A survey of straw use and tail biting in Swedish pig farms rearing undocked pigs

Farmers and data records

A telephone survey of commercial pig farmers in Sweden was conducted from July to November 2014. Their contact information was supplied in January 2014 by the Swedish Board of Agriculture. To be included on the contact list, farms had to have at minimum 50 sows or 300 finishing pig places. The farms were categorised as nursery farms (keeping nursery pigs approximately 5–12 weeks of age; 10–30 kg LW), finishing pig farms (keeping growing pigs from approximately 12 weeks of age until slaughter around 6 months of age; 30–120 kg LW) or integrated farms keeping both nursery and finishing pigs. The final contact list consisted of 747 nursery farms, 892 finishing pig farms and 4618 integrated farms. Farmers were called in a random order generated by a computer.

All telephone calls were conducted by a single person according to the randomised list. The farmers were informed of the purpose of the survey and told it was being conducted on behalf of the Swedish University of Agricultural Sciences. Participation was voluntary, and reasons for not participating were recorded. Each farmer not answering the phone was called a maximum of three times before being excluded as “unable to reach”.

The survey

Calculation of straw usage

Integrated farms were divided into nursery and finishing units. The survey contained no information regarding the number of batches or pens per unit. Thus, to acquire a relevant estimation of straw use, the calculations of straw use were based on the total number of nursery pig places or finishing pigs produced per year.

Statistical analysis

The information obtained in the survey was recorded in Microsoft Excel and transferred to SAS software ver. 9.4 (SAS Inst. Inc., Cary, NC, USA) for statistical analysis. Descriptive statistics were calculated and presented as frequencies, and mean and median values. Because data were not normally distributed, median values were preferred over mean values.

Outcomes from the questions in the survey were classified as either continuous (e.g., straw usage), ordinal (possible to rank; e.g., frequency of tail biting estimated by the farmer) or categorical (not possible to rank, e.g., strategies for avoiding outbreaks of tail biting). Categorical data were solely presented as frequencies and median/mean when feasible.

Response rate and general information about participating farms

A total of 139 farmers were called. Out of these farmers, 60 participated in the survey (43% response rate); 28 (20%) could not be reached; six (4%) did not have the time to participate; six (4%) were unwilling to participate and 38 (28%) did not have pigs anymore or did not have pigs according to the inclusion criteria of the study. The participating farmers included 29 integrated farms, indicating that in total, 46 farmers answered the nursery pig part of the survey and 43 farmers answered the finishing pig questions. Seven of the participating farms produced organic pigs. One survey was excluded because it was incomplete due to technical problems.

Occurrence of bitten pigs and suspected causes

Bitten pigs were reported in 50% of the nurseries and 88% of the finishing pig units. In affected nursery units (n = 23), 78.3% of farmers observed bitten pigs less than two times/year, 17.4% 3–6 times/year and 4.3% at least every month (tail-biting frequency). In affected finishing pig units (n = 38), 21% observed bitten pigs less than two times/year, 37% 3–6 times/year, 34% at least every month and 8% every week (tail-biting frequency).

Farmers reported observing bitten pigs in on average 1.6 pens (median 1, range 1–5) during the last tail biting outbreak in nursery units and in on average 1.5 pens (median 1, range 1–4) in finishing pig units. The finishing pig units had on average 1.6% bitten pigs in each batch at slaughter (range 0.1–6.5%). The number of affected pens per outbreak was positively correlated to the tail-biting frequency (r = 0.33, P = 0.04, n = 38) in finishing pig units. The tail-biting frequency was also positively correlated to the percentage of bitten pigs (r = 0.55, P < 0.001, n = 38) and to the number of slaughtered pigs/year of the farm (r = 0.32, P = 0.05, n = 38), in finishing pig units.

