Sow group formation

The German rules for protection of kept animals (Tierschutz-Nutztierhaltungsverordnung, TierSchNutztV) of 25 October 2001, as amended on 22 August 2006, defines the animal welfare ­requirements for pig farming in Germany. Under this order, the unobstructed floor area allocated to sows kept in groups must be 1.65 m2 for gilts and 2.25 m2 for older sows. For groups of less than six animals, the floor area should be increased by 10?% and for groups of more than 40 the area can be reduced by 10?%. There are very practical reasons for this requirement. Once sows in large groups have established their hierarchy they lie close together, leaving a relatively large space free and enabling social minimum distances to be observed. The pen floor must have a solid area of 0.95 m2 for gilts and 1.30 m2 for older sows, or the slatted floor area should not exceed 15?% of the total.

The EU-wide requirement for group housing of pregnant sows introduced on 1 January 2013 brought about the most radical changes in pig farming for many years. It ultimately forced a structural change that led to many smaller pig farms giving up piglet production altogether. Under the TierSchNutztV, gilts and older sows must be kept in groups during a period starting from five weeks after insemination to one week before the expected farrowing date. The sides of the pen in which the group is kept must exceed 2.8 m in length. For groups of fewer than six individuals, the sides of the pen must exceed 2.4 m in length. Sows and gilts raised on farms with fewer than 10 sows may be kept individually during this period provided that they can turn around easily in their crates.

With regard to the design of the floor, the openings in slatted floors for gilts and sows may not exceed 20 mm. Where a concrete slatted floor is used, the edges must be deburred and the slat width must be at least 80 mm.

Under the TierSchNutztV, every pig must have permanent access to sufficient quantities of material to investigate and manipulate which does not compromise the health of the animals.

EU Directive 2008/120/EC furthermore requires sows and gilts kept in groups to be fed using a system which ensures that each individual can obtain sufficient food even when competitors for the food are present. This formulation allows all existing feeding systems to continue to be used for group-housed sows. Particularly aggressive animals, animals attacked by other pigs or sick or injured animals may be temporarily housed in individual crates provided that they can turn round easily in their crates. The conclusion to be drawn from this is therefore that group housing should be managed in such a way that as little selection needs to take place as possible. Crates should be kept vacant for selected individuals. Where it is necessary to house sows in individual crates to aid recovery, the veterinary surgeon can require this in line with the EU Directive.

However, unlike the EU requirement, under which pigs must be kept in light with an intensity of at least 40 lux for a minimum period of eight hours, the TierSchNutztV additionally requires a light intensity of at least 80 lux in the animal area and access to natural daylight over at least 3?% of the house floor area. 

Domesticated pigs and wild boar display very similar social behaviour patterns. Wild boar live in matrilinial groups: the pack (or “sounder”) consists of a dominant sow and her offspring or of a small group of closely related sows. The young males leave the group when they reach sexual maturity. Wild boar sounders consist of between three and 30 animals. When a pregnant sow reaches term she separates herself off from the group and only returns to it two to three weeks after the birth of her young.

Observations over several years by Stolba and Wood-Gush (1989) of domestic pigs in a large free-range enclosure showed that the social order in the group is similar to that of wild boar. Outsiders introduced into the enclosure were attacked and it took between one and two months for the others to allow them to lie in the group nest.

On pig farms group formation is not left up to the animals but organized by the farmer. When pigs are placed in groups, fights for dominance take place with the aim of establishing the social relationships between the animals in the group. From a biological point of view it is important that this hierarchy is established quickly. Every fight costs the animals energy and puts them at risk of injury. A study by Bauer and Hoy (2002) showed that 78?% of all fights that break out on grouping end after 24 hours and 91?% after 48 hours. Although the social order in a group of sows is determined by fighting, it also serves the purpose of restricting aggressive behaviour once the hierarchy has been determined. A social hierarchy requires the animals to know each other individually and to be able to recognize each other quickly when they encounter each other again.

