a younger litter more closely matching them in body weight. It has been a time-honoured way of adjusting litters, but the research showed it does not work. While 8% increased in weight and 6% did not change, 69% had a decrease in growth rate. Mostly, the pigs fostered back just stopped growing.

No deaths could be attributed to fostering. Big pigs fostered to a lighter litter did comparatively better than pigs fostered to a heavier litter. After fostering the pigs fostered to lighter litters were about 0.4 kg lighter than expected. Those fostered to heavier litters were 1.4kg lighter than expected; they had to fight hard to regain their teat order, their place at the udder and a teat.

Fostering also affected the resident pigs. In the study there were 93 pigs remaining in their original litters. Of these, 62 had no change in growth rate, 23 grew more slowly and 8 grew more quickly. Overall, it was a negative effect. Those pigs, fostered or resident, that kept their own teat were okay. Those that did not keep their own, fell behind.

Limited cross-fostering

In a separate study, researchers investigated the effect of limiting cross-fostering to within the first 24 hours of life using the following rules:

  • cross-fostering between litters to stop before 24 hours of age
  • fostering limited to moving only enough pigs to fill teat spaces
  • restricting fostering to the room the pigs were in (i.e. not fostering between rooms)
  • destroying severely sick or debilitated pigs
  • not moving any poorly pigs back.

Within four weeks, they found preweaning mortality rates had fallen by 2%. When the researchers repeated this work on another farm, they reduced the preweaning mortality rate in the face of a porcine reproductive and respiratory syndrome (PRRS) outbreak from 16.8% to 10%.

Behavioural studies

In a separate study, researchers looked at fostering from a behavioural perspective. Pigs in 24 litters were observed. They were part of a fairly intensive fostering regimen. At birth, all the litters were standardised to 10+1 the day after birth. Every 3 days up to day 16, the piglets were weighed and cross-fosterings were done. The three biggest pigs from one litter were exchanged for the three lightest pigs from another.

There was more fighting and face lacerations in the fostered litters and more non-productive milk let-downs. Typically, little pigs fought and vocalised at the udder as soon as the sow started her milking let-down routine and started to grunt. The sows were more aggressive in the fostered litters and the aggression was always towards a fostered pig. Sows were more restless and stood up or sat down quite suddenly in the two hours after fostering. They spent less time lying down from day 4 to day 16 of lactation.

Earlier studies have indicated that less than half the pigs fostered after two days of age had sucked within six hours of fostering. Others have found that pigs fostered at seven days have reduced weight gain. Clearly, while fostering as a management practice targets the survival of small pigs, there is not much value in it if the best pigs suffer.


From all this work, and taking into account the underlying reasons for fostering (to protect the small pigs), the following guidelines were developed:

  • foster within the first 24 hours
  • never move good pigs back
  • wean a good milking sow early (10 days) and foster little pigs on to her
  • put the weaned litter in a supplementary rearing unit to make sure they continue to grow. At this stage, their growth off the sow can exceed their growth on her.

Destroying pigs that are not doing well or that are debilitated is a difficult recommendation to make. It gets into the heart of people's attitudes towards the animals they care for and demands special considerations involving farm management and staff. If farm policy is to destroy weak or debilitated pigs, it must be accompanied by clear guidelines.

Early care

To make the fostering process work effectively, it is essential that fostered pigs have colostrum. Little pigs with full tummies usually indicates that they have sucked. Where there is any doubt, artificial colostrum will help. Replacement milk formulas (e.g. SURVIVE) are helpful for pigs that have had colostrum but need additional milk.

Small farms

These rules for fostering make it difficult to foster on small farms. The simplest approach is to not even bother if you can not foster within 24-48 hours of birth. It is better just to leave the pigs where they are and, as you are going past, top them up with artificial colostrum or milk replacer two or three times a day. If you have to foster to equalise pigs and teats, do it as early as you can. If you have to move big pigs, make sure they get colostrum and top them up with milk replacer as you move them. Give them a bit more over the next day or two to make sure they do not fall too far behind.

Early weaning

Weaning a litter off a good milking sow early and giving her a new foster litter to nurse can run the risk of keeping her longer in the system. It will reduce litters per sow a year, but it may not hurt too much because the next litter size will be larger because of a longer lactation length. Feeding levels for the foster mother must reflect the increased nutritional drain on her. She will need about 7 kg feed per day.

Fostering patterns

  1. Sow 4 is a good milker and will make a good foster mother. She's a young sow with relatively small teats. Wean her litter at about 10 days of age to a supplementary rearing unit.
  2. Sows 1, 2, 3 have good litters with some small pigs. Within the first 24 hours of life make sure the smallest pigs (shaded) get colostrum (by stomach tube if necessary).
  3. Move the smallest pigs to sow 4. If in doubt about whether they've sucked, top them up with 10 ml of colostrum or by stomach tube to make sure their energy reserves aren't depleted. If supplementary feeding is the only source of milk, give them 12 ml/kg/hour.
  4. Use careful judgement as to whether you move the largest pig from sow 3 to sow 1 to balance numbers. It's breaking the rules and the fostered pig will fall behind but, you might do it if you doubt the capacity of sow 3 to rear her litter.
  5. The supplementary rearing unit must be well heated, kept very clean and the pigs provided with a high-quality ration.