National Hog Farmer is part of the Informa Markets Division of Informa PLC

This site is operated by a business or businesses owned by Informa PLC and all copyright resides with them. Informa PLC's registered office is 5 Howick Place, London SW1P 1WG. Registered in England and Wales. Number 8860726.

Management to Control Aggression in Group Housing

Grouphoused sows

One of the greatest concerns when sows are housed in groups is the incidence of aggression. This is somewhat surprising, given that sows in the wild live fairly harmoniously in groups. The reason for this discrepancy between commercial and wild pigs is that sows in the wild essentially never form new groups and their feed resources are generally scattered throughout their environment, greatly reducing competition among individuals.

In commercial conditions, we generally observe two distinct types of aggression: 1) during group formation and, 2) feeding-associated aggression. Feeding-associated aggression will be addressed somewhat within the sections on feeding systems and group composition. The remainder of this section will examine aggression during group formation.

It may be surprising to some, but on average, newly regrouped sows spend less than 10 minutes fighting, and the vast majority of the fighting occurs within the first 24 hours of group formation. Less than 1% of the injuries received will be considered severe, with most animals receiving only scratches to the neck and shoulders. Although some sows, particularly the older and larger ones, fight more than others, they are rarely injured.

On the other hand, smaller, younger sows will have more injuries than the time they spend fighting would suggest. The injuries will also be more pronounced if the pen does not provide opportunities to retreat.

Management to control aggression is divided into two strategies: social and non-social (physical). Because regrouping aggression is all about introducing unfamiliar animals to each other, we can reduce the level of aggression simply by minimizing the proportion of new animals in a group. 

Once the breeding list is complete — and for most systems, the desired feeding level is known — it is time to allocate sows to their gestation groups. Sows on the “edge” of a feeding level should be sorted in such a way as to maximize the number of familiar animals within a group (See Figure 1). 


Sows will be familiar with each other if they were housed together in their previous gestation, or, in the case of gilts, in the same development group. One limitation is generally given for this approach. As the unfamiliar animals in a pen will receive the majority of the aggression, care should be taken to ensure that no one animal becomes a victim to the rest. If unfamiliar pigs must be included in a group, they should make up at least 10% of the new group, so that aggression is directed to multiple pigs.

Pigs housed in large groups (>40 animals) develop a different attitude toward others in their social group. They become more tolerant of other pigs, and are less inclined to fight when encountering a stranger. Thus, housing gilts in large groups during finishing or development should reduce their inclination to fight when they are moved to a gestation group. Similarly, pigs that have experienced several social groups are less likely to fight when placed into a new group than animals that have never experienced regrouping. Regrouping animals during development will reduce aggression in newly formed gestation groups.

A third strategy for managing aggression through social means is to select animals based on their temperament. We are finding that pigs can be classified along two behavioral descriptors: the active-passive and the confident-fearful continuums. 


Keep the most in-depth pork production information available at your fingertips! Download our Blueprint app today.


Passive animals are less aggressive than active ones, and would be the preferred personality type in group-housing situations. However, they should also be confident enough to explore their new surroundings, rather than being fearful of moving about the pen. Research on these temperament traits is relatively new and may eventually lead to selecting animals either to achieve a good social mixture at regrouping, or as a longer- term strategy of genetic selection.


The Mixing Pen

The first physical means of controlling aggression relates to the mixing pen — which may be the gestation pen, but could be a separate area. Aggression is reduced if animals can avoid and/or flee from a potential aggressor (an unfamiliar animal).  One way to do this is to provide adequate distances between barriers in the pen design. Generally, it is recommended that alleys between rows of stalls, or openings into lying-down areas be at least 7 ft. and preferably 10 ft. wide.

Another way, particularly in large groups, is to provide several loafing areas in the pen, separated by dividers (See Figure 2). These features allow animals to avoid those that are a threat or are pursuing them. In this regard, it is also important to provide good flooring in the mixing pen.


Most lameness in group housing is observed during the initial 2-3 weeks of being together, and may be attributed to slipping on floors or being injured by slats when fighting or trying to avoid a fight. Although these features would be beneficial during the entire gestation, they are most critical during group formation. Additional space may also be beneficial to achieve a better pen design for new group formation.