In the aforementioned study by Bauer and Hoy (2002) it was shown that the number of fights per sow on re-encounter rises as the interval between the first and second encounter increases (e.?g. 28 days compared with 7). This echoes the findings of Olsson and Svendsen (1995) and Spoolder et al. (1996), who established that individual group members can recognize one another over a period of between one and four weeks. Nonetheless, relationships in a sow group are more complex than is generally assumed. For example, up to 32?% of sows in a group never fight other group members when they meet for the first time, so it is not possible to identify their position in the hierarchy. Relationships between individual sow pairs remain undecided in a small percentage of cases (3.6?%), since each one of these animals wins as often as she loses (Hoy and Bauer 2004).

The more balanced the sow group is in terms of age and live weight, the more vicious the fighting can be. Older literature gives the optimum group size as between 8 and 20 sows, although it is not known how many members of the group sows can recognize individually. This number most likely differs from animal to animal, as a pig’s ability to recognize or remember other pigs depends both on their individual abilities and on the length of time they have belonged to or been separated from the group.

When new animals join an existing group (e.?g. when subgroups are integrated into an existing larger group – dynamic groups at demand feeding stations), renewed fights for dominance will occur, leading to stress, injuries and, in the worst case, loss of embryos or even repeat breeder syndrome (Arey and Edwards 1998). However, there is no clear indication of an ideal group size or floor area per animal in the literature, as results are often completely contradictory.

A distinction is drawn between frontal and lateral combat. In lateral combat, the opponents stand shoulder to shoulder and attempt to thwart their opponent and throw them on the ground. This shoulder contact also protects them against an attack involving bodily injury. In a frontal fight the sows bite their opponents on the head, the ears or the neck.

The individual distances between animals in small groups are shorter than in larger groups with the same amount of space available per animal. This has the disadvantage that there is less distance between subordinate and high-ranking animals than in large groups in which subordinate sows can keep as far away from high-ranking sows as possible and can “hide” in the group.

 Sow fights can last for anything from less than one minute to half an hour. They either end when the subordinate sow retreats or when both sows are exhausted. Sometimes it is not clear which sow has prevailed as the two opponents part without a clear result (victory or defeat). Various ways of reducing the number and intensity of fights in sow groupings have been explored. However, the following results have not succeeded in significantly reducing injury in fights for dominance:

• Use of straw
• Administration of relaxants (e.g. Stresnil)
• Use of odour-concealing substances (e.g. camphor, soap or eucalyptus oil)
• Diversion feeding
• Group formation in the dark
• Grouping in the presence of a boar.

Sows are synchronous feeders, meaning that they endeavour to eat at the same time. Competition to access feed can trigger aggressive encounters. When forming groups it is important to distinguish between fights for dominance in the pen and agonistic interactions at the feeding point (feeding sows being pushed away or feeding points being defended when attacked).

Keeping sows in groups raises concerns – and not without reason – regarding loss of performance due to a higher rate of repeat breeder sows and smaller litters of live born piglets, as well as a risk to health (e.?g. injuries to claws or limbs) (Hoy et al. 2009a). It is also harder to identify abortions in the group pen. These problems are caused both by the direct contact between the animals and by the fights for dominance once the group is formed (Borberg 2008, Borberg and Hoy 2009, Hoy et al. 2009b). The inevitable fights between sows should take place in places, at times and in conditions that avoid injury to the sows or their embryos (Hoy 2006, 2011). In the first few days after fertilization the eggs move down the Fallopian tubes to the womb and are relatively well protected. During the second and third week of pregnancy, the embryos are still in the lumen of the uterus and only start to attach themselves to the uterus wall later on. This is a very vulnerable time. Fights for dominance in the first two to four weeks of pregnancy can result in the death of more than 20?–?30?% of embryos and can even result in the loss of a pregnancy followed by repeat breeding (Schnurrbusch and Hühn 1994). With good management, however, this can and should be avoided. Many farm managers demonstrate that it is in principle possible to achieve very high performance with pregnant sows kept in groups.