Prevention of outbreaks and treatment of bitten pigs

Usage of straw and other manipulable materials

Objects other than manipulable materials were provided in 13% (n = 46) of the nursery and 16% (n = 43) of the finishing pig units. These other objects were balls (2), rope (3), silage/hay (1), newspapers (1), sacks (6), chains (1), wood logs (5) and salt stones (5). However, in 69% of the units, pigs had access to the additional objects only when bitten pigs had been observed to prevent tail biting and fighting between pigs at certain occasions, such as when pig groups were mixed.

The ration of manipulable material was modified by farmers depending on the health status in the pen in 35% (n = 46) of the nursery and 33% (n = 42) of the finishing pig units. Some nursery units were given more straw if pigs seemed cold or dirty (5), switched to wood shavings when pigs were sick (7) or switched to peat when pigs had diarrhoea (3). One farmer reported providing less straw when pigs were sick. In finishing pig units, straw rations were increased when bitten pigs were observed (9), when pigs had diarrhoea or it was wet in the pen (3) or when the animals were sick (2). One farmer decreased the ration if it was hot outside. Farmers could give several reasons for altering the manipulable material ration, which is why the amount of answers exceeds the number of responding farmers.

Straw usage and tail biting

In finishing pig units, the reported occurrence of bitten pigs had a negative rank correlation with deep straw systems (r = −0.42, P < 0.01, n = 38) compared to systems that had partly slatted flooring, and there was a negative rank correlation between size of straw ration and tail-biting frequency (r = −0.32, P < 0.05, n = 37). In nursery units, the coefficient of correlation between reported tail-biting frequency and straw ration was r = −0.328 (P < 0.01, n = 38).

Manure handling systems

Response rate and farmer self-reporting

According to the Swedish agricultural statistical compilation 2016 [16], there were 1228 pig businesses in Sweden in 2015, meaning that 5% of Swedish pig farms are included in this study. All farmers were called in a randomised order. The vast majority of reached farmers that fit into the inclusion criteria (84%) wanted to participate in the study, which indicates that we have a representative sample of Swedish farmers.

The results from this study are based on farmers answering questions and reporting about how pigs with intact tails are reared. Self-reporting of routines and how often bitten pigs are seen is not exact. Farmers report their perceptions, but reality may differ for different reasons such as the farmer not being aware of the true prevalence of tail biting outbreaks and reporting only detected outbreaks. The aim of this study was to investigate how undocked pigs are reared, how farmers handle bitten pigs, how straw is managed and how its use is linked to tail biting and manure handling problems. Due to the nature of the aims, a survey in which farmers report their experiences gives more information in this stage than a controlled on-farm study would. The fact that farmers reported similarly (e.g., similar causes of tail biting outbreaks, similar frequency of tail biting outbreaks) indicates that the results are reliable. Furthermore, the results of this study are in accordance with those of previous studies (e.g., number of bitten pigs/batch and that tail biting increases with increased age), which also indicates that the results from this study are reliable. However, the results of this study should be considered for what they are: farmers’ opinions and not absolute facts.

Occurrence of bitten pigs and suspected causes

In the present study, suspected causes of tail biting varied between farms and pig age groups (nursery or finishing pigs), which is in accordance with previous studies concluding that different production systems have different risks for developing tail biting [9, 10]. Previous findings show that pigs with permanent access to straw are mainly exposed to climatic risk factors for tail biting (e.g., temperature and air humidity), whereas pigs with restricted access to manipulable material were mainly exposed to the risk factor “strawless system” [10].

In the present study, farmers suggested stocking density as the main risk factor in nursery units and feed composition/feed equipment as the main risk factor in finishing pig units and did not suggest factors related to the housing climate, which have also previously been considered less important [10]. This finding is partly in accordance with a survey showing that pig farmers in The Netherlands (rearing docked pigs) consider stocking density to be the main risk factor for tail biting, along with stable climate [17]. Stocking density has been considered to be one of the most common risks associated with tail biting in pigs reared in partly slatted systems with straw, as seen in previous studies [12]. Although the maximum stocking density in Sweden is lower than specified in the EU directive, stocking density was considered the most common cause of tail biting outbreaks in nursery pig units in this study and was also highly ranked in finishing pig units. It has been shown that tail biting significantly increases when the feeding space decreases in nursery units [4] and as the pen facilities remain unchanged, the stocking density will increase in the pens, decreasing feeding space per pig. Therefore, increased stocking density is associated with an increase in other pen-related risk factors. Stocking density and not only provision of additional manipulable material may be an important preventive factor for rearing undocked pigs with low incidences of tail biting.

