animals Article Welfare and Performance of Post-Weaning Sows and Piglets Previously Housed in Farrowing Pens with Temporary Crating on a Spanish Commercial Farm: A Pilot Study Heng-Lun Ko 1, * , Déborah Temple 2 , Janni Hales 3 , Xavier Manteca 1 and Pol Llonch 1 1 Department of Animal and Food Science, School of Veterinary Science, Universitat Autònoma de Barcelona, Bellaterra, 08193 Barcelona, Spain; xavier.manteca@uab.cat (X.M.); pol.llonch@uab.cat (P.L.) 2 AWEC Advisors SL, Parc de Recerca de la Universitat Autònoma de Barcelona, Bellaterra, 08193 Barcelona, Spain; deborah.temple@uab.cat 3 SKIOLD A/S, La Cours Vej 1, 7430 Ikast, Denmark; jhp@skiold.com * Correspondence: henglun.ko@uab.cat Simple Summary: Social pressure, especially in Europe, towards the ban of farrowing crates has driven research on alternative farrowing systems. However, few studies comparing the welfare and performance of pigs in different types of farrowing pens with temporary crating have been carried out under commercial conditions. Additionally, the weaning stress response of pigs that are previously housed in alternative farrowing systems is overlooked. Therefore, the present study aimed to compare how the sows and piglets from three types of farrowing systems adapt to the weaning challenge. Behavioral observation, number of skin lesions, and salivary stress biomarkers were carried out at different time points around weaning, to assess the adaptability of sows and  piglets to weaning. Our results found that the post-weaning aggression level in piglets from three  farrowing systems was similar. Piglets’ saliva samples from one of the alternative farrowing systems Citation: Ko, H.-L.; Temple, D.; showed a reduced stress response at weaning, which may suggest better weaning adaptability of Hales, J.; Manteca, X.; Llonch, P. these pigs. Sows from the same alternative farrowing system vocalized most on the day of weaning Welfare and Performance of and decreased drastically over time. This vocalization frequency pattern after weaning might indicate Post-Weaning Sows and Piglets Previously Housed in Farrowing a greater effect of abrupt separation from the piglets in this type of farrowing system. Pens with Temporary Crating on a Spanish Commercial Farm: A Pilot Abstract: The study investigated the effect of farrowing environment on the weaning adaptability Study. Animals 2022, 12, 724. https:// of sows and piglets. One farrowing crate (FC) and two farrowing pens with temporary crating doi.org/10.3390/ani12060724 (TC: SWAP and JLF15) were compared. Sixty-four sows and 663 piglets were followed until 5 days post-weaning. At weaning (D24), sows and piglets were moved to group pens and nursery pens, Academic Editor: Michaela Fels respectively. Sows and piglets’ behaviors were observed on D24, D25, and D26. On D23, D25, and Received: 25 February 2022 D26, piglets’ skin lesions were counted, and sows and piglets’ saliva samples were collected for stress Accepted: 11 March 2022 biomarkers (cortisol and chromogranin A, CgA). Piglets were weighed on D23 and D29. All the Published: 13 March 2022 piglets’ skin lesions increased on D25 and decreased on D26 (p < 0.05). Compared to D23, cortisol Publisher’s Note: MDPI stays neutral of JLF15 and CgA of FC piglets increased, whereas those of SWAP piglets remained similar after with regard to jurisdictional claims in weaning (p < 0.05). Post-weaning performance in piglets was similar across farrowing systems. SWAP published maps and institutional affil- sows vocalized more than FC and JLF15 on D24 and D25 (p < 0.001). Results suggested that SWAP iations. piglets showed a lower weaning stress response. Frequent post-weaning vocalization in SWAP sows might be linked with a negative effect of the abrupt separation from the piglets. Keywords: aggression; behavior; farrowing system; pig; stress biomarker; temporary crating; Copyright: © 2022 by the authors. vocalization; weaning Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// 1. Introduction creativecommons.org/licenses/by/ In modern pig farming, farrowing crates are used to reduce the risk of piglet mortality 4.0/). caused by sow crushing [1]. Currently, approximately 95% of the sows are housed in the Animals 2022, 12, 724. https://doi.org/10.3390/ani12060724 https://www.mdpi.com/journal/animals Animals 2022, 12, 724 2 of 13 farrowing crates during farrowing and lactation in the European Union (EU) [2]. However, farrowing crates jeopardize the welfare of the sows [3]. Farrowing crates limit the behavioral repertoire of body movements and restrict the expression of natural behaviors like nest- building and mother–young interactions [3]. Along with the growing societal trend of eliminating farrowing crate policies in several EU countries, and the well-received European citizen initiative “End the Cage Age” launched in 2018, there are many alternative farrowing systems being studied and implemented lately. A farrowing pen with temporary crating (TC) is one of them. It allows the producers to crate the sows temporarily for a few days when the piglets are newly born and vulnerable, to prevent piglet crushing, and to set loose the sows for the rest of the lactation period, to ensure freedom of body movements of the sows. While there is increasing research on welfare assessment of the alternative farrowing systems in sows and piglets during lactation, few studies followed these animals after weaning. Weaning in pig production is usually performed abruptly which makes it one of the most stressful events in a pig’s life on commercial farms [4]. Yet, several studies have found benefits of the enriched early environment to mitigate weaning stress in piglets [5], including pre-weaning socialization [6–8] and environmental enrichment [8–10]. However, little research has focused on how farrowing systems that facilitate the mother–young interactions play a role in coping with weaning stress. TC has shown to facilitate the mother–young interactions [1,11], and as Telkänranta and Edwards [12] stated, a sow is “the most important social figure in the early life of a pig” for piglets, which underlines the importance of the social experiences with the sow during suckling. This paper is a follow-up study of Ko et al. [11], in which we compared the welfare and performance of sows and piglets in three farrowing systems: one conventional system with a farrowing crate (FC) and two TCs. This paper focuses on the same group of sows and piglets in post-weaning, until 5 days (D) after weaning. We hypothesized that piglets raised in an early environment that allows more mother– young interactions would better cope with abrupt weaning stress, due to