The best time to form a group is immediately after the piglets have been weaned. The sows are not pregnant, so fights for dominance cannot harm the pregnancy. Another option is at the beginning of the fifth week of pregnancy – the time required throughout the EU. The embryos are already attached to the uterus wall by this point, and fights for dominance on group formation should not impact dramatically on eggs or sows. Starting group housing at a later stage would be even more beneficial in terms of this aspect, but this has been banned since 1 January 2013. Grouping sows immediately after insemination and after the abating of heat symptoms should only be considered if the sows had already been grouped beforehand (after weaning) and therefore already know each other. Otherwise the fights for dominance that would occur at this point could cause harm to the pregnancy. The least favourable time for grouping is in the second to third week of pregnancy.

Curtis et al. (2009) undertook a general comparison of group and individual housing of pregnant sows focusing on the aspects of behaviour, feeding, reproduction and health along with various other factors, based on a comprehensive literature analysis. The authors quoted McGlone et al. (2004), who performed a meta-analysis of ten scientific studies and concluded that sows kept individually achieved higher or equivalent performance to sows kept in groups. However, Curtis et al. (2009) also pointed out that the results in the literature are not consistent and depend to a large degree on housing and management factors (e.?g. Schmidt et al. 1985, Lynch et al. 2000, Lammers et al. 2004, McGlone et al. 2004, Estienne et al. 2006, Karlen et al. 2007, van Wettere et al. 2008, Spoolder et al. 2009, Bierman and Kohler 2009, Hoofs and Schulte 2010), although they underlined that the most critical period in terms of embryonic mortality and therefore fertility was the first month after insemination.

There is very little scientific analysis available on the influence of the start of group housing on fertility performance. Cassar et al. (2008) compared sows kept in groups that were 2, 7, 14, 21 or 28 days pregnant when their groups were formed with sows kept individually. The results clearly showed that the farrowing rate among sows put into group housing 2, 7 or 14 days after insemination was much lower (72.3 to 77.5?%) than among sows grouped in their 3rd or 4th week of pregnancy or among sows kept individually (82 to 83.2?%). In addition, the litter size of sows put into group housing when between 2 and 7 days pregnant fell by 0.2 to 0.6 piglets/litter (table 1). When the farrowing index was calculated (farrowing rate times litter size = total number or number of live born piglets per 100 sows served – Hoy 2008), significant differences were revealed between the variants tested. The lowest number of piglets born per 100 inseminated sows was produced by sows placed into group housing between the 2nd and 14th day of pregnancy (839 to 853 total births/100 sows served). There was no difference between sows kept individually and sows put into group housing from the 4th week of pregnancy onwards (or from the 21st day of pregnancy).  These sows achieved around 100 total born piglets per 100 sows served, or 1 piglet per sow, more than the comparison sows placed into group housing at an early stage. 

Table 1: Farrowing rate, average litter size (+ standard deviation) and farrowing index (own calculation) of sows kept individually (control) or in groups starting at different stages of pregnancy (Cassar et al. 2008)

¹ Farrowing index = total number of piglets born per 100 sows served (farrowing rate times litter size of total born piglets) – own calculation and added into table

In the Netherlands (Sterksel), studies have been conducted on static and dynamic groups, with sows being introduced into the dynamic group at three different times:

time I: Introduction into group one week after insemination, 

time II: introduction into group two weeks after insemination,

time III: introduction into group four weeks after insemination (table 2).

Table 2: Repeat breeding and pregnancy rates in sows in static groups or introduced into a dynamic group at different times (anonymous 2004)

The sows in the dynamic groups displayed more skin injuries and had more claw problems than animals in static groups. Sows in dynamic group III (introduced four weeks after insemination) showed the lowest repeat breeder rate and the highest pregnancy rate and achieved an average litter size of 11.8 live born piglets (anonymous 2004). The highest repeat breeder rate and lowest pregnancy rate were achieved by sows that were introduced into the group two weeks after insemination (litter size 11.76).

Levis (2006) reports on the impact of different grouping times in comparison with individual housing on the farrowing index (live born piglets per 100 sows served – table 3). The results show that group formation before the 7th day of pregnancy has a marked detrimental impact on fertility performance compared with individual housing and grouping at a later stage. Group formation after the 35th day of pregnancy achieves the same or even better fertility results than those of sows housed individually.

