Animal welfare is a major challenge that most European pig producers have been facing in recent decades to comply with EU legislation and to meet the increasing societal and market demand for pork produced in a sustainable way. Although pig welfare has been governed by EU legislation since 1991 
, some major welfare issues still remain, such as the lack of space allowance, enrichment materials and bedding, and the practice of tail docking if carried out routinely 
. Housing conditions are deemed by stakeholders as particularly important to safeguard animal welfare, as well as the use of animal-based and farm-level indicators to monitor the progress of animal welfare 
Animal-based measures were developed to directly assess the effective welfare state of the pig by measuring, for example, its behaviour, fearfulness, health, or physical condition 
. Nevertheless, the European legislation for the protection of pigs is based on housing and management risks which can be assessed by using resource-based measures (i.e., non-animal-based measures) rather than animal-based measures 
. Resource-based measures are indirect measures of animal welfare because measuring the ability of the farming system (housing and management) to provide pigs with conditions to which the pigs can adapt without endangering their welfare; therefore, monitoring resource-based measures can be useful to identify risk factors that lead or may lead to actual welfare problems in pigs, which can be measured by animal-based measures. Therefore, monitoring both animal-based and non-animal-based measures is a promising approach to advice pig farmers to control and improve the welfare conditions of pigs. “Age of the animals, type of floor, feeding system, stocking density and environmental temperature can be useful to predict the appearance of a given welfare measure of ‘good housing’ on a farm” 
European legislation sets to 1 m2
/pig the minimum floor space allowance for pigs over 110 kg of body weight without any further indication for heavier pigs, such as the Italian heavy pigs slaughtered at the minimum age of 9 months and at the live weight of 160 kg ± 10%, according to the Parma ham Protected Designation of Origin (PDO) scheme 
. The allometric equation A = k × BW0.67
was used with k = 0.03 to calculate the minimum legal space allowance for growing and fattening pigs 
although EFSA recommended k = 0.036 for pigs up to 110 kg of live weight and k = 0.047 above 110 kg, to allow all pigs to rest simultaneously in lateral lying posture 
. One study showed that less stocking densities and reduced pen size can lead to more pigs laying at the same time, less pig lesions, less pen dirtiness and higher average daily growth 
The influence of group size on pig welfare is controversial; no significant effect was proved on fattening pigs according to some authors 
, whereas an increase in group size would result into unfavourable effects on welfare and performance, according to other studies 
Pig dunging behaviour is affected by space allowance because different functional areas are used by pigs for resting and for dunging, unless the pigs are heat stressed or sick or the stocking density is too high 
. Pig and litter cleanliness in straw bedded pens was also found as negatively affected by hot climate 
. One study showed that pen soiling increases with increasing age in pigs kept on solid floor, as floor soiling and wallowing behaviour was more prevalent in the late growth period 
. More pig soiling was observed in pigs that were liquid fed, compared to pigs fed with wet and dry feed 
. Italian heavy pigs are traditionally liquid fed during the entire growing-fattening phase, which is approximately 6 month long 
Free access to water of good quality is mandatory in fattening pigs 
and needed even if liquid or wet feed is provided 
. To this end, a maximum number of fattening pigs per functioning drinker is recommended, depending on the type of drinker 
: 12 pigs per nipple or 15 pigs per water bowl.
Slatted floors systems are widely used for pig housing throughout the EU 
. Fully slatted or partially slatted floors are generally used to house heavy pigs in Italy 
. They promote pig cleanliness and hygiene by allowing the quick and effective removal of faeces and urine from the pen, although fully slatted floors were found to limit the use of straw as bedding or manipulable material to allow pigs to perform explorative behaviour 
Manipulable materials are needed to enrich the pen environment for pigs intensively kept in order to meet their exploration behavioural needs and avoid tail biting and skin lesions in growing and fattening pigs kept at high stocking densities 
. One study found decreased exploration of enrichment material with increasing live weight 
, so special attention should be given to providing effective enrichment as the pigs’ live weight increases.
Enrichment materials are categorized as 
: (a) optimal materials which can be used alone because they are “edible, chewable, investigable, manipulable, of sustainable interest, accessible for oral manipulation, given in sufficient quantity, clean and hygienic”; (b) suboptimal materials, possessing most of the previous characteristics but not all of them so that their use should be combined with other materials; (c) “materials of marginal interest providing distraction for pigs which should not be considered as fulfilling their essential needs”.