previously learned social skills from the mother [12], heavier body weight [13], larger farrowing pen size to play in, and the nesting material to manipulate with [9,14] (and possibly earlier expo- sure/consumption of solid food [15], i.e., the nesting material). We also hypothesized that sows that can interact with the piglets during lactation, would have more stress after abrupt weaning due to a stronger bond with the piglets [16]. The aim of the study was to study the effect of farrowing systems on behavior, aggression-related skin lesions, salivary stress, and performance in post-weaning sows and piglets from FCs, compared to two different TCs. 2. Materials and Methods 2.1. Housing and Experimental Design The study took place on a farrow-to-finish commercial farm in Girona, Spain. During lactation, three farrowing systems were used, one FC and two TC, including Sow Welfare and Piglet protection pen (SWAP) and JLF15 (both produced by SKIOLD A/S, Ikast, Denmark). In each batch, there were five FC pens, six SWAP pens, and six JLF15 pens during lactation. Five FC pens were installed in one room, and six SWAP pens and six JLF15 pens were installed in another room. Sows entered the rooms a week before the expected farrowing date. FC sows were confined from entry to weaning, whereas TC sows were confined from 1D before expected farrowing to 3D after farrowing. Four batches of crossbred Duroc sows (n = 64; 18, 23 and 23 in FC, SWAP and JLF15, respectively) with a total of 663 piglets (183, 243 and 237 in FC, SWAP and JLF15, respectively) were followed in the study from lactation to 5D after weaning in four seasons between 2018 and 2019: autumn batch (November), winter batch (February), spring batch (May) and summer batch (July). Regarding the technical details (dimension of the pens, crates, and creep areas) of the three farrowing systems used in the study, see Figure 1 and Table 1 in Ko et al. [11]. piglets (155, 168 and 168 in FC, SWAP and JLF15) in the end. The group pen size for the weaning sows was 2.2 m × 3.5 m (four sows/pen on average) and was installed with one feeder and one drinker. The nursery pens of the autumn batch were 2.4 m × 2.53 m (36.7 piglets/pen on average) (installed with two feeders and three drinkers per pen) and 1.2 m × 2.55 m (16.5 piglets/pen on average) (installed with one feeder and two drinkers per pen). Animals 2022, 12, 724 3 of 13 The nursery pens of the rest of the batches were 2.4 m × 2.8 m (35 piglets/pen on average) (installed with two feeders and four drinkers per pen). (a) (b) Figure Figure1.1.Distribution Distributionofof(a) (a)sows sowsand and(b) (b)piglets pigletsafter afterweaning weaningininfour fourbatches, batches,including including14 14group group pens pensfor forthe thesows sowsand and1616nursery nurserypens pensfor forthe thepiglets, piglets,which whicharearehighlighted highlightedininthick thicklines. lines.(FC: (FC:pigs pigs that were housed in the conventional farrowing pens with a farrowing crate before weaning; that were housed in the conventional farrowing pens with a farrowing crate before weaning; SWAP SWAP (Sow Welfare and Piglet protection) and JLF15: pigs that were housed in two types of farrowing (Sow Welfare and Piglet protection) and JLF15: pigs that were housed in two types of farrowing pens pens with temporary crating before weaning.) n indicates the number of the sows or piglets in one with temporary crating before weaning.) n indicates the number of the sows or piglets in one pen. pen. A pen that has sows/piglets originating from more than one farrowing system is considered as A pen the that has “surplus sows/piglets sows/piglets”, originating and from morefrom they are discarded thanthe one farrowing study system is considered as the after weaning.5 “surplus sows/piglets”, and they are discarded from the study after weaning.5. 2.2. Animals and Management Table 1. Behavior categories for post-weaning piglets. Sows in the group pens and piglets in the nursery pens had ad libitum feed and water Behavior access. Sows had home-mixed feed which was barley- and soybean-based. Piglets had Description commercialAny nursery physicalfeed (Nuscience, interaction Ghent, Belgium) that indicates and with social conflict werebrute supplemented withtwo force involving com- or plementarymore liquid feed (Re-hydralab; individuals, Labiana including head Lifepushing knocking, SciencesandS.A., Terrassa, fighting. Spain) during The number of the Aggression the first 5 days after fighting weaning. event is recorded according to the number of individuals involved (adapted from Ko The group penset al. for[8]). the sows after weaning were in a semi-open building which was dependentPiglet on natural orally in contact and lighting with ventilation. another piglet,The nursery including for thechewing, nibbling, weaningsucking pigletsand was Biting biting. Biting during fighting is excluded (adapted from Day et an indoor building, in which light was on from 07:00 to 18:00 and the room temperature al. [17]). was programmed at 25nosing Repetitively ± 2 °C.with a rhythmic pattern towards another piglet’s abdomen with its Belly-nosing The study snoutof the sows (adapted ended from 2DGonyou Li and after weaning, [18]). and the piglets ended 5D after wean- ing. Sows and piglets followed their production Any physical interaction with minimal or moderate cycle after theinvolving force study. On twotheor3rd and 4th more Other social interaction individuals with no reaction or avoidance response from the recipient(s), including sniffing, nudging, mounting and chasing (adapted from Ko et al. [8]). Locomotor play behavior See Martin et al. [9] for different locomotor play behaviors. Behaviors including sniffing or manipulating pen features, enrichment objects or other Object play and exploratory behavior items; feed and water are excluded. Weaning occurred in the morning on D24. Sows and piglets of the four batches were removed from the farrowing pens and were moved to 14 group pens (5, 3, 3 and 3 pens in each batch) and 16 nursery pens (7, 3, 3 and 3 pens in each batch), respectively. Pigs were randomly regrouped but were kept with those from the same farrowing system together. Pigs that were regrouped with those from different farrowing systems after weaning, were considered as surplus sows/piglets and were removed from the study. Due to different availabilities of the pens for sows and piglets, and the production cycle of the sows (e.g., some were culled or were served as nurse sows), the number of pigs per pen after weaning was not consistent across farrowing systems and across the four batches (as specified in Figure 1a,b), making it 56 sows (16, 21 and 19 in FC, SWAP and JLF15) and 491 piglets (155, 168 and 168 in FC, SWAP and JLF15) in the end. The group pen size for the weaning sows was 2.2 m × 3.5 m (four sows/pen on average) and was installed with one feeder and one drinker. The nursery pens of the autumn batch were 2.4 m × 2.53 m (36.7 piglets/pen on average) (installed with two feeders and three drinkers per pen) and 1.2 m × 2.55 m Animals 2022, 12, 724 4 of 13 (16.5 piglets/pen on average) (installed with one feeder and two drinkers per pen). The nursery pens of the rest of the batches were 2.4 m × 2.8 m (35 piglets/pen on average) (installed with two feeders and four drinkers per pen). 