Growth performance
Re-grouping of pigs at weaning requires the establishment of a new social order [6] through aggressive behaviour, which results in skin lesions, psychological stress [1] and compromised productive performance [25]. We hypothesized that apart from the initial fights for hierarchy establishment at re-grouping, MIXED pigs would show a poorer social stability along the production cycle, displaying more conflicts and chronic aggression when dealing with a nutritionally low-density diet. Pigs in MIXED groups showed higher levels of aggression and had more body lesions post-weaning compared to pigs kept in LITTER groups. Additionally, the results support that a low-density diet interacts with mixing at weaning, as aggression was more prevalent in MIXED groups fed the LOW diet. In line with previous reports [18,19,20,21], nursery pigs fed a low energy and protein diet were unable to increase their voluntary feed intake to compensate for the reduced dietary nutrient and energy density and so their growth was reduced. Consequently, the FCR of pigs fed the LOW diet was 10.3% poorer during the nursery period.
Although pigs expend a high amount of energy fighting to establish the social order [9] and the associated stress response is energy costly, mixing did not affect weight gain of pigs. Mixing caused a slight numerical increase in feed intake, which resulted in a worsened FCR during the nursery period. These findings are in contrast to previous studies that identified a negative effect of regrouping involving an impairment of growth. Camerlink et al. (2021) [25] reported poorer growth during the first week post-weaning after mixing weaned pigs compared to keeping them in littermate pens. Additionally, other authors associated the regrouping of pigs at the beginning of or during the growing-finishing period with a reduction in growth [5, 9,10,11,12,13]. It may be that under the conditions of the current study that the aggression exhibited due to mixing was not sufficiently intense to compromise growth, but this is difficult to confirm since most previous studies did not include behavioural data.
One of the goals of mixing pigs at weaning is to reduce the variability in pig BW within the pen. Previous studies observed that while the co-efficient of variation (CV) for within pen pig weight was initially reduced due to mixing, it then gradually increased until the end of the growing and finishing period when mixed groups had similar CV for pig weight as pen-groups of littermates (i.e. not remixed) [4, 26]. Indeed, Tindsley and Lean (1984) [26] proposed that a certain degree of variation in BW between individuals within a group is a necessary component of group social dynamics and that groups of animals will therefore tend towards such variation. Accordingly, the CV for pig weight increased among pigs in MIXED groups and converged with the CV in BW of LITTER groups fed the CON dietary regime. However, the CV in BW of LITTER groups fed the LOW diet did not decrease to the levels observed in the other three treatment groups. This result is probably explained by the fact that the lighter pigs in LITTER pens had reduced feed intake capacity. Therefore, they could not ingest sufficient nutrients and energy from the low-density diet to catch up the heaviest littermates. In the same line, Douglas et al. (2014) [27] found that light birth weight pigs benefited more from a high specification post-weaning diets than their normal birthweight counterparts. In growing pigs, Hastad et al. (2020) [28] demonstrated that increasing the dietary energy density for pigs from 30 kg BW mainly favored the growth of the lighter half of the pigs and reduced the within-pen CV of bodyweight at slaughter. Aymerich et al. (2022) [29] also showed that severely limiting dietary SID Lys:NE below nutritional requirements can negatively affect the within-pen CV of pig weight of growing pigs (28–63 kg BW), mainly because the dietary challenge restricted the growth of the lightest pigs. This interaction between the initial BW homogeneity of the pen and dietary regime density was also observed by Magowan et al. (2011) [30]. These authors observed the highest within-pen CV for ADG when they provided a low energy density dietary regime to pigs heterogeneously grouped from weaning to 20 weeks of age, while the lowest CV for ADG was observed when uniformly grouped pigs were fed an energy and nutrient-rich dietary regime.
