Sow grouping strategies for your barn.
Gestating sows can be managed in static or dynamic groups. The ideal grouping strategy is largely based on the housing system implemented.
Mixing aggression
One of the greatest concerns with group housing is aggression among sows. Injuries sustained during fighting are a significant concern for producers, as severe injuries can result in the need to cull valuable animals from the herd. However, research on aggression in sows has found the majority of injuries sustained in group sow housing are superficial scratches around the neck and shoulders, and less than 1% of the injuries observed are considered severe. Overall, sows in groups generally ‘get along’ and there are many management techniques to reduce the incidence and severity of aggression. To effectively reduce and manage sow aggression, it is important to understand what causes it in the first place.
Some aggression will naturally occur at group formation as sows establish their social status. Aggression at group formation can be intense, but is short lived with most fights occurring during the first 2 hours after mixing. Fighting can also take place during daily feeding, especially in competitive feeding systems. Daily aggression over feed access is a more serious issue, causing chronic stress and potentially the removal of poor-doing sows from the group.
In the wild, groups of sows include related individuals and their offspring with each group having its own territory and social order, and will drive away unfamiliar animals. When penned together on-farm, sows do not have the option to leave and the animals will fight to establish their dominance within the group. The initial mixing aggression is short lived and intense, typically involving the more dominant and unfamiliar animals. Afterwards, social tolerance develops as sows communicate using more subtle ‘avoidance’ behaviors. The tolerance of new group members develops gradually in a process lasting several weeks. Thus newly introduced animals will often be seen lying away from the main group of sows, often in less preferred areas of the pen.
Techniques for reducing mixing aggression include grouping familiar sows (when possible), providing sufficient floor space per sow, and the use of pen dividers (partitions) to give sows divided space and hiding areas to escape aggression. Pen partitions are typically seven feet in length, and help to define lying areas as sows prefer to lie against a solid wall. While partitions are typically four feet high, studies completed in Denmark indicate that short walls (e.g, two foot) can also be effective. Short walls can improve air flow and make it easier to see sows in the pen, but care should be taken to ensure that they are not placed in areas where sows may jump over them causing injury. Large, rectangular pens will also allow more room for sows to avoid one-another and good flooring (eg. partially slatted, gaps no greater than 0.8 inches) will reduce the likelihood of hoof and leg injuries.
Static versus dynamic groups
Gestating sows can be managed in either static or dynamic groups. For static groups, all sows in a group are mixed on the same day, and remain in that group for the rest of gestation with no new animals added. Static groups are used in competitive feeding systems, and in some non-competitive systems. In competitive feeding, sows groups should be formed of sows uniform in size, as this helps to even out competition. With only one mixing at group formation, static grouping allows sows to form a stable group hierarchy, and helps to reduce competition and allow all sows to feed. In competitively fed pens, sows should be observed daily at feeding and any that are injured or losing body condition should be removed promptly to comfort pens. No new sows can be added to small static groups, as any new animals will be targeted for aggression from other sows.
In dynamic groups, small groups of sows are added to a larger existing group periodically throughout gestation, and groups are also removed periodically as sows move to farrowing. Each time a new group is added a new bout of aggression will occur. However, with large dynamic groups it has been shown that sows adopt a more tolerant and passive response to unfamiliar animals. A pen layout that includes adequate space, and the addition of dividing walls for escape and to hide behind, helps to reduce the aggression.
Group size
Group size can also influence sow aggression within the group. Smaller groups of sows (8-10) will form a social structure with stable, linear hierarchies. Larger groups of sows (≥40 animals) develop a different social structure in which there is a greater tolerance and less aggression. It is believed that the cost of establishing dominance in a larger group is greater, and so animals learn to adopt a more tolerant approach rather than trying to dominate. Instead, sows in large groups will form smaller sub-groups, or cliques of sows that lie together.
With dynamic grouping, maintaining large groups is one way to help control aggression. In the dynamic setting, the addition of a smaller group of sows to the larger group lends itself to the natural formation of sub-groups of sows. To help reduce the stress to animals added to a dynamic group, it is recommended that animals added to a new group should make up at least 10% of the total group size.
Many producers use an introduction pen that allows incoming sows to be penned together and become familiar with one-another before joining the larger group. These sows typically remain together as a sub-group once they join the larger group. A specially designed mixing pen can also offer better conditions to minimize aggression and stress, including features such as extra space per sow, better flooring to help prevent injuries and provision of enrichment or bedding.