2.2. Animals and Management Sows in the group pens and piglets in the nursery pens had ad libitum feed and water access. Sows had home-mixed feed which was barley- and soybean-based. Piglets had commercial nursery feed (Nuscience, Ghent, Belgium) and were supplemented with complementary liquid feed (Re-hydralab; Labiana Life Sciences S.A., Terrassa, Spain) during the first 5 days after weaning. The group pens for the sows after weaning were in a semi-open building which was dependent on natural lighting and ventilation. The nursery for the weaning piglets was an indoor building, in which light was on from 07:00 to 18:00 and the room temperature was programmed at 25 ± 2 ◦ C. The study of the sows ended 2D after weaning, and the piglets ended 5D after weaning. Sows and piglets followed their production cycle after the study. On the 3rd and 4th days after weaning, sows were artificially inseminated and were moved to a larger pen with a dynamic group of gestating sows altogether. On the 6th day after weaning, the nursery feed for piglets changed. 2.3. Direct Behavioral Observation in Sows and Piglets after Weaning Direct behavioral observations were conducted on D24 (the day of weaning), D25 (1D post-weaning), and D26 (2D post-weaning) by one observer. It was conducted hourly between 14:00 and 17:00 on each observation day, making it three sessions per day (30-s scan-sampling for 3 min per pen). Each session started observing the sows in the group pens and then the piglets in the nursery pens. Behavior categories for the piglets and the sows after weaning are listed in Tables 1 and 2, respectively. Table 2. Behavior categories for post-weaning sows. Behavior Description Aggression See the definition of Aggression in Table 1. Sow orally in contact with another animal, including nibbling, chewing and biting. Biting during Biting fighting is excluded. Other social interaction See the definition of Other social interaction in Table 1. Oral contact is excluded. Pen investigation Sow sniffing or manipulating pen features (adapted from Averós et al. [19]). Object investigation Sow sniffing or manipulating the enrichment object (adapted from Averós et al. [19]). Sow produces a certain type of high-pitched call. Normal grunting, vocalization during fighting or Vocalization being bitten is excluded. Locomotion Any locomotor behavior not listed above, for example, walking and pacing. 2.4. Pre-Selection of the Piglets for Lesion Scoring and Saliva Collection Of each litter, six piglets were selected: the heaviest, the middle, and the lightest on D3 (3 days after farrowing) of males and females. To assess the intensity of aggression in piglets after weaning, and to study the effect of weaning stress in sows and piglets, three sampling days were determined: D23 (1D pre-weaning), D25 (1D post-weaning) and D26 (2D post-weaning). 2.4.1. Skin Lesion Scoring in Piglets around Weaning The number of skin lesions was scored as suggested by Turner et al. [20] by one observer. The piglet’s body was divided into six parts: the left and the right sides of the front (i.e., from head to front leg), middle (i.e., the trunk), and rear (i.e., hind leg to rump) parts of the body. In each body part, only fresh and unbroken linear lesions were scored. Animals 2022, 12, 724 5 of 13 2.4.2. Sows and Piglets’ Saliva Collection around Weaning and Stress Biomarker Analysis Saliva samples were collected to determine two salivary stress biomarkers, cortisol (CORT) and chromogranin A (CgA). Saliva samples were collected with the swab in the Salivette® tube (Sarstedt, Aktiengesellschaft & Co., Nümbrecht, Germany) to the pig’s mouth for 1 min. Sow samples were collected between 09:00 and 10:00, and piglet samples between 10:30 and 14:00. Samples were centrifuged (Heraeus™ Labofuge™ 200 Centrifuge; Thermo Fisher Scientific GmbH, Dreieich, Germany) for 10 min at 3000 rpm and stored at −20 ◦ C until analysis. CORT was detected by the automated chemiluminescence immunoassay (Immulite 1000 cortisol; Siemens Medical Solutions Diagnostics, Los Angeles, CA, USA) and CgA by the time-resolved immunofluorometry assays (TR-IFMA). 2.5. Weighing of Piglets All the piglets were weighed. To calculate the average daily gain (ADG) of each individual piglet, two weighing points were set: one on D23 (1D pre-weaning) and the other one on D29 (5D post-weaning). Bodyweight on D3 (3 days after birth, i.e., the initial weight after litter stabilization) was obtained. Weighing on D23 was carried out after saliva sampling to avoid the confounding effect of handling stress. 2.6. Statistical Analysis Data were analyzed in RStudio version 1.2.5033 (R Foundation, Vienna, Austria). The experimental unit for all the analyses is the pen. Statistical significance was accepted when p ≤ 0.05, and the tendency was considered when 0.05 < p ≤ 0.10. 2.6.1. Behaviors in Piglets and Sows around Weaning For each behavior category, the behavioral data on each observation day were summed up. The proportion of each behavior category was obtained by dividing the amount of the behavior of a pen by the total amount of sample points of that observation day. Each behavior category was expressed as a proportion of total active behavior. The categories “Other social interaction” and “Object play and exploration” (piglet), and “Pen investigation” (sow) were normally distributed, so they were analyzed in linear mixed models (LMM). There were 30%, 95% and 93.3% of 0 s in Biting, Belly-nosing and Locomotor play (piglet), respectively, so Biting was merged into the category of “Aggres- sion”; 84.4%, 68.9% and 62.2% of 0 s in Biting, Object investigation and Locomotion (sow), respectively, so Biting was merged into the category of “Aggression”. Belly-nosing and Locomotor play (piglet), and Object investigation and Locomotion (sow) were not analyzed. “Aggression + Biting” (piglet), “Aggression + Biting”, and Other social interactions (sow) were log(1 + x) transformed and analyzed in LMMs. Vocalization (sow) was analyzed in a GLMM with a Poisson distribution. In all models, the behavior was as the response variable, farrowing system, day and the interaction between farrowing system and day were the fixed effects, and the batch was the random factor. 