Straw is considered as one of the best enrichment materials 
; it has been demonstrated that its distribution in racks in fully slatted housing systems is possible and does not compromise the effectiveness of the manure removal system 
Pig welfare is also affected by stockperson’s action 
, so the ratio of number of pigs to number of stockpersons was acknowledged as a predictor variable for severe tail lesions in heavy pigs 
Most pigs in Europe are tail-docked despite the fact that the practice of routine tail-docking was banned in 1994 
. Tail docking aims at reducing the frequency of tail biting and the related tail lesions, but it is painful for pigs and can lead to neuroma formation 
However, the occurrence of tail biting depends on a wide range of factors such as the lack of environmental enrichment, stocking density, presence of slatted floors, microclimate discomfort, high levels of dust and noxious gases (i.e., ammonia), competition for resources, social instability and genetic, dietary and health factors 
. Additional risk factors to predict farms having severe tail lesions were identified in: pig age, live weight at slaughter, space allowance for 100 kg of live weight, number and type of drinkers, pen size and number of pigs in the farm 
Tail biting can occur in all production systems, including free-range and organic 
. Particular attention should be paid by farmers keeping pigs with intact tails through frequent observation and timely intervention in case of tail-biting outbreaks, which can spread rapidly and become difficult to stop 
The prevalence of physical conditions in pigs varies between herds 
. Tail, skin, and ear lesions are used widely as animal-based measures to directly assess animal welfare of growing and fattening pigs 
The farm average pig’s mortality rate is a common measure of health and welfare for pig herds. Mortality is defined as “the uncontrolled death of animals (as distinct from culling/euthanasia). Any animal which is found dead on the floor in the house, or out on the field is considered a mortality” 
. Pigs may be culled (i.e., emergency killing) if they are injured or sick to avoid exposing them to severe pain or suffering, or if no other practical way is available to relieve the pain 
. One study shows that emergency killing is more frequently implemented on piglets rather than on older pigs, such as growing and fattening pigs 
2. Current Insights
Lower farm size (AVP) and number of pigs per Annual Work Unit (PWU) were found in farms of the BED group, compared to farms of the other two clusters, suggesting that the higher labour needed per pig in these farms could be related to a higher workload for the management of bedding materials, as well as to less economies of scale in place in these smaller sized farms. These farms are featured by no or limited slatted floor, presence of proper manipulable material and roughage, higher space allowance and no or limited tail docking; almost all pigs observed in these farms were found in reach of optimal enrichment materials, according to the EC Recommendation 336/2016 
, and most active pigs were manipulating them in the observed pens. Lower prevalence of ear and body lesions was found in these farms, compared to lean pig farms without bedded solid floored resting area (NBL), but not compared to heavy pig farms without bedded solid floored resting area (NBH), where the higher age of the observed pigs and the higher pig space allowance could have mitigated the occurrence of these lesions.
The group of heavy pig farms without bedded solid floored resting area (NBH) is characterized by the presence of fully or partially slatted floor, liquid feeding, and limited availability of drinkers, which is considered as a risk factor for pig welfare, particularly in summertime when the water nutritional need tends to increase 
, as well as the competition of pigs to access water.
Higher space allowance detected in the NBH and BED groups, compared to the HBL group, can be related to the higher live weight in NBH farms and the higher space allowance needed to house pigs with bedded solid floored resting area. It is worth noting that the pig space allowance of 1.15 m2
/pig in the heavy pig farms of the NBH group exceeds the minimum legal requirement of 1 m2
/pig for pigs over the live weight of 110 kg, although it is lower than the value on 1.48 m2
/pig, calculated through the allometric formula for pigs of 172 kg of live weight with k = 0.047, as recommended by EFSA 
. However, more space allowance for heavy pigs could be further used in the next few years by heavy pig farmers in an attempt to house pigs with intact tails, in compliance with EU legislation.
The lower presence of roughage and of bedding material in the laying area in farms of the NBH e and NBL groups, compared to the BED group, can also be related to the higher prevalence of slatted floor, which is likely to limit the use of a large quantity of organic materials (e.g., straw, wood shavings) because of the inability of the most common slurry systems (e.g., vacuum system) in place in European fattening pig farms to handle and evacuate these organic materials, together with the liquid manure under the slats, as confirmed in previous studies 
. The relatively low dirtiness score in the laying area of NBH heavy pig farms can be related to the presence slatted floor in these farms.
Lean pig farms without bedded solid floored resting area (NBL) were featured by lower pig space allowance, mortality, medication cost, number of pigs per drinker and feed conversion rate and by higher average daily growth and prevalence of ear and body lesions, compared to NBH heavy pig farms.
No statistical difference was found between the farm percentages of observed pens with at least one tail lesion in one pig in the three farm groups, although tail docking was performed in both NBH and NBL farms on almost all pigs and not performed on most pigs in BED farms; this outcome suggests that the prevalence of tail biting in undocked tail pigs can be similar to docked tail pigs housed in intensive systems, if undocked tail pigs are housed with bedded solid floored resting area, plenty of manipulable material that most pigs are able to access, and larger space allowance above the minimum EU legal requirements.
Low prevalence of tail lesions in the NBH group can be related to the majority of pigs with tails shortened by more than 50% (STT), which is likely to expose the pigs to less severe tail lesions but also to more painful tail docking and more frequent formation of neuromas afterwards 
. Low prevalence of tail lesions in NBH farms can be explained by the higher liveweight (AW) and age of the observed pigs, confirming the outcomes of a previous study showing more severe tail lesions in younger pigs, compared to older ones 
The observation of more prevalent ear and skin lesions in pigs of the NBL farm group could be ascribed to the lower space allowance at the start of the growing phase when more frequent fights may occur in recently grouped pigs to establish a hierarchy 
. Higher pig mortality rates and medication costs per pig in NBH heavy pig farms can be explained by the longer duration of the fattening period and the higher age at slaughter of at least nine months in heavy pigs for Parma Ham PDO. Mortality rate and medication cost per pig could also be biased by different culling rates across farms, due to a different degree of implementing this practice to reduce animal suffering and the spread of diseases on farms.
In conclusion, the set of animal and non-animal-based measures used in this study, was found suitable and useful to assess, analyse, and compare most of the housing risk factors for pig welfare on farms. Greater validity of the statistical model used in this study could result from a greater availability in the future of pig farm cases in the SusPigSys database.
Additional non-animal-based measures of pig welfare could also be considered to assess the microclimate pig comfort and the presence on noxious gases and dust as risk factors for tail biting and for overall welfare assessment. Animal-based indicators were used to monitor pig welfare directly and to investigate the effect that the resources and management have on the animals. Both animal and non-animal-based measures provided different types of information, which are needed for routine official controls and are suitable for use in farm assurance schemes.
Housing risk factors for pig welfare, such as the lack of space allowance, bedding, and environmental enrichment, as well as the presence of fully slatted floor and the availability of drinkers to ensure pigs have permanent access to drinkable water are likely to become more challenging for pig farmers to keep pigs intensively with long undocked tails, once the ban of routine tail docking is finally applied across all EU Member States.