Journal of ISSN: 2377-4312JDVAR

Dairy, Veterinary & Animal Research
Review Article
Volume 4 Issue 3 - 2016
A Review: Aggression Concerns with Group-Housed Sow Well-Being
Mayra Lopez and Janeen L Salak-Johnson*
Department of Animal Sciences, University of Illinois, USA
Received: September 25, 2016 | Published: December 15, 2016
*Corresponding author: Janeen L Salak-Johnson, University of Illinois, 1207 West Gregory, Urbana, IL, USA Tel: 217-333-0069; Fax: 217-333-8286; Email:
Citation: Lopez M, Salak-Johnson JL (2016) A Review: Aggression Concerns with Group-Housed Sow Well-Being. J Dairy Vet Anim Res 4(3): 00122. DOI: 10.15406/jdvar.2016.04.00122


The global trend toward housing gestating sows in group pens is driven by welfare concerns associated with the use of individual gestation stalls. Despite transition toward group housing systems, no housing system has scientifically been identified as being better than another based on current notions of sow well-being. Both benefits and drawbacks are associated with housing gestating sows either individually or in groups. The main concern associated with group-housing is increased sow aggression that occurs at mixing and around limited resources, including feed and water. Aggression is inevitable upon mixing of unfamiliar sows and is necessary for establishment of social hierarchy which helps reduce overall aggression and tension among the group. Unfortunately, if aggression is prolonged and intense it can lead to stress and compromised sow well-being. With few differences being reported between gestation housing systems, it seems plausible that the consequences on sow well-being is not only about the stall or the pen the sow is confined to, but other housing components. Inpens, some of the most critical factors to consider include space allowance, group size, time of mixing, and feeding systems/strategies which may directly or indirectly affect various welfare metrics (e.g., lesion scores, aggressive behavior, reproduction) of sows housed in groups. It is imperative to minimize aggression among grouped sows and feeding and management strategies can be used to reduce aggression among group-housed sows, but cannot eliminate aggressive encounters. Other housing components must be considered as well (i.e., group size). Therefore, the focus of this review is to examine the different feeding and management strategies that can be used to mitigate the drawbacks of group sow housing while improving sow performance, productivity, and well-being.

Keywords: Behavior; Gestation; Group-pens; Physiology; Sows; Stress; Welfare


The use of the gestation stall, within in the US, has become increasingly controversial due to consumer and legislative pressure to transition to alternative group housing systems. Legislative and ballot initiatives have limited the use of the individual stalls in some US states, whereas, in the European Union and other countries across the globe have completely eliminated the use of this individual housing system. There are both plusses and minuses associated with sows being housed either individually or in groups and limited data indicate an improvement in sow well-being, in any system [1,2]. Group-housing systems are extremely complex, and difficult with which to come to grips, simply because there are so many factors that must be considered. Animal accommodations are not jigsaw puzzles, meaning the pieces do not necessarily fit together perfectly or even acceptably. Yes, group-pens facilitate social living and freedom of movement, but not without welfare tradeoffs or consequences.

The major welfare concern associated with managing sows in groups is increased aggressive interactions, especially at mixing and during feeding, which results in more skin lesions and injuries, and variability in body condition scores, which may negatively impact sow well-being. Aggression is inevitable among unfamiliar sows for establishment of social hierarchy which helps reduce overall aggression and tension within the group. However, if aggression is intense and prolonged, it can lead to stress and compromise well-being. Sows housed in group-pens experience acute stress, but some sows may experience chronic stress caused by ongoing social stress. Moreover, other housing components such as feeding method, floor-space allowance, group size, and behavioral management-just to name a few-may exacerbate the impact on sow well-being. In fact, when a group-keeping system fails to work well, some sows inevitably experience a poor state of wellness [3,4]. The focus of this review is to discuss the effects of stress on animal welfare and to examine few of the potential housing and management strategies that may be used to mitigate the drawbacks of group sow housing, especially in terms of minimizing aggression, and maintaining good welfare.