2.6.2. Bodyweight and Average Daily Gain in Piglets around Weaning Bodyweight on 1D pre- (BW1D pre-weaning ) and 5D post-weaning (BW5D post-weaning ), and ADG around weaning were analyzed in LMMs: BW1D pre-weaning , BW5D post-weaning , or ADG (the means of each nursery pen) as the response variable, farrowing system as the fixed effect, bodyweight on D3 (BW3 ) as the covariate, and batch as the random factor. 2.6.3. Number of Aggression-Associated Skin Lesions in Piglets around Weaning The total number of skin lesions was summed by the individual due to a low number of lesions in each body part. The number of skin lesions was not normally distributed after square root transformation. A GLMM with a Poisson distribution was used to analyze the untransformed data: number of skin lesions (the means of each nursery pen) as the response variable, farrowing system, day, and the interaction between farrowing system and day as the fixed effects, and batch as the random factor. Animals 2022, 12, 724 6 of 13 2.6.4. Salivary Stress Biomarkers in Piglets and Sows around Weaning Salivary stress biomarkers, including CORT and CgA, were log-transformed and analyzed in LMMs: concentration of CORT or CgA (piglet model: the means of each nursery pen; sow model: the means of each group pen) as the response variable, farrowing system, day, and the interaction between farrowing system and day as the fixed effects, the basal level (i.e., samples on 1D pre-weaning) as the covariate, and batch as the random factor. 3. Results We present the results by piglets and sows. Two JLF15 sows in the winter batch, one JLF15 sow in the spring batch, one FC sow in the summer batch, and two SWAP sows in the summer batch were culled after weaning and therefore were excluded from the analysis. Information regarding the crating period (in days), average litter size, and the reproductive parameters of the sows in each farrowing system can be found in Ko et al. [11]. 3.1. Piglet 3.1.1. Post-Weaning Behavior The proportion of each active behavior observed in piglets after weaning is presented in Table 3. Overall, we did not find any difference between farrowing systems in the post-weaning behaviors. Table 3. Proportion (number of behavior/total active behaviors) of active behaviors in piglets (mean ± SE) in the post-weaning period in three farrowing systems (FC, n = 4: conventional farrowing crate; SWAP, n = 6, and JLF15, n = 6: farrowing pens with temporary crating). Values with a different superscript (a , b ) correspond to a significant difference (p ≤ 0.05) between observation days in the same behavior category and farrowing system. n indicates the number of nursery pens. Aggression and Biting 1 Other Social Interaction Object Play and Exploration Day of weaning (D24) FC 0.07 ± 0.02 0.44 ± 0.09 0.49 ± 0.09 SWAP 0.06 ± 0.01 a 0.37 ± 0.04 a 0.57 ± 0.03 a JLF15 0.10 ± 0.04 0.34 ± 0.08 0.55 ± 0.06 1D post-weaning (D25) FC 0.24 ± 0.09 0.41 ± 0.08 0.34 ± 0.08 SWAP 0.28 ± 0.07 b 0.32 ± 0.05 a,b 0.36 ± 0.07 b JLF15 0.27 ± 0.08 0.26 ± 0.05 0.47 ± 0.07 2D post-weaning (D26) FC 0.23 ± 0.05 0.28 ± 0.05 0.48 ± 0.06 SWAP 0.22 ± 0.05 b 0.22 ± 0.05 b 0.55 ± 0.09 a JLF15 0.18 ± 0.04 0.24 ± 0.05 0.57 ± 0.07 Global p-value 0.99 0.32 0.40 1 Aggression and Biting are presented as back-transformed values. Within each farrowing system, aggression and biting in SWAP piglets increased from the day of weaning to 1D post-weaning (p = 0.009) and the increase persisted on 2D post- weaning (p = 0.05, between D24 and D26), whereas that in FC and JLF15 piglets neither increased nor decreased. Other social interactions in SWAP piglets decreased from the day of weaning to 2D post-weaning (p = 0.03), whereas in FC and JLF15 piglets it remained similar around weaning. Object play and exploration in SWAP piglets decreased from the day of weaning to 1D post-weaning (p = 0.01) and then increased on 2D post-weaning (p = 0.03), while no changes were observed in either FC nor JLF15 piglets. Animals 2022, 12, 724 7 of 13 3.1.2. Aggression-Associated Skin Lesions around Weaning Table 4 presents the number of skin lesions counted in pre- and post-weaning periods in each of the three farrowing systems. No difference in the number of skin lesions was observed between farrowing systems (p = 0.62). Regardless of the farrowing systems, there is a trend in the number of skin lesions over time, which first increases after weaning (between 1D pre- and 1D post-weaning, all farrowing systems p < 0.001), and then decreases a day after (between 1D and 2D post-weaning, all farrowing systems p < 0.01). Table 4. Number of skin lesions of piglets (mean ± SE) counted on D23 (1D pre-weaning), D25 (1D post-weaning) and D26 (2D post-weaning) in three farrowing systems (FC: conventional farrowing crate; SWAP and JLF15: farrowing pens with temporary crating). Values with a different superscript (a , b ) correspond to a significant difference (p ≤ 0.05) between sampling days in the same farrowing system. n indicates the number of nursery pens. Farrowing System n 1D Pre-Weaning 1D Post-Weaning 2D Post-Weaning a b FC 4 0.74 ± 0.20 7.31 ± 2.87 4.93 ± 2.38 b SWAP 6 1.02 ± 0.40 a 7.77 ± 1.96 b 5.77 ± 0.93 b JLF15 6 0.96 ± 0.23 a 9.11 ± 2.54 b 6.69 ± 1.25 b 3.1.3. Salivary Stress Biomarkers Post-Weaning Table 5 presents the concentration of CORT and CgA in piglets in pre- and post- weaning periods in each of the three farrowing systems. No difference in CORT and CgA was observed between farrowing systems (p = 0.55 and 0.58, respectively). Table 5. Concentration of salivary cortisol (CORT) and chromogranin A (CgA) of piglets (mean ± SE) collected on D23 (1D pre-weaning), D25 (1D post-weaning) and D26 (2D post-weaning) in three far- rowing systems (FC: conventional farrowing crate; SWAP and JLF15: farrowing pens with temporary crating). Values with a different superscript (a , b ) correspond to a significant difference (p ≤ 0.05) between sampling days in the same farrowing system. n indicates the number of nursery pens. Farrowing System n 1D Pre-Weaning 1D Post-Weaning 2D Post-Weaning CORT, µg/dL FC 4 1.01 ± 0.16 1.42 ± 0.46 1.20 ± 0.27 SWAP 6 0.84 ± 0.17 1.34 ± 0.46 1.56 ± 0.45 JLF15 6 0.75 ± 0.11 a 1.78 ± 0.39 b 1.83 ± 0.39 b CgA, µg/mL FC 4 0.51 ± 0.09 a,b 0.50 ± 0.20 b 0.76 ± 0.26 a SWAP 6 0.71 ± 0.17 0.71 ± 0.22 0.57 ± 0.17 JLF15 6 0.50 ± 0.14 0.58 ± 0.14 0.61 ± 0.09 CORT in JLF15 increased from 1D pre- to 1D post-weaning (p = 0.02), and the increase persisted for 2 days (p = 0.01, between D23 and D26), whereas that in FC and SWAP remained at a similar level around weaning. CgA in FC increased from 1D post- to 2D post-weaning (p = 0.04), whereas that in SWAP and JLF15 remained at a similar level around weaning. 