To ensure the high welfare of animals, it is important to understand their biological, physiological, and behavioral needs. Pigs are highly intelligent and social animals, the social status of which is determined by their age, body weight, and physical strength ^[1][16]. Among their natural behaviors, rooting seems to be very important and related to multiple roles. Pigs perform rooting in order to forage and to explore, while sows also root to build a nest prior to farrowing ^[2][17].

Pigs housed in barren environments often exhibit signs of boredom and frustration ^[3][18]. Environmental enrichments clearly improve the welfare of pigs by allowing them to express natural, species-specific behaviors and thus play a crucial role in the development of a welfare-friendly farm environment. On the other hand, the real effect of enrichments on pigs depends on many factors, including the type of enrichment, its adequate quantity, location, maintenance, and safety. Not all additives to barren farm environments are suitable enrichments for pigs ^[4][19]. Enrichments provided to pigs should be edible, chewable, safe, and frequently replaced or renewed so that the animals do not lose interest in them ^[5][20]. According to the Commission Recommendation (EU) 2016/336 ^[5][20], straw, green fodder, miscanthus, and root vegetables may be ‘optimal’ materials for pigs. When used as bedding, these materials promote rooting behavior, serve as a comfortable resting area, and absorb excreta ^[6][21]. Unfortunately, in light of African Swine Fever outbreaks in Europe, virus-infected straw, green forage, or hay have been identified as potential sources of the disease ^[7][22]. Furthermore, the use of natural enrichments, such as straw, is limited in some parts of the world due to higher production costs (including additional costs for straw and labor) compared to housing based on slatted floors. Moreover, housing systems with slatted floors may involve problems with slurry system management when substrates are used as environmental enrichments ^[8][23]. As suggested by Nannoni et al. ^[9][24], in no-bedding systems, pigs should be at least provided with hay or silage in racks placed above the floor. According to the Commission Recommendation (EU) 2016/336 ^[5][20], straw provided in racks, peanut shells, fresh wood, corn cobs, natural ropes, shredded paper, and pellets are ‘suboptimal’ enrichments for pigs. Though the deep-bedded systems based on straw appear to be welfare-friendly, they are not free from welfare and health problems. Studies analyzing the relationship between the use of straw as bedding and hygienic problems and development of pathogens have yielded contradictory results ^[8][23]. Moreover, pigs’ preference of floor type (deep-bedding vs. slatted floor) depends on the thermal conditions, as when temperatures are high, the animals will choose to lie on concrete floors to cool off ^[10][25]. With the exception of hot weather conditions, pigs prefer straw to concrete floors; however, substrates such as peat and compost are preferred by pigs over straw ^[8][23].

It has been demonstrated that the housing system determines the behavioral activity (e.g., time spent on rooting, lying down, inactive) of all groups of pigs. Piglets reared outdoor are more active compared to those housed on slatted floors ^[11][26]. Allowing piglets to express their play behavior improves their social skills and their ability to cope with adverse situations later in life ^[12][27]. Extensive outdoor systems enable pregnant sows to express nest-building and nursing behaviors ^[13][28]. All groups of pigs given access to outdoor runs exhibit wallowing, which is associated with multiple functions such as thermoregulation, protection against sunburn, elimination of skin parasites, and expression of social and sexual behaviors ^[14][29].

2. Effects of Housing Conditions on the Welfare of Sows and Piglets

The housing system has been reported to strongly influence the maternal behaviors of sows. Freedom of movement promotes the expression of farrowing behaviors, such as nest-building ^[15][44]. It is also well known that environmental conditions determine the behavior of sows and piglets in the preweaning period and that an undesirable environment may increase the incidence of agonistic behaviors ^[16][17][45,46]. Prior to farrowing, sows exhibit nest-building behaviors such as foraging, rooting, and pawing ^[18][47]. If not provided with appropriate environmental conditions, they will redirect their nesting behaviors to head shaking, sham-chewing, drinker-playing, and drinking excessive amounts of water ^[19][48]. Such abnormal behaviors are mostly observed among sows that are raised under intensive housing conditions ^[20][49]. Furthermore, it has been proven that prepartum environmental stimuli (such as the presence of nesting material) promote nest-building behaviors in sows. Provision of a suitable substrate has a positive impact on the duration of nest-building behaviors and the amount of rooting observed prepartum ^[21][50]. Rosvold et al. ^[22][51] compared different nesting materials provided to sows before farrowing and observed that sows provided with straw and wood shavings expressed a higher number of total nest-building behaviors and nest-building elements (i.e., pawing, rooting, pushing, carrying and arranging material) compared to groups provided with peat and wood shavings and wood shavings only. Based on their results, the authors stated that both straw and peat promoted nest-building behaviors in comparison to no provision of these substrates. Research data also underline that the housing system combined with suitable breed determines the maternal behavior in pigs. Free-range sows always check the bedding for the presence of piglets before laying down and move away from the piglets that are too close ^[23][52]. This behavior prevents piglet crushing, which is one of the major causes of mortality in litters ^[24][25][53,54]. Table 1 summarizes the effects of housing systems on sows’ health, behaviors, and performance based on the reviewed literature.