Stress and Allostasis

Assessing stress is often challenging because animal factors including genetics, age, physiological state, and social relationships affect the biological response(s) to the stressor and the biological consequence to animal well-being [5]. In general, when confronted with a stressor, an animal elicits a biological response(s) in attempt to cope with stress, and if the appropriate response, often times there is minimal consequences to animal well-being. Conversely, if an animal is unable to cope with a stressor, its’ life expectancy and productivity may be compromised [6]. In short, once the threat to homeostasis is perceived, the hypothalamic-pituitary-adrenal (HPA) axis is activated upon the release of corticotropin releasing factor which in turn causes the release of adrenocorticotropic hormone which leads to the secretion of glucocorticoids, primarily cortisol [7,8]. Glucocorticoids regulate basal activity of the HPA axis and terminates the stress response via negative feedback. Hence, activation, modulation, and termination of the HPA axis is essential for adaptation and homeostasis in the short term; however, in the long-term, repeated stress or prolonged stress can lead to high cortisol levels and once the animal fails to cope with the stressor it may experience distress (“bad” stress). Well-being is often compromised when animals experience distress because nutrients must be diverted from other biological functions (e.g, growth, reproduction) in attempt to mitigate and cope with the stressor [5].

Stress can be defined as “a real or interpreted threat to physiologic or psychologic integrity of an individual organisms that evokes physiological and/or behavioral responses,” to achieve a state of equilibrium [9]. Homeostasis refers to the re-establishment of an internal set point by initiating the appropriate physiological adjustment that enables the organism to achieve internal equilibrium [10]. Whereas, allostasis is a process that supports homeostasis in which stability is achieved through change [11]. In general, homeostasis applies to those physiological systems that are essential for life, while allostasis applies to those physiological systems that change as environments and other life history stages change [10,11]. It has been proposed that the concept of allostasis is better to assess animal well-being since the capacity to change is crucial to good health and good animal welfare [12]. The mediators of allostasis (e.g., glucocorticoids, catecholamines) are protective and adaptive, thus increasing survival and health, however too much can also be damaging. At times animals may fail to successfully habituate to repeated challenges, fail to shut-down the physiological response, or fail to initiate an adequate response to mitigate the challenge, this animal is experiencing an allostatic overload and well-being is compromised [10].

It is also important to recognize that other factors may influence the biological response evoked and the biological cost to the animal, including type (physical or psychological) and duration (chronic or acute) of the stressor as well as physiological status of the animal [13]. For example, Salak-Johnson et al. [14,15] found that gestating sows housed in group pens at different floor-space allowances or in individual stalls initiate different behavioral and immunological responses in attempt to cope with the constraints of their housing environment. They concluded that different housing environments evoked different biological response(s), but none of these environments compromised sow well-being in terms of performance, productivity, or health. More specifically, these data imply that if the appropriate biological response is initiated, animal adapts, thus minimal consequences (if any) on well-being.

Aggression welfare concern

Increased aggression at mixing and around limited resources, such as feed and water is a major drawback associated with group-housed sows. Aggression among group-housed sows is evitable to establish a social hierarchy which often reduces aggressive encounters later-on [16,17]. Despite an established hierarchy, aggression can still occur, especially during feeding. Most often higher ranked sows will displace lower ranked sows, thus resulting in sows that are too thin and too fat [18,19]. Moreover, food deprivation leads to an increase in cortisol and if prolonged, animals can lose body weight, experience reproductive failure [20-22] as well as negative effects on embryonic development (i.e. decrease embryonic cleavage rate; [23]). More recently, Pacheco & Salak-Johnson [24] found that dominant sows engaged in more aggressive encounters and other metrics were indicative of chronic stress which may cost them in terms of performance and productivity. It is plausible, that welfare tradeoff in terms of reproductive performance may be greater for dominant sows.

