Small group pens with 10-14 grower-finisher pigs are convenient from a management point of view because they allow for a rapid monitoring of health and welfare issues in pigs, without the need to access the pen. This pen system is normally linked to wet-dry feeders as these optimize feed efficiency (23). Despite no references are available, this is one of the most common systems in growing-finishing units in the EU. To understand how space allowance and mixing influence growth performance and welfare of pigs in this system, pigs were subjected to three space allowances (0.96, 0.84 and 0.72 m2/pig) in a first trial, and two space allowances (0.96 and 0.78 m2/pig) combined with mixing in the second trial. These space allowances were chosen above the 0.65 m2/pig minimum set by European legislation based on live weight (22) which is already criticised from a welfare point of view because of very low amount of shared space (24). The space allowances were adjusted by changing the number of pigs per pen as it would be observed in current field situations. This fact could induce confounding on whether growth performance is affected by space allowance, feeder space or group size (7). However, these factors are usually confused in any commercial conditions. Nevertheless, Schmolke et al. (8) observed no detrimental effect on growth performance of pigs housed at 10, 20, 40 and 80 pigs per pen with a space allowance of 0.76 m2/pig and one single wet-dry feeder provided for every 10 pigs. Moreover, Flohr et al. (1) also reported no effect of group size on ADG, ADFI and FCR in similar conditions to the present study. This results suggests that group size would not affect productive performance in the present study. Restricted feeder space could also impact growth performance (3). Wastell et al. (9) compared group sizes of 20 and 26 pigs per pen and did not find any detrimental effect on growth performance with 10 pigs per wet-dry feeder space compared to 13 pigs per wet-dry feeder space considering a space allowance of 0.65 or 0.78 m2/pig. This results suggest that feeder space would not affect growth performance in the present study. Hence, the present study discusses space allowance as the main factor to affect growth performance regarding the pen system studied.
In terms of space allowance, previous studies observed that decreasing space allowance resulted in a poorer growth performance in pigs with space allowances of 0.65 m2/pig and similar slaughter weights (4,5,9) or 0.80 m2/pig when marketed to slaughter weights up to 138 kg of BW (25). Overall, all these studies can be compared using the allometric approach expressing space allowance as a coefficient (k) (10). Gonyou et al. (11) stated that the critical k value below which growth performance is affected as space allowance is further decreased, ranges from 0.0317 to 0.0348 over all data sets analysed using a broken-line analysis. In our study, pigs with 0.72 and 0.78 m2/pig reached the critical k value by the end of the trial when the first group of pigs reached the marketed weight (i.e. 110 kg of BW) and were sent to slaughter. Thus, the growth performance of the pigs would not be compromised during the grower-finisher period if space allowance is established based on the critical k value at the marketed weight.
Mixing affected growth performance during the whole grower-finisher period in Exp2. The drop in ADG and ADFI is consistent with previous literature which observed that mixed pigs had decreased ADG and ADFI when they were followed in a 4 week experiment at the beginning of the grower-finisher period (17,18). Stookey and Gonyou (26) also observed a depressed ADG in mixed pigs after being mixed for a 2 week period when they had 83 kg of BW. The present study showed that mixing causes a severe effect on growth performance in currently modern facilities and genetics, and strategies to avoid mixing or mitigate it are an important issue for future research (27,28).
The underlying hypothesis in this study was that space allowance and mixing interact with each other in current field situations. The study found that mixing effect on final BW is exacerbated at lower space allowances (i.e. 0.78 m2/pig). However, this interaction did not show up in any of the other variables and should be checked for repeatability in further experiments. Hyun et al. (17) reported that when pigs are subjected to multiple concurrent environmental stressors such as high ambient temperature, regrouping and low space allowance, the final effect over productive performance is additive.
In terms of animal welfare, the current study found that the number of body lesions increased at lower space allowances. This finding is in accordance with previous literature which reported a strong relationship between space allowance and body lesions (7,14,29). Anil et al. (7) stated that animal welfare is enhanced at higher space allowances in terms of postural behaviour, lower injuries and aggression. Space allowance affected the number of body lesions during the whole grower-finisher period. Nevertheless, the number of body lesions decreases as the pigs get heavier which is in accordance with previous studies (30). A possible explanation for this might be related to the pigs’ experience and ability to adapt to their social environment and being in a stable group for a long time which benefits the long term welfare of the pigs (31,32). The present study raises the possibility that there is a threshold between 0.78 and 0.84 m2/pig which an increase in the number of body lesions due to space allowance is observed. Still, the number of body lesions as a proxy for aggression may vary because of other factors not controlled in the present study, and moreover, it could also be related to the pen design and the wet-dry feeder space per pig.
One interesting finding was that highest body lesions counts were seen in the anterior body region which is consistent with fighting for access to the feeder (33). Single space wet-dry feeders may allow to accommodate a high number of pigs per feeder space without having a detrimental effect on growth performance (9,34). However, López-Vergé et al. (35) observed that pigs allotted to more feeder spaces had low body lesions counts and tended to have low BW variability within pen by the end of the grower-finisher period.
The results provided in the present study indicate that animal welfare may be compromised before growth performance is affected. Averós et al. (36) suggested a critical k value of 0.039 for lying behaviour with a broken-line analysis. This k value is higher than the 0.0336 reported by Gonyou et al. (11) below which productive performance is affected. High number of body lesions caused by competition or aggression, are likely associated with detrimental implications for pig health and performance due to immunosuppression caused by the social stress (37–39). This fact may be exacerbated in farms that have more infectious diseases in comparison to the farm where the trial was performed, which is free of the main infectious diseases. Ultimately, compromised animal welfare has detrimental implications to the sustainability of the swine production system (40).
Mixing pigs leads to agonistic social behaviour mainly within the first 24 h (41). However, the current study observed that the number of body lesions due to aggression in mixed groups is the same as the non-mixed groups after one week of being mixed. This finding is consistent with previous studies which may be explained by the establishment of the social hierarchy (42–44). Nevertheless, mixed pigs showed a poor growth performance compared to their counterparts in the study. This could be related to the social network properties and chronic stress that are not shown in body lesions (44–46).
Regarding pen efficiency, this study supports evidence from previous observations (7) which showed that pigs in lower space allowances had higher overall pen efficiency. In addition, pen efficiency showed an interaction between space allowance and mixing which indicates that pen efficiency in lower space allowance may be affected when pigs are mixed. These results may encourage pig producers to seek optimal space allowances to optimize overall efficiency and reduce housing costs. Nevertheless, the present study also observed that pig welfare is aggravated in low space allowances. This and if other environmental stressors such as high ambient temperature, mixing or the farm sanitary status are considered, pig performance and pig producer’s income may be mitigated even with improved pen efficiency. Therefore, further studies considering economic analyses on how environmental stressors affect pig performance and welfare in different space allowances are needed.