Author: Mark White BVSc LLB DPM MRCVS
Reviewed: Mark White BVSc LLB DPM MRCVS 2016
Published: 2004

One of the most frequently recorded causes of death in baby piglets in indoor pig farms is "overlaying" of the pig by the sow crushing it to death, accounting on some farms for 50% of all piglet deaths.

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Depending on how precise the definition is, this may take the form of treading on, trapping between the ironmongery or literally lying over the top of.  However, there is a need to look a little closer at this issue.  In many circumstances, it can be used as an excuse in that it is the sow's fault rather than being within the control of the stockman, particularly given the size disparity between the sow and each piglet in that the sow is typically in the order of 150x the weight of each piglet at birth.

Furthermore, in the outdoor situation, far less control can be exerted by the stockmen; the behaviour of the sow and its interaction with the litter is key and the stockman's influence is restricted to ensuring bed management is correct, primary hypothermia is avoided and sows are calm and not in any way agitated.

To be able to understand and then mitigate the incidence of overlaying, it is necessary to split losses into different categories, which we shall call primary and secondary overlaying, although there is necessarily some overlap.

Primary overlaying is what most stockmen think happens i.e. the piglet is perfectly well albeit small compared to its mother but the sow is in some way very clumsy in not taking care when moving.  There are undoubtedly cases of this and they may be associated with:

  1. Slippery floors making it difficult for the sow to lie down carefully or even get up easily (the latter is more likely to lead to crushing against bars etc).
  2. Fat sows. Excessive condition in the sow renders them less agile and, thus, more likely to flop down and crush piglets underneath her.
  3. Crate construction. The design of the crate, position of lower bars and layout of the pen all affect the interaction between sow and litter. Sophisticated farrowing systems involving floors that raise and lower under the sow on pneumatic or hydraulic rams in response to the sow standing or lying down are designed to ensure the piglets cannot get underneath the sow when she is standing and can cut overlaying deaths dramatically.
  4. Excessive size relative to the crate again restricts ease of movement and leads to uncontrolled action that may trap piglets. Insufficient space at the back of the sow due to poorly designed or incorrectly fitted back bars can trap piglets as they are born.
  5. Inadequate size of sows or gilts. As sows have generally increased in size, the industry has responded to some extent by increasing crate size.  However, this may provide too much space for small sows and effectively removes any benefit of the sow being in a farrowing crate.  Slippery floors will exacerbate this effect.
  6. Excitable sows/gilts e.g. at feeding time may cause uncontrolled movement liable to trap pigs.  Disturbance of the sow at feeding time must be balanced with the potential for increasing overlaying losses.  This is particularly significant with manual feeding where stockmen have to rush around to feed and are unable to oversee the piglets during their period of disruption. Provision of a third feed during the day to improve feed intake can increase losses due to this disturbance.
  7. Clumsy sows. There are undoubtedly individual sows that are inherently uncoordinated or clumsy and record cards will identify such individuals for culling.
  8. Overheating of the creep will drive pigs out of the creep and into the danger zone.
  9. Inadequate creep heat encouraging pigs to lie close to the sow.


Fig 1 Avoiding chlling is key to survival


Fig 2 Size matching the sow to the crate helps reduce overlaying

Increasingly in recent years, producers are looking at farrowing systems indoors which do not fully confine the sow. Whilst in some hands these systems - generally referred to as freedom farrowing systems - can function with low levels of mortality associated with overlaying, this is not yet consistently achievable. In particular it is reported that if the pen area is too large, higher levels of overlaying occur, although construction, airflow and stockmanship still play an important role. Many of these systems allow a compromise whereby the sow is fully confined whilst farrowing but is released from the crate a few days after - something of a throwback to systems of the 1960-70's where conventional crates were only used at farrowing and sows and litters were moved on to loose housing once the litter was established.

Whilst these features are typical of the indoor environment, the typical outdoor arc system can present similar problems:

  1. Sows too big for the arc.
  2. Clumsy sows.
  3. Agitated sows - due to stockmen, foxes, external noise, disturbance etc.
  4. Arc design - vertical sides are far more likely to allow trapping than inward sloping sides.
  5. Bed construction and maintenance is key to reducing mortality outdoors

Secondary overlaying occurs where the "fault" lies more with the piglet than the sow.  Piglets are born out of a temperature of 39-40°C into an environment of at best 25°C (plus heat lamps) and are wet.  The initial effect is that they chill.  They also have little energy reserve and need to acquire milk/colostrum rapidly to provide fuel to generate heat.  Thus, they should be moving around close to the udder early on, often when the sow is still farrowing the litter.  The fact that, in many cases, overlaying occurs in the first 24-36 hours, confirms this danger.

However, on-farm studies have shown that where overlaying accounts for the death of 5% or more of all pigs born alive, a high proportion of these piglets have empty stomachs i.e. have not managed to suck.  The effect is that they will chill, which makes them slower to move and they will desperately try to seek out the udder, putting them in the danger zone.

Failure to find the creep area or failure to provide adequate warmth will encourage pigs to lie close to the sow, placing them in danger.  Management of lamps around farrowing time is critical to reducing chilling and overlaying.

Any other condition affecting young piglets (joint ill, scour etc) will reduce their mobility and tend to put them at risk of being crushed.

In the outdoor situation, the same principles apply. Chilling is far more likely to occur in cold weather making early colostrum intake absolutely critical.  Draughts are fatal to newborn piglets, especially when they are still wet.  It is vital that new straw bedding is packed around the edges of the arc, that the arc's integrity and insulation is maintained and that arcs are orientated to avoid wind blowing straight in.


Fig 3 Overlaying is one of the perils of substandard outdoor conditions

Earthing up outside can be beneficial.  Doors or plastic curtains over the entrance can be very valuable over the farrowing period and beyond.

In all environments management of the bedding is of key importance.  As a general rule, barley straw is preferable to wheat straw and whilst plenty is needed in cold weather, care must be taken to avoid piglets getting trapped in it and thus being unable to get out of the way of the sow.


If the average mortality recorded in the UK pig herd is 11% losses due to overlaying can be assumed to be in the order of 5% of pigs born alive a 500 sow herd producing 28pigs BA per year might expect to suffer 700 overlaying deaths totalling more than £20000 per annum loss. The pain and suffering of piglets crushed but not immediately killed is an added biological cost.

With close attention to detail of management, avoidance of chilling and adequate investment in facilities, these losses can be cut by 50%


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