Sow social rank tends to be related to parity, meaning higher parity sows are often the more dominant sow within the group [25]. Higher parity sows are involved in more and longer aggressive encounters than lower parity sows [26], while pregnant gilts that are housed in mixed parity groups tend to avoid aggressive encounters [14]. While, others have shown that housing only gilts together, higher rank gilts tend to show more aggression, while lower rank gilts receive more aggression [27]. When sows are individually fed, social rank influences the order feed is consumed among the group, with dominant sows enter feeding area first when housed in pens that use an electronic sow feeding system and often they enter the first stall of a feeding stall system [25,28]. In addition, when sows are floor-fed higher ranked sows will “guard” feed by staying in the center of the pile allowing lower ranked sows access to feed on the outer-edges of the pile [18]. Furthermore, dominant sows will displace submissive sows from the feeder, resulting in higher ranked sows consuming more feed than lower ranked sows [19,29]. Thus, higher ranked sows gain more body weight during gestation than lower ranked sows resulting in greater variability in body condition score among the group. Sows that consume more than their daily feed allotment tend to be heavier and obese, while those sows that consume less tend to be lighter. It is apparent that the biological cost to the sow in terms of well-being may be dictated by social rank within the group.

Other housing components impact welfare

Many other factors, such as group size, floor space allowance, and type of feeding system either independently or interactively may also affect aggressive behavior and well-being of group-housed gestating sows. When comparing group sizes of 5, 10, 20, and 40 sows per pen more aggressive behavior was observed more often among larger groups [30], while Hemsworth [31] found less injuries among sows housed in groups of 10 compared to housing sows in groups of 30 or 40. Moreover, they found that as space allowance increased from 1.4 m2 to 3.0 m2 per sow, aggression during feeding was reduced. Similarly, Salak-Johnson [14] demonstrated that sows allotted in pens at 1.4 m2 of floor-space per sow had greater lesion scores and lower body condition scores compared to sows that were allotted at 2.3 m2 or 3.3 m2 of floor space per sow. Also, total lesion severity scores were higher for sows kept at 1.7 m2 as opposed to 2.3 m2 of floor-space per sow [32]. Anil [33] found that using an electronic sow feeding system and managing sows dynamically leads to higher injury scores compared to sows maintained in static groups or only mixed twice. Similarly, sows managed in a static group of 35-40 sows have less lesions than if managed in a dynamic group with more than 100 sows [34]. In addition, the type of feeding system used can affect the aggressive behavior and sow body condition score. There is more aggression and injuries when sequential feeding systems (e.g. electronic sow feeder) are used compared to simultaneous feedings systems (e.g. trickle-feeding system) are used [35,36]. Yet, there is more sow displacement in the latter system leading to greater variability in body condition score because some sows eat more feed than other sows.

The time of mixing post-breeding can affect aggressive encounters and reproductive performance. Mixing sows at 35 days post-breeding resulted in less aggressive encounters, lesion scores, and cortisol concentrations than if sows were mixed prior to day 35 [26,37]. Conversely, Knox [38] found that mixing sows 13 to 17 days post-breeding resulted in less fighting bouts compared to sows that were mixed at 3 to 7 days or 35 days post-breeding. Often times mixing sows before implantation can negatively impact reproductive performance, such that farrowing and conception rates were reduced among sows that were mixed prior to day 9 post-breeding [34,38].

Managing aggression group-housing feeding strategies

In general, some believe that stereotypic behavior compromises well-being of sows, but others believe that these behaviors also serve as coping mechanisms or are part of pre- and post-feeding behavioral sequences. Sekiguchi & Koketsu [39] found sows that perform more sham-chew behavior had less piglets born compared to sows that did not perform this behavior; yet, other reproductive performance measures were not different. However, stereotypic behaviors may be used as a coping mechanism in an attempt to cope with a stressful environment [40,41]. Furthermore, stereotypic behaviors can develop due to lack of satiety when sows are fed a limited amount of feed [42]. Terlouw [43] showed that gilts that were fed 2.2 kg per day had an increase in activity, chain manipulation, and pre-feeding sham chewing compared to gilts that were fed 4.0 kg per day. Also, when sows were fed 2.5 kg per day they performed more ONF behaviors compared to those that were fed 4.0 kg/day [44].