3.1.4. Bodyweight (BW) and Average Daily Gain (ADG) around Weaning Table 6 presents the BWs in 1D pre- and 5D post-weaning and the ADG around weaning in each of the three farrowing systems. There was no difference in BW1D pre-weaning , BW5D post-weaning , and ADG around weaning between three farrowing systems (p = 0.43, 0.61 and 0.16, respectively). Animals 2022, 12, 724 8 of 13 Table 6. Bodyweight (BW) (kg) on D23 (1D pre-weaning) and D29 (5D post-weaning), and average daily gain (ADG) (g/day) between D23 and D29 in piglets in three farrowing systems (FC: conven- tional farrowing crate; SWAP and JLF15: farrowing pens with temporary crating). n indicates the number of nursery pens. Farrowing System n Mean SEM p-Value FC 4 5.77 0.34 BWD23 (1D pre-weaning) , kg SWAP 6 5.65 0.43 0.43 JLF15 6 6.13 0.40 FC 4 6.42 0.45 BWD29 (5D post-weaning) , kg SWAP 6 6.14 0.37 0.61 JLF15 6 6.68 0.40 FC 4 108.00 20.73 ADGD23-29 , g/day SWAP 6 81.70 14.09 0.16 JLF15 6 91.69 12.97 3.2. Sow 3.2.1. Post-Weaning Behavior The proportion of each active behavior observed in sows after weaning in each of the three farrowing systems is presented in Table 7. Except for vocalization (p < 0.0001), there was no difference between farrowing systems in other post-weaning behaviors. The trend over time of the vocalization counts after weaning was similar in all farrowing systems: Sows vocalized the most on the day of weaning, and then rapidly decreased on 1D post- and 2D post-weaning (all farrowing systems, p ≤ 0.0001, between D24 and D25, and between D24 and D26). Table 7. Proportion (number of behavior/total active behaviors) of active behaviors in sows (mean ± SE) in the post-weaning period in three farrowing systems (FC, n = 4: conventional farrow- ing crate; SWAP, n = 5, and JLF15, n = 4: farrowing pens with temporary crating). Values with a different superscript (a , b , c ) correspond to a significant difference (p < 0.05) between sampling days in the same farrowing system. Values with a different superscript (x , y ) correspond to a significant difference (p ≤ 0.05) between farrowing systems on the same sampling day. n indicates the number of pens. Other Social Vocalization Aggression and Biting 1 Pen Investigation Interaction (Average Counts) Day of weaning (D24) FC 0.03 ± 0.01 0.22 ± 0.08 0.42 ± 0.04 25.50 ± 9.54 a,x SWAP 0.12 ± 0.04 0.24 ± 0.06 0.34 ± 0.07 52.80 ± 6.45 a,y JLF15 0.17 ± 0.05 0.37 ± 0.08 0.32 ± 0.06 19.25 ± 7.58 a,x 1D post-weaning (D25) FC 0.13 ± 0.11 0.23 ± 0.10 0.35 ± 0.14 6.00 ± 3.34 b,x SWAP 0.05 ± 0.03 0.24 ± 0.11 0.46 ± 0.14 19.80 ± 8.21 b,y JLF15 0.16 ± 0.03 0.30 ± 0.17 0.43 ± 0.15 7.75 ± 6.12 b,x 2D post-weaning (D26) FC 0.12 ± 0.04 0.24 ± 0.07 0.33 ± 0.14 5.25 ± 5.25 b SWAP 0.10 ± 0.05 0.31 ± 0.11 0.27 ± 0.09 3.60 ± 3.60 c JLF15 0.07 ± 0.06 0.19 ± 0.12 0.49 ± 0.22 3.75 ± 3.75 c Global p-value 0.64 0.97 0.88 <0.0001 1 Aggression and Biting are presented as back-transformed values. Animals 2022, 12, 724 9 of 13 3.2.2. Salivary Stress Biomarkers Post-Weaning Table 8 presents the concentration of CORT and CgA in sows in pre- and post-weaning periods in each of the three farrowing systems. No difference in CORT and CgA was observed between farrowing systems (p = 0.81 and 0.22, respectively). Table 8. Concentration of salivary cortisol (CORT) and chromogranin A (CgA) of sows (mean ± SE) collected on D23 (1D pre-weaning), D25 (1D post-weaning) and D26 (2D post-weaning) in three far- rowing systems (FC: conventional farrowing crate; SWAP and JLF15: farrowing pens with temporary crating). n indicates the number of group pens. Farrowing System n 1D Pre-Weaning 1D Post-Weaning 2D Post-Weaning CORT, µg/dL FC 4 1.49 ± 1.08 1.24 ± 0.41 1.49 ± 0.39 SWAP 4 0.89 ± 0.19 1.42 ± 0.46 1.72 ± 0.52 JLF15 4 0.67 ± 0.12 1.28 ± 0.49 1.65 ± 0.65 CgA, µg/mL FC 4 0.57 ± 0.15 1.22 ± 0.52 1.10 ± 0.40 SWAP 4 0.81 ± 0.44 1.11 ± 0.56 1.29 ± 0.41 JLF15 4 0.94 ± 0.31 1.39 ± 0.51 0.66 ± 0.21 4. Discussion This paper studies the effect of farrowing systems (i.e., early environment) on the adaptability to weaning in sows and piglets on a commercial farm. It is a follow-up study from Ko et al. [11] of the post-weaning period, meaning the same group of animals from Ko et al. [11] were followed. Sows and piglets were no longer housed in litters but regrouped in weaning facilities. As the first published pilot study involving farrowing pens with temporary crating in Spain, it is interesting to follow these sows and piglets from lactation to a few days after weaning, and how they adapted to weaning challenges in commercial conditions. 4.1. Piglets Similar numbers of skin lesions after weaning between farrowing systems indicated that the level of aggression within a group after regrouping was similar, regardless of farrowing systems. The result agrees with Chaloupková et al. [14] and Verdon et al. [21], where they found that piglets raised in FC were as aggressive (e.g., frequency of aggression and fights, fighting duration, and number of skin lesions) as those raised in farrowing pens (i.e., without sow confinement throughout the lactation period) but a single litter after weaning. As stated in Chaloupková et al. [14], a series of stressful events at weaning, including the change of the environment, maternal separation, and mixing with unfamiliar individuals, may mask the differences of the pre-weaning housing environment. Providing a socially enriched pre-weaning environment, such as early exposure to non-littermates, may play a more important role to reduce aggression after regrouping, especially at wean- ing [7,8,14,22]. We hypothesized more frequent mother–young interactions in TC during lactation may lead to an additional increased weaning stress response in sows and piglets, due to the separation of a closer sow-piglet bonding compared to FC [16]. This was, however, not the case in sows or piglets. SWAP piglets showed a similar level of CORT and CgA post-weaning, suggesting that weaning, known as a stressful event in pig production [23], did not activate the HPA (Hypothalamic-Pituitary-Adrenal) and the SAM (Sympathetic- Adrenal-Medullary) axes in SWAP piglets. We did not expect to see the difference in salivary stress biomarkers between SWAP and JLF15 as both systems are TC. However, some of the differences in pen features between SWAP and JLF15 may explain it. Firstly, SWAP pens have a slightly larger (1.04-times larger) space allowance than JLF15. Secondly, a larger coverage of the solid flooring