Table 3: Impact of group formation time in comparison with sows kept individually on the farrowing index (number of live born piglets per 100 sows served) – after Gonyou 2004 (cited in Levis 2006)

Levis (2007) also cites data from 71 farms in northern Italy with individual or group housing or various combinations of the two. The largest number of piglets weaned per sow per year was achieved with sows kept constantly in individual housing during insemination and pregnancy. The worst results were observed in sows kept in individual crates during insemination and grouped between the 14th and 28th day of pregnancy. 

A report by the Scientific Veterinary Committee entitled “The welfare of intensively kept pigs” (anonymous 1997) refers to various studies on the impact of the start of group housing on reproduction parameters. For example, Brake and Bressers (1990) showed that starting group housing around 10 days after insemination led to a higher repeat breeder sow rate and a smaller litter size than starting after the 10th or 31st day of pregnancy. 

There are no known scientific comparative studies on the health of sows kept in groups or individually, despite the fact that there seems to be a higher risk of inter-animal infection in group housing than in sows kept individually (Curtis et al. 2009, Hoy 2011). Sows kept in groups during early pregnancy have a higher incidence of lesions, more frequent repeat breeding and a higher cortisol concentration, whereas sows kept individually suffer more lameness in late pregnancy (Salak-Johnson and Curtis 2007).

5.1 Group formation in practice

All group housing starts with the formation of groups. The differences lie in practice in the time when the group is formed, the type of group formed, the size of group and whether the farm works with dynamic or static groups.

Time of group formation

Besides being kept in groups continuously (family pens), sows are also kept in a social context (group) after being kept in confinement in the farrowing and insemination crates. A distinction is drawn between groups formed before and after weaning and those formed before and after insemination.

Group formation during the suckling phase is an exception as this procedure is very labour- and capital-intensive. In this procedure, several sows and their litters are housed together for about two weeks after farrowing. Wild boar also return to the group between one and three weeks after farrowing, so fights for dominance can take place before the sows become pregnant again. However, the stress of being moved can cause some sows to come into heat during lactation. These sows will cause problems after weaning as they will no longer fit in with the group cycle. Furthermore, group suckling has other negative aspects, including cross-suckling of individual piglets, greater separation of piglets, the amount of labour involved (more intensive inspection and servicing work, additional rehousing and cleaning) and the large amount of space required. From the point of view of breeding, the rearing performance of sows housed in groups with piglets is harder to assess if some piglets are suckling on different sows (multi-suckling).

In practice, group formation after weaning is the usual method. In this instance, a distinction is drawn between group formation immediately after weaning and group formation during pregnancy. The aim should be to allow fights for dominance to take place at a time and in a manner that poses the least risk to the sow and her foetuses. Group formation during the critical period in the second and third week of pregnancy should be avoided at all costs. The group should be mixed before or after this time (table 4).

As a rule, sows are transferred straight to individual insemination crates in the insemination area after weaning, where they usually remain until at least the 28th day of pregnancy. After the fourth week of pregnancy they are either introduced into an existing group (dynamic group) or form a new, separate group (static group). As the TierSchNutztV requires sows to be housed in groups from the fifth week after insemination (see above), later times for group formation cannot be considered.

Table 4: When should sows be grouped?

+ = more favourable         – = less favourable

It has proven worthwhile to form later small groups (during pregnancy) of about 8?–?16 sows in a special pen (stimulation pen) or “arena” pen (see below) immediately after weaning. Any aggressive behaviour then takes place at a time when the sows are not pregnant or lactating. The group hierarchy will be established within 48 hours at most, after which the sows can be moved to individual crates for insemination. The hierarchy will remain static if the sows are subsequently regrouped in the original configuration, either in the first week of pregnancy or, at the latest, from the fifth week onwards. On account of the higher risk of injury from mutual mounting or avoidance behaviour while in heat, sows should be kept individually during heat and insemination. 

Where to form the group

Groups are in practice usually formed in the dry sow area, although the conditions there are not always ideal. The amount of space available, the structure and the layout of dry sow pens are not specifically designed for this purpose. Subordinate pigs in particular have problems in a suboptimal group pen, which can lead to a drop in performance. Group formation for small groups should take place in an open pen designed specifically for this purpose (see “Stimulation pen” and “Arena pen” below). 