Feeding strategies, such as modified gestational diets, can be used to reduce stereotypic behaviors by improving sow satiety. Gestating sows are fed a restricted diet based on metabolizable energy to meet their maintenance and reproductive needs while preventing excessive fat. It has been shown that when sows are restrictedly fed they develop stereotypies which increase sow activity levels and may be the expression of high levels of motivation to feed or forage [43-45]. In an attempt to limit these behaviors many researchers have fed high fiber gestational diets to sows. Feeding a high fiber diet allows sows to feel “fuller” because these diets are bulkier and sows take longer to consume them [46,47] and have been shown to reduce sham chewing, bar biting, and foraging behaviors [32,42,46]. Moreover, feeding fiber to sows has been shown to increase time spent lying and decrease aggressive behavior among group housed sows. de Leeuw [48] found that sows fed a high fiber diet spent less time active and made less frequent postural changes for several hours post-feeding. Similarly, others reported a decrease in activity and an increase in time spent lying among sows fed a high fiber diet [49,50]. Furthermore, sows fed a high fiber diet had less severe vulva lesion scores and engaged in less aggression compared with sows fed a low fiber diet [32,42]. Also, fiber source and amount used in diets are important factors that need to be taken into account because not all sources provide the same amount of fiber or have the same physiological effects. Robert [49] found that feeding sows a wheat bran and corn cobs fiber diet was more effective at reducing stereotypic behaviors, while increasing time spent lying compared to feeding sows an oat hulls and oat fiber diet. Certain fiber sources do not reduce stereotypic behaviors, for instance feeding a supplemented fortified sorghum-soybean meal with 25% beet pulp did not reduce ONF behavior in gilts [51]. Similarly feeding sows a high fiber diet composed of corn-soybean meal with 40% soybean hulls did not decrease stereotypic behaviors, but the diet did increase the time sows spent eating [52]. Feeding high energy diets can also reduce stereotypic behaviors such as sham chewing and chain manipulation, but this strategy has not been well documented [43].

The use of feeding partitions may also help reduce aggressive encounters among group housed sows. When sows are restricted fed they fight to access feed, thus feeding partitions provide partial protection for sows during feeding. Feeding sows in pens equipped with partial feeding partitions have been shown to reduce the occurrence of aggressive interactions among sows during feeding as opposed to feeding sows on the floor without partitions [53]. The length of the feeding partitions can also have an effect, the use of full body partitions result in reduced aggression and displacement and allowed submissive sows to spend more time at the trough than does shoulder length feeding partitions [54]. Conversely, Lopez [55] showed that sows housed in pens with shoulder length feeding partitions had shorter aggressive encounters during the first 48 hours post-mixing and less severe lesions compared to sows housed in pens with full body length feeding partitions.


Numerous factors that comprise a group-housing environment can influence behavior, physiology, performance, and overall well-being of gestating sows. However, the housing system per se may not independently be the most critical factor (stall vs. pen), but other constraints, especially biological factors (e.g., parity, social status) of the housing system may have greater impact on well-being. The major welfare concern associated with group housing of gestating sows is increased aggression which is a welfare trade-off of housing sows in group-pens. Aggression is evitable because it is needed to establish social rank, unfortunately, if aggression is prolonged and intense it can lead to stress and compromised sow well-being. Multiple factors such as space allowance, group size, social status, and feeding system/management can affect not only the level of aggression among the group, but can exacerbate some of the consequences. Managing these factors adequately and implementing new strategies (e.g. high fiber diets and feeding partitions) may be viable opportunities to minimize the level of aggression and potential consequences.


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