in SWAP pens can retain the nest-building materials Animals 2022, 12, x FOR PEER REVIEW 10 of 13 Animals 2022, 12, x FOR PEER REVIEW 10 of 13 Animals 2022, 12, 724 10 of 13 larger coverage of the solid flooring in SWAP pens can retain the nest-building materials larger coverage of the solid flooring in SWAP pens can retain the nest-building materials (hay in (hay in this this case) case) on the onthe floor thefloor for floorfor piglets pigletstoto forpiglets manipulate tomanipulate during manipulateduring the duringthe suckling thesuckling period. sucklingperiod. Thirdly, period.Thirdly, Thirdly, (hay in this case) on a metal-barred metal-barred gate is installed installed between the adjacent SWAP pens, pens, allowing the piglets to a metal-barred gate is installed between the adjacent SWAP pens, allowing the piglets to perform nose perform nosecontact contactwithwithnon-littermates non-littermatesand and theneighbor neighbor sow before weaning (Figure perform nose contact with non-littermates and the the neighborsow sowbefore beforeweaning weaning(Figure (Figure 2). 2). Lastly, Lastly, the the fixed-side fixed-side cratecrate of of SWAPSWAP pens pens doesnot does notblock blockthe thesows’ sows’direct directcontact contact with with 2). Lastly, the fixed-side crate of SWAP pens does not block the sows’ direct contact with creep area thecreep creep area (Figure (Figure 3). 3). All in all, an insignificant weaning stress response in SWAP the area (Figure 3). All in all, an insignificant weaning stress response in SWAP piglets may be due to piglets to the the additional additional facilitation facilitationofofexploration explorationand andsocial socialbehaviors behaviorsinina piglets may be due to the additional facilitation of exploration and social behaviors in a larger and a more enriched (both physically and a larger and a more enriched (both physically and socially) earlysocially) early environment, which is is larger and a more enriched (both physically and socially) early environment, which is known to known tomitigate mitigatethe theweaning weaning stress stress (physical (physical enrichment: enrichment: environmental environmental enrichment enrichment [10], known to mitigate the weaning stress (physical enrichment: environmental enrichment [10], space space allowance allowance [24]; social [24]; social enrichment: enrichment: nose contacts nose contacts [25], early [25], early socialsocial playearly play [26], [26], [10], space allowance [24]; social enrichment: nose contacts [25], early social play [26], socialization [8]). [8]). early socialization However, However, we acknowledge we acknowledge thatthat in Ko et et in Ko al.al. [11], [11],we wediddidnot notfind find aa early socialization [8]). However, we acknowledge that in Ko et al. [11], we did not find a difference in social social interactions interactions between between piglets (defined as SB in the the paper), paper), exploration exploration difference in social interactions between piglets (defined as SB in the paper), exploration in piglets piglets (defined (defined as as PPE), PPE), andand piglet-initiated piglet-initiated mother–young mother–young interactions interactions (defined as in piglets (defined as PPE), and piglet-initiated mother–young interactions (defined as NNC and SC) betweenbetween SWAP and JLF15 during the suckling period. Better SWAP and JLF15 during the suckling period. Better adaptability adaptability NNC and SC) between SWAP and JLF15 during the suckling period. Better adaptability to weaning weaning in inSWAP SWAPpiglets, piglets,therefore, therefore,deserves deserves a deeper a deeper understanding understanding by different by different ap- to weaning in SWAP piglets, therefore, deserves a deeper understanding by different ap- approaches, proaches, and and certainly certainly moremore studies studies comparing comparing the the welfare welfare andand performance performance in sows in sows and proaches, and certainly more studies comparing the welfare and performance in sows and and piglets piglets in different in different TC areTC needed. are needed. piglets in different TC are needed. Figure 2. 2. A metal-barred metal-barred gate gate installed installed between between the the adjacent adjacent SWAP SWAPpens. pens. Figure Figure 2. AAmetal-barred gate installed between the adjacent SWAP pens. (a) (b) (a) (b) Figure 3. Different distances for the sow to access the creep area in (a) SWAP and (b) JLF15 pens. Figure Figure3.3.Different Differentdistances distancesfor forthe thesow sowtotoaccess accessthe thecreep creeparea areain in(a) (a)SWAP SWAP and and (b) (b) JLF15 JLF15 pens. pens. 4.2. Sows 4.2. Sows 4.2. Sows According to According to the the farmer’s farmer’s observations, observations,sows sows sowson on this onthis farm thisfarm commonly farmcommonly produce commonlyproduceproducehigh-high- high- According to the farmer’s observations, pitched calls pitched calls after weaning calls after weaning for a few days when they were moved from the farrowing pitched weaning for foraafew fewdaysdayswhenwhen theytheywere were moved movedfrom the farrowing from the farrowing pens pens to the to the group pens. In the present study, we found that SWAP sows vocalized more pens togroup pens.pens. the group In the In present study, the present we found study, we found that that SWAP sowssows SWAP vocalized moremore vocalized than than FC FCJLF15 and and JLF15 sows sows on theonday the day of weaning of weaning and and a daya day after. after. Moreover, Moreover, a similar a similar trend trend than FC and JLF15 sows on the day of weaning and a day after. Moreover, a similar trendin in the change vocalization of vocalization counts was found across farrowing systems, which peaked inthe thechange changeofof vocalization counts counts was was found found across across farrowing farrowing systems, systems,which which peaked peaked on on the the day day of of weaning weaning and and rapidly rapidly decreased decreased over over time time (with (with a a 5-time 5-time reduction reduction in FCinand FC on the day of weaning and rapidly decreased over time (with a 5-time reduction in FC and JLF15, JLF15, aand and and a 15-time 15-time reduction reduction in SWAP, in SWAP, from the from daythethe day of weaning of weaning to 2-dayto to 2-day post- post-weaning). and JLF15, a 15-time reduction in SWAP, from day of weaning 2-day post- weaning). As As Manteuffel Manteuffel et al. [27] et al. [27] reviewed, reviewed, louder, louder, longer, longer, and and high-pitched high-pitched calls calls (e.g., (e.g., squeals weaning). As Manteuffel et al. [27] reviewed, louder, longer, and high-pitched calls (e.g., squeals and and screams) screams) in pigs are in pigs are related related to the state of the excitement. Additionally, Xin squeals and screams) in pigs are to the state related of the to the state excitement. Additionally, of the excitement. Xin et al.Xin Additionally, [28] found that high frequency and long duration of calls can imply more severe stress in pigs, Animals 2022, 12, 724 11 of 13 although it was not physiologically validated. It is known that a close bonding between the sow and the piglets is established already in early lactation, with piglets recognizing the odor and the vocalization from their mother by 3 days of age [29]. In Ko et al. [11], we observed that more mother–young interactions were found in SWAP and JLF15 than FC during lactation, including both sow- and piglet-initiated. We therefore suspect the relation between this high-pitched call with the separation from the piglets, although we acknowledge that SWAP sows vocalized significantly more than JLF15. However, to the best of our knowledge, we did not find studies about vocalization in sows related to separation from the piglets or weaning, so it is difficult to demonstrate the association. Yet, further investigation on the high-pitched call produced after weaning by sows is warranted to confirm the suspicion. A similar level of salivary CORT and CgA in sows around weaning indicated an insignificant activation of the HAP and SAM stress axes. This could be due to several reasons that may overshadow the stress of separation from the piglets. Having a dominant boar [30] in the adjacent pen helps to minimize aggression and salivary stress at regrouping, as suggested in many studies [31–33]. Regardless of farrowing systems, sows from the same batch were kept in the same gestation pen before being assigned to different farrowing systems, which would not induce much aggression at regrouping after weaning as they already recognized each other [30,34]. It is also true that the regrouping procedure varied in different batches due to different availability of the group pens, for instance, the stocking density and the distribution of the boars in the adjacent pens. Further research to confirm the stronger mother–young bonding during lactation and its impact on the separation after weaning in loose pen sows and piglets is therefore necessary. 4.3. Limitations of the Present Study We acknowledge that there are two main limitations of the present study: low sample size and commercial settings. A low sample size gives a relatively weak statistical power to interpret the results. However, using farrowing pens with temporary crating is becoming a trend in the European Union, and studies regarding this topic are rather scarce. We therefore would like to encourage more research on the effect of this system, especially on the weaning adaptability in piglets. In addition, conducting studies in commercial farms where the control over different variables is rather limited (availability of the housing facilities after weaning which led to different regrouping procedures in different batches, in this case). The results of the present study ought to be interpreted with caution. 5. Conclusions Based on the results collected from this pilot study, farrowing pens with temporary crating which allow more mother–young interactions, did not seem to reduce aggression in piglets after weaning. However, salivary stress biomarkers showed a lower weaning stress response in SWAP than FC and JLF15 piglets. Increased high-pitched calls in SWAP sows after weaning might be associated with the stress of the separation from the piglets, but more research in sow vocalization after weaning should be studied. Due to the growing popularity of using alternative farrowing systems, more research to study the effect of the farrowing/early environment, which promotes close contact between the mother and the young, on a pig’s life-long adaptability towards social challenges (e.g., weaning or regrouping) is needed. Author Contributions: Conceptualization, H.-L.K., X.M. and P.L.; methodology, H.-L.K., D.T. and P.L.; formal analysis, H.-L.K., D.T. and P.L.; investigation, H.-L.K.; resources, D.T.; data curation, H.-L.K., D.T. and P.L.; writing—original draft preparation, H.-L.K.; writing—review and editing, HH.-L.K., D.T., J.H., X.M. and P.L.; supervision, X.M. and P.L.; project administration, P.L.; funding acquisition, P.L. All authors have read and agreed to the published version of the manuscript. Animals 2022, 12, 724 12 of 13 Funding: The study was funded by Proyectos I + D + I Convocatoria RETOS 2015 (AGL2015-68373- C2-1-R) from the Spanish Ministry of Economy, Industry and Competitiveness (MINECO), and the Technological Transfer of the Rural Development Program 2014–2020 (56 30062 2017 2A) from the Catalan Government. HLK received the University, Research Center and Hospital Foundation Grants for the Contracting of New Research Staff (FI) from Agency for Management of University and Research Grants (AGAUR) of the Catalan Government. PL received the Juan de la Cierva postdoctoral fellowship from the Spanish Ministry for Science and Innovation (MICINN). Institutional Review Board Statement: The study was conducted according to the guidelines of the Declaration of Helsinki, and approved by the ethical committee of Universitat Autònoma de Barcelona (UAB) (FUE-2016-00441221). Informed Consent Statement: Informed consent was obtained from all subjects involved in the study. Data Availability Statement: The data presented in this study are available on reasonable request from the corresponding author. Acknowledgments: The authors gratefully acknowledge all the collaborators who made this study possible, including Gerfam Rubirola, L’Ekip, SKIOLD and Rotecna. We thank Eva Mainau, Josep Gasa, and David Solà for providing advice on experimental design, Francisco Benitez for participating in the pilot study, David Arias, Gabriela Rojas, Alejandra Feld, and Lourdes Llonch for assisting with data collection during the study period. We also thank the statistical consultation provided by Applied Statistics Service (SEA) of UAB, and the technical support from Department of Animal and Food Science of UAB and Interdisciplinary Laboratory of Clinical Analysis of University of Murcia. Conflicts of Interest: Janni Hales works at SKIOLD which produces the SWAP and JLF15 pens, but she only participated in the housing design of the SWAP and JLF15 pens of the study, interpretation of data, and assisted in the writing of the manuscript. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results. References 1. Chidgey, K.L.; Morel, P.C.H.; Stafford, K.J.; Barugh, I.W. Observations of sows and piglets housed in farrowing pens with temporary crating or farrowing crates on a commercial farm. Appl. Anim. Behav. Sci. 2016, 176, 12–18. [CrossRef] 2. Johnson, A.K.; Marchant-Forde, J.N. Welfare of Pigs in the Farrowing Environment. In The Welfare of Pigs, 1st ed.; Marchant-Forde, J.N., Ed.; Springer: Dordrecht, The Netherlands, 2009; Volume 7, pp. 141–188. [CrossRef] 3. Baxter, E.M.; Andersen, I.L.; Edwards, S.A. Sow welfare in the farrowing crate and alternatives. In Advances in Pig Welfare, 1st ed.; Špinka, M., Ed.; Woodhead Publishing: Sowston, UK, 2018; pp. 27–72. [CrossRef] 4. Campbell, J.M.; Crenshaw, J.D.; Polo, J. The biological stress of early weaned piglets. J. Anim. Sci. Biotechnol. 2013, 4, 19. [CrossRef] [PubMed] 5. Godyń, D.; Nowicki, J.; Herbut, P. Effects of Environmental Enrichment on Pig Welfare—A Review. Animals 2019, 9, 383. [CrossRef] [PubMed] 6. Camerlink, I.; Farish, M.; D’Eath, R.B.; Arnott, G.; Turner, S.P. Long term benefits on social behaviour after early life socialization of piglets. Animals 2018, 8, 192. [CrossRef] 7. Salazar, L.C.; Ko, H.L.; Yang, C.H.; Llonch, L.; Manteca, X.; Camerlink, I.; Llonch, P. Early socialisation as a strategy to increase piglets’ social skills in intensive farming conditions. Appl. Anim. Behav. Sci. 2018, 206, 25–31. [CrossRef] 8. Ko, H.L.; Chong, Q.; Escribano, D.; Camerlink, I.; Manteca, X.; Llonch, P. Pre-weaning socialization and environmental enrichment affect life-long response to regrouping in commercially-reared pigs. Appl. Anim. Behav. Sci. 2020, 229, 105044. [CrossRef] 9. Martin, J.E.; Ison, S.H.; Baxter, E.M. The influence of neonatal environment on piglet play behaviour and post-weaning social and cognitive development. Appl. Anim. Behav. Sci. 2015, 163, 69–79. [CrossRef] 10. Yang, C.H.; Ko, H.L.; Salazar, L.C.; Llonch, L.; Manteca, X.; Camerlink, I.; Llonch, P. Pre-weaning environmental enrichment increases piglets’ object play behaviour on a large scale commercial pig farm. Appl. Anim. Behav. Sci. 2018, 202, 7–12. [CrossRef] 11. Ko, H.L.; Temple, D.; Hales, J.; Manteca, X.; Llonch, P. Welfare and performance of sows and piglets in farrowing pens with temporary crating system on a Spanish commercial farm. Appl. Anim. Behav. Sci. 2022, 246, 105527. [CrossRef] 12. Telkänranta, H.; Edwards, S.A. Lifetime consequences of the early physical and social environment of piglets. In Advances in Pig Welfare, 1st ed.; Špinka, M., Ed.; Woodhead Publishing: Sowston, UK, 2018; pp. 101–136. [CrossRef] 13. Nowland, T.L.; van Wettere, W.H.E.J.; Plush, K.J. Allowing sows to farrow unconfined has positive implications for sow and piglet welfare. Appl. Anim. Behav. Sci. 2019, 221, 104872. [CrossRef] 14. Chaloupková, H.; Illmann, G.; Bartoš, L.; Špinka, M. The effect of pre-weaning housing on the play and agonistic behaviour of domestic pigs. Appl. Anim. Behav. Sci. 2007, 103, 25–34. [CrossRef] Animals 2022, 12, 724 13 of 13 15. Oostindjer, M.; Bolhuis, J.E.; Mendl, M.; Held, S.; Gerrits, W.; van den Brand, H.; Kemp, B. Effects of environmental enrichment and loose housing of lactating sows on piglet performance before and after weaning. J. Anim. Sci. 2010, 88, 3554–3562. [CrossRef] [PubMed] 16. Newberry, R.C.; Swanson, J.C. Implications of breaking mother–young social bonds. Appl. Anim. Behav. Sci. 2008, 110, 3–23. [CrossRef] 17. Day, J.E.L.; Burfoot, A.; Docking, C.M.; Whittaker, X.; Spoolder, H.A.M.; Edwards, S.A. The effects of prior experience of straw and the level of straw provision on the behaviour of growing pigs. Appl. Anim. Behav. Sci. 2002, 76, 189–202. [CrossRef] 18. Li, Y.; Gonyou, H.W. Analysis of belly nosing and associated behaviour among pigs weaned at 12–14 days of age. Appl. Anim. Behav. Sci. 2002, 77, 285–294. [CrossRef] 19. Averós, X.; Brossard, L.; Dourmad, J.Y.; de Greef, K.H.; Edge, H.L.; Edwards, S.A.; Meunier-Salaün, M.C. A meta-analysis of the combined effect of housing and environmental enrichment characteristics on the behaviour and performance of pigs. Appl. Anim. Behav. Sci. 2010, 127, 73–85. [CrossRef] 20. Turner, S.P.; Farnworth, M.J.; White, I.M.; Brotherstone, S.; Mendl, M.; Knap, P.; Penny, P.; Lawrence, A.B. The accumulation of skin lesions and their use as a predictor of individual aggressiveness in pigs. Appl. Anim. Behav. Sci. 2006, 96, 245–259. [CrossRef] 21. Verdon, M.; Morrison, R.S.; Hemsworth, P.H. Rearing piglets in multi-litter group lactation systems: Effects on piglet aggression and injuries post-weaning. Appl. Anim. Behav. Sci. 2016, 183, 35–41. [CrossRef] 22. D’Eath, R.B. Socialising piglets before weaning improves social hierarchy formation when pigs are mixed post-weaning. Appl. Anim. Behav. Sci. 2005, 93, 199–211. [CrossRef] 23. Escribano, D.; Ko, H.L.; Chong, Q.; Llonch, L.; Manteca, X.; Llonch, P. Salivary biomarkers to monitor stress due to aggression after weaning in piglets. Res. Vet. Sci. 2019, 123, 178–183. [CrossRef] 24. Lange, A.; Gentz, M.; Hahne, M.; Lambertz, C.; Gauly, M.; Burfeind, O.; Traulsen, I. Effects of different farrowing and rearing systems on post-weaning stress in piglets. Agriculture 2020, 10, 230. [CrossRef] 25. Camerlink, I.; Turner, S.P. The pig’s nose and its role in dominance relationships and harmful behaviour. Appl. Anim. Behav. Sci. 2013, 145, 84–91. [CrossRef] 26. Horback, K. Nosing Around: Play in Pigs. Anim. Behav. Cogn. 2014, 2, 186. [CrossRef] 27. Manteuffel, G.; Puppe, B.; Schön, P.C. Vocalization of farm animals as a measure of welfare. Appl. Anim. Behav. Sci. 2004, 88, 163–182. [CrossRef] 28. Xin, H.; De Shazer, J.A.; Leger, D.W. Pig Vocalizations Under Selected Husbandry Practices. Trans. ASAE 1989, 32, 2181–2184. [CrossRef] 29. Horrell, I.; Hodgson, J. The bases of sow-piglet identification. 2. Cues used by piglets to identify their dam and home pen. Appl. Anim. Behav. Sci. 1992, 33, 329–343. [CrossRef] 30. Marchant-Forde, J.; Marchant-Forde, R.M. Minimizing inter-pig aggression during mixing. Pig News Inf. 2005, 26, 63–71. 31. Séguin, M.J.; Friendship, R.M.; Kirkwood, R.N.; Zanella, A.J.; Widowski, T.M. Effects of boar presence on agonistic behavior, shoulder scratches, and stress response of bred sows at mixing. J. Anim. Sci. 2006, 84, 1227–1237. [CrossRef] 32. McLeman, M.A.; Mendl, M.; Jones, R.B.; White, R.; Wathes, C.M. Discrimination of conspecifics by juvenile domestic pigs, Sus scrofa. Anim. Behav. 2005, 70, 451–461. [CrossRef] 33. Gieling, E.T.; Nordquist, R.E.; van derStaay, F.J. Assessing learning and memory in pigs. Anim. Cogn. 2011, 14, 151–173. [CrossRef] 34. Grandin, T.; Bruning, J. Boar presence reduces fighting in mixed slaughter-weight pigs. Appl. Anim. Behav. Sci. 1992, 33, 273–276. [CrossRef]