Dynamic or static groups

Whether a farmer chooses to work with dynamic or static groups depends mainly on the herd size and the feeding system in the dry sow area. If the sows in the dry sow area are fed from demand feeding stations, the farmer will usually opt for a dynamic group so as to utilize the feeding system to the full. If static groups are the preferred option in this situation, very large herds and/or weaning rhythms over several weeks will be required. For example, the herd size for a three-week rhythm with a demand feeding station is 420 sows (7 weekly groups times 60 sows). Both static and dynamic groups can also be achieved with a mash nozzle, self-catching feeding crates and, to a certain extent, with a liquid feeding system, although small static groups are preferred for technical reasons. The use of dynamic groups with these feeding systems makes sows difficult to sort as these systems do not have an automatic sorting gate. The mash nozzle can be fitted with a device to mark the sows a particular colour. Static groups are therefore the norm with dribble feeding systems, quick feeders and self-catching feeding crates.

In dynamic groups, sows approaching their due date are removed from the group and moved to the farrowing house at set intervals, with their places in the group being taken by sows in the early stages of pregnancy. The advantage of dynamic groups is that the feeding technology should be well utilized, even in smaller herds. On the other hand, the group is disrupted every time a sow is introduced or removed, as dominance has to be re-established each time. If a herd of 420 sows (see above) is managed in a weekly rhythm, 21 sows approaching their due date must be removed from the dynamic group and 21 sows scanned and confirmed as pregnant must be introduced every week. It takes at least a week for the social hierarchy in the new group to stabilize, by which time the next change will be taking place.

In static groups, sows at the same stage of pregnancy are grouped in a pen and stay there until they farrow. This creates a static hierarchy throughout the pregnancy and also makes the group easier to manage since all the sows in the pen are at the same stage of pregnancy. Another option is to form body condition groups. Depending on the feeding technology, however, larger herds may be needed to make the best use of the equipment. In addition, spare places should be kept available for varying group sizes (caused by repeat breeder syndrome, losses etc.). The advantages and disadvantages of static and dynamic groups are summarized in table 5.

Group size

The stress on the sows on introduction into existing or new groups depends primarily on the size of the group. The larger the group, the easier it is for new animals to assimilate: introduced sows find it easier to “disappear” into a larger group. Sows are able to recognize each other individually in groups of up to around 20 animals. In larger groups they can easily lose track of the hierarchy and can no longer decide whether a sow belongs to the group or not. The smaller the group, the more sensible it is not to allow the group to meet for the first time in the pregnancy pen as there is too little space for subordinate sows to retreat or avoid other sows. Effects on pregnancy and sow health (especially limbs) cannot be ruled out. With a group of less than 12 animals, it is always a good idea to form the group in a pen earmarked for this purpose, such as a stimulation or “arena” pen (see below).

Table 5: Advantages and disadvantages of static and dynamic groups

Handling problem animals

Sows should never be introduced into a group individually but always in groups. Experience shows that sows introduced together stay together in the larger group later on. They will often seek out the feeding station together and rest close together. During the first few days after introduction, the “new” sows will nearly always lie with other “new” sows and the “old” sows will keep company with other “old” ones. It takes at least three weeks for sows to mix properly and become fully integrated. It is therefore advisable to match the width of the lying area to the size of the group (0.5 x 2 m per sow). 

If introduced animals display highly aggressive behaviour, it is important to wait until the fighting phase has ceased before taking action. If there is no sign of the fighting abating even after several hours or days, the aggressive sows should be removed from the pen in order to protect the other animals. For this reason, it is advisable to keep spare places available in all forms of group housing (with regard to crate places in the dry sow area and depending on the housing process). Aggressive, sick or incompatible sows can be kept here, either until the end of the pregnancy (in accordance with the veterinarian’s instructions), or alternatively a new attempt can be made to integrate these sows into the group at one of the next moving dates.

