Detection, implications and risk factors for lameness in group-housed gestating sows

Lameness, the most general clinical sign of musculoskeletal problems, is a serious welfare and health problem to sows and a cause of considerable economic loss to pig producers. Since 2013, group-housing of sows is mandatory in the European Union, mainly because of welfare concerns. Yet, the prevalence of lame sows tends to be higher in group-housing. A decline in the prevalence of sow lameness by using proper preventive strategies is necessary to improve animal welfare, enhance farmer's working pleasure and diminish financial loss, all of which may contribute to a more sustainable pig production. A critical review of the literature on sow lameness (chapter 1) revealed that to enable optimal prevention of sow lameness, more research is needed on three main areas. First, lameness detection in sows so far has been based on subjective visual scoring systems characterized by a low sensitivity and high inter-observer variability. More objective, precise and sensitive detection methods based on kinematic and kinetic techniques have been successfully applied in other animal species but not yet in sows. Secondly, awareness of the economic implications is recognized as a potential powerful driver to motivate farmers to take action against lameness. The financial loss related to lameness is partly attributed to the (in)direct effect on reproduction. The indirect effect of lameness on sows' reproductive performance has been studied extensively but inconsistency still exists on the direct impact. Finally, critical for the development of preventive strategies is to identify risk factors for lameness. Although risk factors have been studied in stall-housed sows and finishing pigs, research on the risk factors for group-housed gestating sows is scarce. The aim of this thesis (chapter 2) was to enhance prevention of lameness in group-housed sows through improvement of lameness detection, clarification of the reproductive implications and identification of risk factors. To improve the detection of lame sows, a system, based on force and visual stance variables derived from balance analysis and image processing respectively, was developed: SowSIS (Sow Stance Information System) (chapter 3). The demountable and transportable device is practical for on-farm use and detects lameness in sows while standing. Accuracy of the force stance variables was evaluated using reference masses ranging from 5kg to 175kg. SowSIS proved to be highly accurate (R² ≈ 1) for measuring weight, irrespective of the position of the mass relative to the load cell and the duration of measurements. To determine precision of the force and visual stance variables, five consecutive measurements with SowSIS were carried out for each of 20 pregnant sows that were scored healthy on visual lameness assessment. A sufficient level of precision (within-animal CV ≤ 15%) was shown for both the force and visual stance variables, except for the number and duration of kicks (i.e. lifting leg off the ground) and weight shifts. A proof-of-concept study was performed on four healthy and four unilaterally lame sows, matched by age and gestation stage to investigate the impact of lameness on leg weight distribution. All sows were lame on the right hind leg. Weight was removed from the lame leg and was primarily transferred to the contralateral leg. The number of kicks significantly increased for the lame leg (mean of 48 versus 23 times/5min; P < 0.001). Lame sows showed a lower leg weight symmetry than non-lame sows between the hind legs (64% versus 93%; P < 0.05) and between the right legs (54% versus 76%; P < 0.05). It was concluded that SowSIS is practical, accurate and precise in measuring stance variables and that force stance variables are able to distinguish lame from visually healthy sows. Future research should establish the accuracy of visual stance variables and the results of the proof-of-concept study need to be confirmed in a large scale validation study including visual stance variables. To enable classification of sows into lame and healthy ones, an algorithm based on the force and visual stance variables should be developed and validated (chapter 6). To encourage pig producers to control and prevent lameness, the economic implications of lameness need to be elucidated, especially regarding the direct effect on reproduction. To achieve this goal, a total of 491 sows from five commercial herds were followed-up to investigate the impact of lameness and claw lesions throughout one reproductive cycle on the reproductive performance of sows (chapter 4). No significant associations were found between lameness and sows' breeding performance (i.e. weaning-to-oestrus interval and presence of sows not showing oestrus post-weaning, returning to service and aborting). The impact of lameness on the sows' farrowing performance was restricted to an effect on the presence of mummified foetuses. Lame sows were 2.4 times (95% CI: 1.19 – 4.75; P = 0.014) more likely to have mummified foetuses. Claw lesions did have an effect on farrowing performance. Sows with white line lesions and skin lesions above the claw respectively, were 1.9 times (95% CI: 1.04 – 3.51; P = 0.036) and 4.1 times (95% CI: 1.23 – 13.8; P = 0.021) more likely to have stillborn piglets. Twenty-two per cent of all sows left the group throughout the study and almost half of these sows were culled or euthanized. Lameness was the second most important reason for culling. Sows culled due to lameness were significantly younger compared to sows culled for other reasons (parity: 2.6 ±1.3 versus 4.0 ±1.8). In conclusion, lameness mainly affects reproduction indirectly, through the increased culling rate of young sows. The overall herd performance is rather affected than the reproductive performance of an individual sow and the economic implication of sow lameness is thus largely determined by the effect on sow longevity. Knowledge on the stage in the reproductive cycle at which lameness is most prevalent has a key value regarding risk factor analysis. Each of the 491 sows from the five commercial herds in chapter 4, was visually assessed the day sows were moved from one stage on the farm to another (i.e. farrowing crates, insemination stalls, group-housing). A mean lameness prevalence of 5.9% ± 3.56% was found although the prevalence significantly differed between herds in the range 2.3% to 11.0% (P = 0.002) and varied throughout the reproductive cycle (P = 0.027). The highest prevalence of lame sows was found after sows were housed in the individual stalls of the insemination stable (8.7% ± 6.1%), the lowest at the end of group-housing (4.1% ± 4.4%). These results indicate that to minimize the prevalence of sow lameness, researchers, veterinarians and farmers should focus on housing in the insemination stalls and further research should be carried out to identify influential factors that are manageable (chapter 6). Finally, knowledge on the risk factors for lameness development is crucial to develop preventive strategies. This thesis focussed on the risk factors for lameness development shortly after introduction into group-housing as for this period, the highest incidence of lameness was reported by other researchers. A longitudinal study on 15 commercial herds was performed to investigate the incidence of sow lameness and possible risk factors within the first three to five days of group-housing (chapter 5). The median prevalence of lameness in each herd was 13.1% (0 – 27.3). Sows with more than 10% of the body covered with manure were associated with a higher odds (OR = 2.33, P < 0.001) for lameness development. An increase of space allowance from 1.8 m² to 3 m² (OR = 0.40, P = 0.03) and of herd size from 144 to 750 sows per farm (OR = 0.71, P = 0.02) decreased the odds for lameness development. The degree of aggression, indicated by skin lesions, nor the floor characteristics (i.e. wetness, slipperiness and quality) influenced lameness development. This study demonstrated that lameness development in sows within the first days of group-housing may not be increased by hierarchical aggressive encounters but that sows may benefit from a higher floor area/sow. Further research should reveal the optimal floor space allowance necessary to prevent lameness development. The relationship between sow lameness on the one hand and sow dirtiness and herd size on the other hand should be elucidated and influential factors need to be defined to find measures to prevent sow lameness (chapter 6).