Description
Whilst the evolutionary purposes of a farrowing nest –to protect the piglets from a harsh climate, to hide them from predators and to prevent them from going astray – are no longer imperative within the highly controlled environment of most commercial indoor breeding units, sows are still highly motivated to perform nest-building
behaviours (Wischner et al., 2009). In fact, in a naturalistic environment, nest-building behaviour is effectively unchanged between domesticated sows and their wild boar ancestors (Stolba and Wood-Gush, 1989; see Section 2.3.1. for a description of natural sow nest-building behaviour). Nest-building may be a behavioural need for the sow (Damm et al., 2003), which is why restricting sow movement and substrate- directed behaviour during the pre-partum period can be so damaging for her welfare. Welfare benefits
Although the expression of nesting behaviour is highly dependent on environmental constraints and feedback from the nest site (Damm et al., 2000), the onset of nest- building is primarily dependent on internal cues (Algers and Uvnäs-Moberg, 2007). As nest-building is internally-motivated, preventing the sow from performing nest- building can cause physiological and behavioural indicators of stress and frustration, such as increased plasma cortisol, an increased heart rate and an increased
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occurrence of bar-biting or oral manipulation of pen fixtures (Jarvis et al., 1997; Damm et al., 2003; Burri et al., 2009). Furthermore, repeated removal of the pre- partum nest before parturition increases the latency for piglets to first suckle after parturition (Pedersen et al., 2003).
Beyond the main purposes of a farrowing nest for safeguarding piglets, allowing the sow to perform pre-partum nest-building improves post-partum maternal behaviour. Individual sows which perform more pre-partum nest-building exhibit reduced restlessness during parturition (Thodberg et al., 1999), have shorter farrowing durations (Thodberg et al., 2002a; Westin et al., 2015b) and a lower incidence of piglet crushing events (Andersen et al., 2005; Ocepek and Andersen, 2017;
Wischner et al., 2009b). An increased duration of pre-partum nest-building behaviour is associated with both increased post-partum pre-lying carefulness scores and a decreased average duration of successful nursing bouts (Yun et al., 2014a).
The latency between nest-building activity and farrowing is also important. Thodberg et al. (2002b) found an earlier onset of rooting behaviour, and an earlier peak activity of restlessness and rooting behaviour, to be associated with reduced activity and dangerous posture changes during parturition. Four hours pre-partum, increased nest-building activity is associated with a longer latency until the first nursing bout, a shorter average nursing bout duration and lower piglet body weight gain (Illmann et al., 2015), whilst increased nest-building activity two hours pre-partum is associated with increased restlessness during parturition and therefore an increased frequency of piglet crushing events (Illmann et al., 2016). Sows which continue to perform nest- building during parturition perform more posture changes and spend less time in lateral recumbency (Damm et al., 2000), have increased dangerous situations for piglet crushing (Burri et al., 2009) and are less responsive during a piglet scream test (Illmann et al., 2015).
System effects
During the pre-partum period, crated sows that are unable to perform appropriate nest building behaviour exhibit increased plasma cortisol levels (Jarvis et al., 1997; Jarvis et al., 2001), increased heart rates (Damm et al., 2003), increased
restlessness (Jarvis et al., 2001) and perform more oral-nasal stereotypies (Damm et al., 2003; Yun et al., 2015) in relation to penned sows. Furthermore, penned sows exhibit an earlier commencement of pre-partum rooting (Thodberg et al., 2002a) and an earlier peak intensity of nesting behaviours (Jarvis et al., 2001). Penned sows
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also exhibit a longer duration of rooting (Thodberg et al., 2002a) and more varied forms of nest-building behaviours (Damm et al., 2003), whilst the total duration of pre-partum nest-building behaviours may be longer (Yun et al., 2014a) or shorter (Damm et al., 2003) than observed for crated sows.
The provision of appropriate nesting materials also influences the performance of nest-building behaviours, as sows with pre-partum access to straw performed less nesting behaviour during farrowing, had fewer piglets born before the final posture change during parturition and a lower incidence of piglet crushing or near-crushing events (Thodberg et al., 1999; Damm et al., 2010). The quantity of nesting material may also be important. Sows in a free farrowing pen provided with a large quantity of straw two days before farrowing, rather than multiple smaller quantities over
consecutive days, spent more time nest-building, whilst nest-building also started earlier before parturition and was performed less once parturition had begun (Westin et al., 2015b). However, in a similar experiment by Damm et al. (2005a), an
additional provision of long-stemmed straw during the days around parturition had no effect on nest-building behaviour, posture changes during parturition inter-birth intervals or percentage of the litter who nursed during eight hours post-partum. Finally, more complex nesting materials may also stimulate improved maternal behaviour. In comparison to gilts provided with straw alone, gilts provided with straw and branches had a longer latency between the termination of nest-building and parturition and a lower occurrence of nesting behaviour during parturition (Damm et al., 2000). Furthermore, Yun et al. (2013, 2014a, 2014b) performed a series of experiments comparing the behaviour of sows in an open temporary confinement crate with either sawdust or complex nesting materials, with the latter showing an increased duration of nest-building, increased post-partum carefulness behaviours, increased sow serum prolactin and oxytocin concentrations and increased piglet serum IgG and IgM concentrations.