Only one nursery farmer thought that observed tail biting outbreaks were caused by boredom. This result could indicate that farmers are satisfied with the provided straw ration and outcome. Dutch farmers report similar a prevalence of problems with tail biting (approximately 50% have tail biting problems, ranging from 1 to 6% in docked pigs) but consider provision of manipulable material to be less efficient than docking for preventing tail biting [17]. The manipulable materials used in the Dutch non-organic farms were mainly objects such as chains and ropes, while straw, sawdust or wood shavings were used by only 2–3% of participating farms. Similarly, in a survey of Finnish farmers rearing undocked pigs, manipulable material was considered less important than feeding space, animal health and indoor climate [18], but there was no information about what type of manipulable material was provided daily to the pigs in that study.

It is difficult to evaluate and compare the percentage of bitten pigs between countries and studies due to the different scoring criteria used. A review comparing the prevalence of tail biting from different sources found a wide range both within and between countries depending on data source [19]. This variation may partially be attributed to differences in definition, as definitions range from swollen tails to tissue loss [19, 20]. In this survey, the “bitten” was not defined in the protocol or by surveyed farmers. According to finishing pig farmers, the percentage of bitten pigs was on average 1.6% at slaughter, which is in agreement with tail biting recorded by National Food Agency standards in two Swedish abattoirs (1.5–1.9%) [20]. In addition, the finding that tail biting increases with age is in agreement with the findings of previous studies [3, 21]. We therefore argue that the farmers’ definition of bitten was similar to that used by the National Food Agency: “evident biting damage” or “at least half the tail missing” [22]. According to this definition, only severely bitten pigs were recorded; milder tail lesions without blood that are not easy to detect when looking at the pig from outside the pen were excluded from this study. Both abattoir and farmer scoring may therefore be under-reporting tail lesions compared to a scoring scheme that includes a swollen tail or superficial scratches as the lower limit for bitten [20, 23]. However, according to Swedish regulations, pigs with swollen tails or unhealed tail wounds are not allowed to be transported to the abattoir, which is why these types of injuries might not appear in slaughterhouse data even if the criteria were altered. Also, less severe wounds may be of less importance when investigating the current aim (i.e. how pigs with intact tails are reared and managed). Tail docking is conducted to reduce the problems with injured pigs and carcass losses due to more severe tail wounds. Hence, the lack of information about tail lesions that are less severe than those reported at the abattoir is of less importance for investigating whether or not it is possible to rear pigs with intact tails although less severe wounds may also affect the production and welfare of the pigs [e.g. 12].

Usage of straw and other manipulable materials

All participating farmers reported providing access to manipulable material; of these farmers, 99% provided straw in various amounts. The wide usage of straw in this study indicates that straw usage is considered feasible and functional under commercial conditions in Sweden.

Permanent access to straw has previously been defined as the permanent presence of more than one litre of unsoiled straw in the pen before new straw allocation [2]. Permanent straw access was achieved at 80–290 g straw/pig per day for 30–80 kg LW pigs at a stocking density of 0.7 m² per pig according to a Danish study [2]. According to this definition, many Swedish farmers do not provide permanent access, as the reported median straw ration was 29 g/pig per day for nurseries and 50 g/pig per day in partly slatted flooring systems.