Grouping by size
Grouping sows by size (and/or parity) can help to even out competition within the group, helping in particular the smaller, younger parity sows to do better by not having to compete against larger, older sows. Grouping by size is especially important in competitive feeding systems where smaller sows will be at a distinct disadvantage when competing for feed.
Research has also shown advantages to grouping sows by parity in non-competitive feeding systems. Grouping sows by parity was found to be advantageous for parity 1 sows managed in a static ESF system. Younger sows in the study were able to maintain backfat when housed together, but lost backfat during gestation when kept in mixed parity groups. It is likely that maintaining the sows in uniform parity groupings reduces competition for entry to the ESF feeder, helping to ensure all sows receive their daily feed allowance. Research indicates that grouping sows by parity can also benefit young sows fed in free-access stalls. When housed in mixed parity groups, younger sows spent the majority of time in the free-access stalls, but when housed with other young animals the sows spent more time out of stalls in the common loafing area.
Why do sows fight when re-grouped?
Our reluctance to keep sows in groups seems somewhat misplaced as in the wild pigs live quite harmoniously in groups of numerous sows and their litters (Gonyou, 2001). The difference between commercial production and living in the wild is that sows in the wild rarely, if ever, incorporate new sows into their group. If any sow attempts to join a group, she will be attacked by the resident sows and forced to leave. That is really what happens when we mix sows in commercial conditions: the resident sows attempt to drive away the intruders. Difficulties arise because the new animals cannot leave (Mendl and Held, 2001). Although we often attribute the aggression of newly mixed pigs to the need to establish their dominance order, that is likely a secondary aspect of the aggression. Subordinate animals cannot just submit and accept a low position in the dominance hierarchy; they must also find a way to be tolerated within the group.
The key to remaining in the group is to stay on the periphery. Moore et al., (1993) and Kraus and Hoy (2011) studied the lying position of sows after new animals were added to an established group. The new animals slept together, apart from the resident animals for several weeks after being introduced. Gradually they were able to integrate into the main group. Once established, the stability of a group of sows is maintained more by avoidance than by aggression (Jensen, 1982). Maintaining separate sleeping areas contributes to this avoidance.
Within a well managed group housing operation, many animals within a group will be familiar with each other from their previous gestation period. Sows are able to remember previous pen-mates even after several weeks of separation during farrowing and nursing (Arey, 1999). Consequently, when sows are grouped for a subsequent gestation period, the group consists of previously acquainted sows (an established group) and a number of new animals. The new animals will generally be gilts or 1st parity animals recently added to the breeding herd. Thus, the challenge often associated with younger animals, being the least able to dominate, is confounded by the fact that they are also new. Younger animals (Strawford et al., 2008) and newly introduced animals (Moore et al., 1993) end up sleeping in the least preferred areas of the pen.
How much do sow fight, and how severe are the injuries?
Re-grouping aggression is described as intense but short-lived. Fighting is greatest during the first three to four hours after mixing, and decreases to very low levels by 3-4 days after re-grouping. Reports differ in how aggression has been defined, but the average number of fights during the initial three hours is generally less than three per animal (Moore et al., 1993, Strawford et al., 2008, Kraus and Hoy, 2011). Extrapolating the data of Kraus and Hoy (2011) we estimate that new sows added to a pre-existing group of familiar animals will fight less than 30 times during the first four days. The length of fights have been found to range from eight to 420 seconds but average at around 70 seconds (Arey, 1999). In the first three hours after mixing, the average time spent fighting by sows was reported to be 68 seconds by Strawford et al., (2008). A very similar value was reported by Moore et al., (1993) for new sows in a group, but new gilts were much lower, and fought for a shorter duration. In general, new sows are involved in more aggression (Kraus and Hoy, 2011), and older animals fight more than younger ones (Moore et al., 1993).
Fighting is sometimes assessed by the degree of injuries received over a period of time. Hodgkiss et al., (1998) reported that only 0.16% of injuries received (when studying sows in an ESF system) were considered severe, and of the severe injuries, half were to the vulva. The vast majority of injuries due to aggression are in the form of surface scratches to the skin, generally on the neck and shoulder. However, the incidence of scratches is greater for younger, smaller animals (Hodgkiss et al., 1998) even though they are involved in less fighting than older sows (Strawford et al., 2008).
It needs to be recognized that most reports on aggression and injuries report average values for either the entire group of animals, or particular sub-groups (such as gilts or newly introduced animals). The extremes in terms of number of fights, time spent fighting and severity of injuries may not be reported. Nevertheless, it can be said that the majority of sows in a group are involved in few fights and for a short period of time after regrouping.