At the beginning of the growing-finishing period, all treatment groups were moved to finisher accommodation without further mixing and all groups were fed an amino-acid and energy rich finisher diet. For the rest of the trial, MIXED and LITTER pens did not show any differences in growth parameters. This is in contrast with the results of Jones et al. 2011 [31], who reported improved growth during the growing-finishing period in mixed groups of pigs that included full siblings compared to those that did not, although contrary to our study, these pigs underwent another regrouping at the beginning of this growing-finishing period. After providing a growing-finisher diet to meet the dietary requirements of all pigs during the realimentation period, pigs that were fed the LOW diet during the nursery period exhibited the same BW gain and tended to show a lower ADFI than those fed the CON regimen during the nursery period, which translated into a lower FCR for the former. The improved efficiency of pigs fed the LOW diet during the growing-finishing period can be attributed to two potential factors. First, their lower initial BW at the start of this period may have played a role, as pigs generally become less efficient as they grow [32]. Second, this finding could indicate a growth compensation mechanism exhibited by the pigs following the reduction in dietary density [33].
Previous reports showed that pigs can exhibit complete compensatory growth during the recovery period by increasing feed intake after a period of reducing dietary energy and nutrient density [16, 17]. In the present trial, animals fed the LOW diet were still 2.6 kg lighter at slaughter. Menegat et al. (2020) [33] suggested that a recovery period of > 55–60% of the overall period (63% in the present study) was long enough for complete compensatory growth. However, in this trial the nutrient limitation affected the pigs during an early stage of growth when they were still partially growing through cell proliferation [34], potentially influencing the final cell number and limiting the potential for compensatory growth.
In addition, the dietary change at transfer to growing finishing accommodation reduced the within-pen variation in BW of the LITTER pigs that had been fed the LOW diet during the nursery period. By the end of the study, within pen variation in pig weight was no different to that of the other groups, most probably because the lighter pen-mates had the opportunity to show some degree of compensatory growth.
Behaviour observations and lesion scores
Establishment of the dominance hierarchy in mixed groups was reflected in an increased number of aggressive behaviours, aggressive behaviours around the feeder, body lesions (358% increase) and ear injuries relative to pigs kept in litter groups in the 24 to 48 h post weaning. The number of aggressive behaviours, including aggression at the feeder, showed an interaction with the diet, which is difficult to explain given the short time it was offered to the pigs before the behavioural observations were performed and the low feed intake in the first hours post-weaning. However, it has been suggested that after abrupt weaning, piglet sill have dependence on high oleic acid lipids as found in milk and prefer feeds with a higher lipid inclusion than typically offered in weaner diets [35]. Therefore, the superior fat content of the CON diet (12.18%) compared to the LOW diet (5.03%) might have helped piglets to cope better with the transition from sow’s milk to the starter diet, which might have reduced the stress level and the social tension among pigs. However, the dietary regimen had no effect on the number of body lesions which are a good proxy for aggressive behaviour on commercial farms [36, 37]. In the current study, the body lesions probably showed a more realistic picture of the aggressive interactions that occurred after mixing the pigs, as aggressive behaviour caused by mixing generally subsides within 24 h [6], while in the current study behaviour observations were only performed after this time.
During the same post-weaning observation, LITTER pigs showed a lower frequency of damaging oral behaviour towards other pigs, including belly nosing, tail biting and ear biting than MIXED pigs. The expression of damaging behaviours likely reflected the more stressed state of the mixed animals after weaning [38]. Mixed pigs showed more ear lesions, although, in this stage, they were more likely caused by aggression than by oral ear manipulation. Tail scores were non-existent at the post weaning inspection, indeed they were low throughout the study and did not differ between treatments probably reflecting the fact that the pigs were docked [39]. Additionally, mixed groups showed more mounting behaviour than litter groups after weaning. Mounting may occur when the dominant pig settles its rank [40] or to demonstrate the dominance status [41] and probably was part of the set of behaviours associated with establishment of the dominance hierarchy. These results are in line with Camerlink et al. (2021) [25], who also identified an increase in sexual mounting in regrouped pigs at weaning. However, they did not observe increases in the performance of damaging behaviour in response to mixing, probably because of the large between-pen variability found in their observations.