A second non-social means of reducing aggression is to feed the sows — perhaps even double feeding — immediately before regrouping. These animals will be less active and will either not initiate a fight or will end it more quickly than hungry sows. In some less-competitive systems, such as short stalls, it may also be helpful to feed the animals again several hours after mixing. Other methods that have been reported as helpful include mixing just before the lights are turned off for the evening, or mixing at 4-5 weeks into pregnancy, rather than earlier.


Group Size and Composition

The social hierarchy is well-recognized within pig groups, but its stability will vary with group size. Small groups (up to 6-8 sows) typically have a very stable hierarchy, with a dominant sow and various ranks down to the most subordinate individual. Positions within the hierarchy rarely change, and aggression is minimal. 

In slightly larger groups (10-30 animals) the sows will often challenge the next animal above them, and the hierarchy is less stable. In this situation, more fighting occurs, but if successful, the challenging animal moves up in rank and obtains more resources, such as food or space. 

In large groups (40-60 animals), the cost of achieving and maintaining a high rank may not be worth the additional feed or better space that it brings.

Therefore, animals become more tolerant of each other and aggression is reduced. The point at which the cost of aggression falls below its benefits will be dependent upon the competitiveness of the feeding system and the nature of the individual pigs. We believe that this situation exists when there are more than 40 animals per group. This information results in a preference for either small, stable groups or large, tolerant groups.

A limiting factor to group size is the need to even out competitiveness within groups when using a competitive feeding system (systems other than electronic sow feeding or lock-in stalls). If a group, even a small group, has one or more sows that are much larger than the others, they may become bullies and monopolize feed and space. For this reason we try to even out the size and age of animals in a group for floor or short-stall feeding. Sorting by size will generally result in smaller group sizes than not sorting.


Feeding System and Pen Design

Another factor affecting group size is feeding system and pen design. ESF systems favor large groups for three reasons: 1) the system can accommodate sows with different feed requirements in the same group, so no sorting is required; 2) sows are not required to compete for feed (only access to the feeder), so competitiveness (and uniformity) is less important; and 3) it is more cost-efficient to maintain larger groups to make better use of the expensive feeding station and maintenance required.

However, two important limitations should be kept in mind for ESF systems: if you exceed the number of sows that can easily get feed from the feed stations available, the system becomes very competitive and stressful; and, sows that have just recently been trained on the system may not be assertive enough to maintain daily feeding visits. 

Most ESF systems will handle 60 sows per station with good management. Exceeding that level should be done with extreme care and vigilant assessment of individual perfor-mances.

Stall systems, both short stalls and lock-in stalls, may also limit group size. The most efficient systems in terms of space will have two rows of stalls backing onto each other. The minimum group size would be at least 10 animals, and likely will be 20 or more in order to make the alley between the rows long enough for good sow movement. An outline of group size and composition considerations for various feeding systems is in Table 1.



Feeding Problems

Detecting sows that are off feed in gestation stall systems is a simple matter of checking the feed trough anytime during the day. Group systems can be considerably more difficult to monitor, but it varies among the feeding systems used. 

For floor and short-stall feeding systems, it is necessary to observe feeding to determine if any sows are not eating. For lock-in stalls, it is a matter of manually locking the sows in during feeding and returning an hour or so later to check the trough before releasing the animals. However, this would probably not be done on a daily basis. In ESF systems, check the daily attention list, which is generated by the system software, to determine which sows did not eat the previous day. Regardless of the system, body condition should always be monitored on a regular basis as well.

The second step in managing sows that are off-feed is to determine the reason for their condition. Illness generally results in a poor appetite, and these sows will not attempt to eat. Lame sows are often non-competitive. They will not force their way into the feed drop, and will essentially fall down the hierarchy ranks. 

Other reasons for non-competitiveness are that the sow is too small or timid to hold her position among larger penmates; or in the case of ESF, the animal may not have been fully trained to use the feeding station on her own. All off-feed sows should be checked for injury and lameness. In ESF systems, a sow will be denied feed if her radio frequency identification (RFID) tag is lost or becomes defective.

Body condition is the primary means of determining if the sow has been assigned to the correct feeding level. Sows that are eating all of their feed, but are not reaching their optimal body condition, may be gettingfed too little or too much. A correction in feeding level is required.