In order to be able to replace aggressive sows and animals removed from the group for health reasons, it is important to have a sufficient supply of gilts available at all times. The most reliable way is to pre-order regularly, for example by way of annual purchase contracts. The restocking rate on a farm should be between 30 and 35?%. With an average herd performance of six litters, roughly every sixth sow will leave the group after weaning and will be replaced by a gilt.

Integrating gilts

When integrating gilts, it is important to allow an adequate settling-in phase to begin with. During this time, which usually lasts for around six weeks, the sow will be introduced to the germ spectrum on the farm with the aim of fully immunizing her well before her first insemination. Gilts spend the first 14 days after delivery in an isolation house. This phase serves to protect the farm from any pathogens the animals may have brought with them. All contact with the existing herd must be avoided during this time. They should not be housed with slaughter sows that could bring the gilts into contact with farm-specific germs until after this period. It is important for the pig keeper to regularly spend time with the animals in the isolation house during the settling-in phase in order to build up a good relationship between man and animal. 

The gilts are moved to the insemination area from week 6 onwards. They will be inseminated for the first time during their second heat, once they are at least 240 days old and weigh at least 130 kg. After insemination the animals are returned to a gilt group. It is considered good practice not to integrate gilts into an existing sow group until they have had their first litter, since the animals are much larger by then and can stand up to the older sows more easily. This phase is also an opportunity for the gilts to familiarize themselves thoroughly with the feeding process.

A gilt group using a demand feeding system will ideally have their own feeding station to learn from during the first week, and in smaller herds there should be at least one separate pen available for the gilts.

After the settling-in phase, the gilts can be integrated into the larger group. Where dribble feeding, self-catching feeding crates and quick feeder systems are used, sows are kept in body condition groups so that gilts can automatically form their own group or encounter other sows at the same stage of development. The sows in this group will remain there during the whole of their pregnancy. 

Providing materials for rooting is not a particularly successful method of reducing fights for dominance, since the attractiveness of the materials (earth, sand, straw etc.) soon diminishes and fights for dominance continue anyway. In addition, these materials are regarded as a hygiene risk. However, suitable enrichment toys and manipulable materials should be provided in this area of the housing as well.

Because of the large amount of space needed, an indoor version of the “arena” pen would be very costly (for example, a group of 30 weaned sows would need around 180 m2 of space). For this reason, more cost-effective accommodation is often used, such as a solid-surface outdoor area between two buildings or an old clamp, enclosed with stable fences around the top ends. Outdoor conditions also have a positive impact on heat stimulation. If sows are left outdoors for the entire two days, insulated lying areas must be provided. Of the 6 m2 space allocation per sow, at least 1 m2 should be roofed. It is not necessary to provide a larger area since the risk of manure build-up rises if the sows do not leave the lying area to defecate. Camouflage nets or old terrace awnings are suitable for use as sun protection. To avoid introducing salmonella, a bird net should be used to prevent migratory birds from entering and soiling the arena. Wallowing pools should not be provided for hygiene reasons (table 6).

The outdoor climate conditions can have a detrimental impact on disease prevention measures and on cleaning and disinfection of the pen. Effective disinfection is particularly difficult in frosty conditions. The German “Pig Keeping Hygienic Requirements Ordinance” prohibits the keeping of pigs outdoors in areas at risk of swine fever.

During these two days, pigs can be fed using simple dry feeding systems or, where the pen floor is solid, with ground feeding. During this time only, feed should be provided ad libitum so that the sows do not lose too much body substance. Where automatic feeding systems are used, more than one feeding point should be provided per group in order to reduce aggression in the feeding area.

Feeding crates should not be used in “arena” pens since these can cause dead ends in which subordinate sows can be cornered. In addition, it should be ensured that there are no items of equipment protruding into the pen, as these could cause injury. Drinking nipples should be recessed into the wall so that they do not project. Drinking systems should furthermore be frost-proof.

Table 6: Advantages and disadvantages of the “arena” pen

An indoor method of group formation is the stimulation pen (fig. 2).

Table 7: Advantages and disadvantages of the stimulation pen

B Group formation after individual housing in insemination area for five weeks 

• Stimulation pen
• Small group 

* Turnover group in stimulation pen