There is normally a reduction in aggressive behaviour between mixed pigs once a new stable social order is established, approximately two weeks after mixing [5]. In agreement, the number of body lesions did not differ between treatment groups two weeks after weaning in the current study. However, in socially stable groups chronic aggression may persist, refining previously established social relationships and are often triggered during competition for limited resources [42]. Prior studies revealed a long-term negative effect of mixing growing-finishing pigs on their performance [12, 13], suggesting that there may be long-term implications for their welfare. While there was an initial increase in the number of body lesions immediately after mixing, this increase in body lesions was not followed by an effect on performance which appeared weeks later [12]. To gain further insights into this finding, in the present trial, we incorporated behaviour observations to uncover any potential link between mixing and chronically altered behavior that could elucidate the adverse performance outcomes observed in the earlier studies. In the present study, the number of aggressive behaviours and aggression around the feeder at the end of the nursery period increased where the LOW dietary regimen was fed to MIXED groups of pigs. Pigs fed the LOW diet, especially the ones that were mixed performed more aggression associated with the feeder than pigs fed the CON diet at the end of the nursery period. At the time of the observations (d70), pigs were close to reaching 30 kg BW and probably already had some capacity to increase their physical feed intake to increase their energy and amino acid intake on the LOW regimen. The low-density diet, especially when provided via a single-spaced feeder, might have stimulated increased competition for access to feed and, consequently, increased the number of aggressive behaviours around the feeder. This could have increased the risk of conflicts given the limited feeder space [43], resulting in high levels of stress and aggression [44]. The level of aggression and aggression around the feeder was lowest in MIXED pigs fed the control diet. However, the nutritional treatment only had a significant effect in pigs in the MIXED pens. This result suggests that when pigs are fed the control diet, mixed animals might perform less chronic aggression compared to groups of littermates. However, when they are fed a reduced nutrient and energy density diet and potentially face an increased competition for access to feed, they may display more chronic aggressions compared to groups of littermates. Nevertheless, the differences in the aggressive behaviour observed among groups were not reflected in the number of skin lesions.
Pigs fed the LOW dietary regimen showed more damaging behaviour towards pen mates at the end of nursery period, which might reflect that these pigs were nutritionally limited. When growth or immune functioning are limited by nutrient availability, pigs can increase their foraging behaviour to satisfy their nutritional needs [45]. If rooting substrates are not sufficiently available, pigs can redirect their foraging and exploratory behaviour to nosing, chewing, or sucking certain body parts of their pen mates, which could end up in vigorous biting leading to wounds [46]. Others have described an increase in the occurrence of damaging behaviours such as ear and tail biting when protein requirements were not fulfilled [45, 46]. Our findings revealed that pigs in LITTER pens fed the LOW dietary regime exhibited higher ear lesion scores compared to MIXED pigs fed the CON diet at this stage. However, it is important to note that the overall scores were generally low. The observed difference in scores was primarily influenced by two pens where an outbreak of ear attacks occurred performed by a single pig in each case. Therefore, it is likely that the role of the experimental treatments in the higher ear lesion scores was probably minimal in the current study.
When pigs were moved to the grower-finisher rooms, there was a general increase in aggressive behaviour. This effect was already described by Moore et al. (1994) [47] who found that pigs from static groups fought more after rehousing than pigs of similar size housed in dynamic groups. Nevertheless, this increase in aggressions was minor in the pigs kept in litter groups, suggesting they preserved a better social group stability than those in MIXED groups. Despite this, the total skin lesion counts at the beginning of growing-finishing period did not show a difference between pigs in MIXED and LITTER pens.
After substituting the low-density nursery diet by the growing-finishing diet with a higher nutritional density, the differences between groups in aggression around the feeder disappeared. Even pigs that received the LOW diet during nursery period showed a tendency to perform less aggression after the dietary change. This emphasizes the significant role that competition for feed played in the development of chronic aggression. However, pigs previously fed the LOW dietary regime during the nursery period showed more skin lesions. The explanation for this contrasting finding may be that these lesions possibly resulted from the aggression around the feeder performed during the last days of nursery phase, which was higher in the pigs fed the LOW diet.
At the end of growing-finishing period, there was an interaction between mixing and diet for the counts of aggressive behaviour. LITTER pigs fed the LOW diet showed more aggressive behaviours than LITTER pigs fed the CON diet and MIXED pigs fed the LOW diet. LITTER pigs also performed more damaging behaviours than MIXED pigs. These differences are difficult to interpret but given the passage of time since weaning/mixing and the dietary challenge it is likely that these findings are not biologically relevant. In addition, the lesion scoring did not corroborate these behavioural differences between treatments.
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