Treating or correcting feeding problems also varies by feeding system. Injured or sick individuals may have to be removed and housed in relief pens (hospital pens) until recovered. The number of pens required should be less than 5% for ESF systems, but may approach 15% for more competitive systems such as floor feeding. Once these animals have recovered, they may be able to return to their established group if it is within a few days of removal.

However, many of the recovered sows will have to be added to new groups that are being formed from subsequent breeding weeks. This way they enter just as any other sow, but they will have to be removed for farrowing earlier.

Non-competitive sows are unlikely to be able to return to their original group. They should be given relief for a few days, and in the case of ESF systems, retrained before being added to another group being newly formed. Again, care must be taken that they are removed before their farrowing date.

If the cause of poor body condition is determined to be an incorrect feeding level, there are several options: for dealing with the situation. In ESF systems, it is a matter of adjusting the sow’s individual feeding level in the system. For lock-in stalls, it is possible to lock the sow in manually at feeding, and provide additional feed by hand. For competitive systems, and lock-in stalls if preferred, the animals can either be individually penned, or grouped with a subsequent breeding group when one is formed. Again, these sows will have to be removed from the group prior to their farrowing date.


Sorting for Treatment, Vaccination and Farrowing

The various feeding systems represent three distinct situations as they pertain to sorting and treatments. In floor and short-stall feeding systems, the sows are never confined; the stockperson must enter the pen to identify and work with the animals. If an individual animal must be treated or sorted, identify the sow, label her and perform the treatment. 

For procedures involving the entire group, such as vaccinations, you would treat the animal, label her, and record the treatment. For farrowing, either label and sort an individual that must be removed, or move the entire group to the farrowing room. Sorting an animal out from the pen typically works best with two persons, but can be done by one in a small group. It is generally best to treat and sort the animals after feeding, when the sows are quieter.

Lock-in stalls allow the herdsperson to identify and treat animals within the feeding stall. The animals can be locked in manually during feeding, and held until the treatment is performed. If the treatment requires that the animal be loose, the herdsperson can leave the other sows in their stalls and only release the one being treated. Similarly, to remove a single sow for farrowing, she can be released and moved while the others remain locked in their stalls.

ESF systems are able to identify animals as they eat and label them with a spot of paint, or sort them into a separate holding area. If the animal(s) that need treatment are eating regularly, she or they can be labeled or sorted during a single feed cycle. If the animal is not eating, she will have to be identified in the loafing area. This would normally be done with a handheld electronic tag reader.

If sows have been grouped shortly after breeding, it may be necessary to pregnancy check them after 4-6 weeks. Many managers will prefer to pregnancy-check in a stall, and so for all systems except lock-in stalls, the animals will be removed, stalled, checked and returned. However, it is possible to pregnancy-check sows when they are either standing or lying down in the group pen. Each animal must be identified, preg-checked and labeled, and the results recorded. Identifying an individual sow in a large group can be time-consuming. 

However, if all animals must be checked, the process can be relatively efficient. Pregnancy-checking sows in a group is a skill that requires practice, and herdspersons will improve with time.


Stockperson Skills

All group-housing systems require animal skills to handle and sort sows in groups. Stockpersons must be comfortable walking through groups, and possess skills in assessing injuries and illness. However, there is a shift from these animal skills to technical skills as a system moves from floor feeding, to short- and lock-in stalls, to ESF systems.

In competitive systems, the stockperson must have keen observation skills to identify non-feeding sows, to monitor body condition, and to assess sow compatibility. ESF systems require that sow data, including feeding level, be entered into the system in a timely manner. Stockpersons must check the daily attention list to identify potential problems. The ESF system requires the stockperson to rely on the technical system to help him or her manage the sows. 


An individual who is uncomfortable working with computers will likely struggle with managing an ESF system. In operations where the size warrants it, having two persons in the gestation area with somewhat different skill sets may work very well.  

Read more sow housing stories:

Group Sow Housing: Practical Considerations

Group Sow Housing Alternatives: Building New vs. Remodeling Existing Facilities

Management to Control Aggression in Group Housing

Defining Ideal Sow Body Condition

Hide comments


  • Allowed HTML tags: <em> <strong> <blockquote> <br> <p>

Plain text

  • No HTML tags allowed.
  • Web page addresses and e-mail addresses turn into links automatically.
  • Lines and paragraphs break automatically.