In this study, however, all calculations regarding daily ration were based only upon the amount of straw, even though 37% of the farmers also provided additional material to various extents. Therefore, the actual ration of accessible manipulable material may be underestimated here. A more detailed analysis of the other manipulable material was not possible because the survey did not request further information on this matter. When developing the survey and collecting experiences from researchers and the Swedish trade organisation for pigs, the wide use of additional material was not foreseen. However, it was shown that provision of additional material was common only during certain periods of time, such as when pigs are newly moved into the stable or due to poor pen hygiene, while straw was given on a daily basis. Additional material may therefore have been of less importance in preventing tail biting outbreaks on participating farms.

If the provided additional material is of high value for the pig, it may affect tail biting behaviour. High-value materials that stimulate pig exploratory behaviour are destructible, manipulable, changeable and complex [7]. Furthermore, the most used objects during the first 24 h after presentation are odorous, deformable, rootable, unattached and chewable according to previous studies [24]. The most used objects 5 days after presentation are ingestible, destructible, contained (e.g., straw provided in a box as opposed to directly on the floor as in this study), particulate and rootable [24]. In this study, sawdust and wood shavings were the most commonly provided complements to straw and they may possess the previously identified characteristics. Additionally, the pigs’ usage of sawdust or wood shavings is reduced when they are combined with another material with greater manipulative value [25], and both chopped and long straw are used more than sawdust [24]. The effect of adding sawdust or wood shavings to straw is therefore limited, and the usage of additional material along with the straw could be considered to have a limited effect on tail biting behaviour.

Straw length has, however, been suggested to affect its investigative value for the pigs, as reduced straw length decreases the diversity of behaviours [26]. The majority of farmers in this study used chopped straw, with a mean length of 5 cm in nursery and 6.5 cm in finishing pig units. This straw is considerably longer and therefore possibly of higher value than the chopped straw of 1–4 cm used in the previously mentioned study [26]. The straw on Swedish farms may still be long enough to be of high investigative value to the pigs and seems efficient in preventing tail biting behaviour.

Straw usage and tail biting

The amount of straw needed to reduce the prevalence of tail biting behaviour by satisfying the need to explore has previously been determined to be approximately 400 g straw/pig per day for 30–80 kg LW pigs and 400 g/pig per day for what the author describes as nursery-finishing pigs [2, 11]. Again, this ration is far greater than most of the straw rations reported in this study (median 45–53 g/pig per day). However, the reported rates of bitten pigs at slaughter (average 1.6% at slaughter) and tail biting were low, and no bitten pigs were ever seen in 50% of the nursery and 12% of the finishing pig units. Furthermore, the number of affected pens during an outbreak was reported to be low in both nursery and finishing pig units (on average 1 pen/outbreak), indicating that few pigs are affected in each outbreak and thus that the environment is overall sufficient. The use of additional material along with straw and its effect on tail biting was not assessed in this study.

Straw usage and manure handling

Straw provision in pig pens has been suggested to cause blockage of the slatted floor and to cause obstructions in the manure handling system [6, 27]. However, chopped straw reduces the need for manual cleaning of the pen compared to long straw [25]. Previous studies have shown that provision of large amounts of straw in partly slatted farrowing pens does not cause blockage of slats if straw chop length is adjusted to the type and design of the slatted flooring [28]. The results of this study show that the frequencies of reported problems in the manure handling system were low. The majority of farmers (76%) provided chopped straw, which could explain the low frequency of problems caused by straw in manure handling systems.

The main reported restriction on increasing the straw ration was that straw was thought to cause blockages in the manure handling system or the slatted floor, although most farmers reported never having experienced these problems. This finding implies that the argument that straw would be unusable with current manure handling systems is not correct and shows a gap in knowledge where future research is needed. Straw could also be used in fully slatted flooring systems given that the pigs can make use of the straw before it passes through the slats.

It is reasonable to believe that the pipe diameter of the manure handling systems has an impact on the frequency of obstruction. Pipe diameter is not regulated in the EU Directive or in the Swedish national legislation and could therefore not be assessed in this study, but it could be an interesting area of new research.