Impact of straw on pig and pen hygiene in partly slatted flooring systems


 Background: Straw is a beneficial enrichment material for pigs, shown to improve welfare and reduce abnormal behaviour such as tail biting. Straw has previously been identified as one of the means of how to raise pigs without tail docking, through improving natural exploratory behaviour. Straw has however been linked to poor pen hygiene, making farmers reluctant to use straw and has largely not been implemented in commercial farming. Straw is a beneficial enrichment material for pigs, shown to improve welfare and reduce abnormal behaviour such as tail biting. Straw has however been linked to poor pen hygiene, making farmers reluctant to use straw and has not been implemented in many commercial farming.Results: This study investigates the impact of straw on pig and pen hygiene in partly slatted pens in three grower and four finishing pig batches on five commercial farms in Sweden which are providing straw daily. Each batch was divided into two treatments; Control: 50-600g straw/pen/day based on the farm normal straw ration; and Extra straw; (=doubled Control ration). The pens were scored based on cleanliness of the pigs, solid and slatted pen floor every second week. The pig and pen hygiene were mostly scored as clean in both treatments, overall around 1 % of the observations were considered dirty/soiled.Conclusions: As very few pens or pigs were considered dirty, it was concluded that straw provision is possible without risking poor pig and pen hygiene. Few observations in this study were considered dirty regardless of amount of straw that was provided, and had likely to do with other factors in the production rather than straw ration. These results implicates that straw could be used in partly slatted pens in order to improve pig environment.

compared to fully slatted systems [11]. However, fully slatted pens substantially reduce the possibility to provide straw on the oor since most straw will quickly pass through the slats and reduces the behavioural bene ts for the pigs. Pigs in straw-based systems may purposely soil their lying area to enabling wallowing during hot conditions, which may impair pig hygiene [11] but may also be an indication of poor indoor environment.
There is lack of knowledge of how straw provision impact hygiene under commercial conditions. The aim of this study was to investigate the pig and pen hygiene in systems with partly slatted ooring in farms that are providing increased straw ration compare to standard level. The gained knowledge may be used for facilitating straw usage in commercial production in partly slatted ooring systems.

Animals and housing
The study was conducted in ve commercial Swedish pig farms from November 2015 to June 2017. All pigs were progenies of crossbred sows (Landrace and Yorkshire sows; either Norwegian Landrace*Swedish Yorkshire or Norwegian Landrace*Topigs Large White, that were inseminated with Duroc (G1, G2, F2, F3, G5, F5) or Hampshire boars (F4). One batch each of growing pigs (10 to 30 kg) was investigated on three grower farms: one specialized piglet producing farm (G1), and two farrow-to-nish farms (G2, G4). One batch each of nishing pigs (30 kg -120 kg) was investigated on four farrow-to nish farms (F2, F3, F4, F5). On F2 and F4 the same batch of pigs was observed in the grower nishing pig stable. The experiment started as the pigs moved in to the grower/ nisher unit and nished as the pigs were moved from the grower unit to the rst pig was sent to slaughter, respectively. Detailed information about farms and housing is found in Table 1.  3 3 weekends 4

weekends
The pigs had daily straw provision and supervision according to normal farm routines. All pens consisted of both solid and slatted ooring ( Table 1). The pigs were mixed in pens with gilts and castrates and sorted by size; housing the heaviest and the lightest pigs together respectively. Intact litters was practices when possible. This study was part of a larger study investigating the impact of increased straw ration on pig behaviour and prevalence of tail lesions (see [5]).

Experimental design and treatments
The studied pens was divided in two treatments per farm: Control (C), receiving the farm normal straw ration; and Extra Straw (ES), receiving a double C-ration (Table 1). The C-ration was determined and standardized before the experiment by measuring the daily straw ration provided by the herdsmen (Table 1). All pens of the same treatment were located in the same row of the stable unit, to ease for the animal caretaker. Except for straw ration, all pens in the stable unit were managed in the same way on farm level. Any exceptions from the straw provision routine were recorded by the caretaker ( Table 2). If there was blockage in the slatted ooring, and more than 50% of the slatted area was no longer visible, the daily straw provision could be paused until blockage was cleared.

Observations
The pig and pen hygiene was scored every second week, including the rst and last week of the experiment by two observers (taking turns scoring C or ES treatment, no intra-observer reliability was measured) ( Table 3). The recordings were conducted at least one hour after the daily cleaning and straw provision. Pig hygiene Pig hygiene was scored according to the Welfare Quality® protocol applied to growing and nishing pigs manure on the body (Welfare Quality 2009). All pigs were individually assessed for manure on the body, on the side that was visible towards the observer according to a three point scale; 1 if a maximum of 20% of the pig was covered in manure; 2: >20-50% of manure coverage; 3: >50 of manure coverage.

Pen hygiene
The solid and slatted oor of the pen were each divided in to four separate parts (Fig. 1). The solid oor part was considered dirty when at least 50% of the area was covered by faeces, mired straw or was wet. The slatted oor area was considered as blocked, when the slats was covered and no space between the slats was visible for at least 50% of the assessed area. For each past assessed as dirty/blocked, the pen was scored with one point. Subsequently the scores were added to receive the nal hygiene score of the solid and slatted area respectively. A pen could have a maximum of four points (all parts ≥ 50% dirty/blocked) and a minimum of 0 points (all parts < 50% dirty/blocked) per solid/slatted area.

Statistical analysis
All data was recorded in Microsoft Excel 2016 and analysed through SAS 9.4 (SAS Institute Inc., Cary, NC, USA). The descriptive statistics were calculated through means and frequencies on both farm and age category (grower/ nisher) level. Pig and pen hygiene data was ordinal and data had repeated measurements on pen level.

Pig hygiene
The low variability made analysis of variance on age level impossible. To enable analysis the data was rearranged into binomial traits; hygiene score 1 (i.e. maximum 20% soiled body surface) were considered clean while pigs with score 2-3 (> 20% soiled body surface) were considered dirty. Data was then analysed using Fisher's exact test, investigating impact of Treatment (C or ES) on Pig hygiene on pig level for each Farm and observation occasion separately.

Pen hygiene
The low variability made analysis of variance on age level impossible. To enable analysis of variation, the data was rearranged into binomial traits on the solid/slatted oor separately. Pens with the score 0 (no soling/blockage of the oor) were considered clean while pens with score 1-4 (soling/blockage of 25-100% of the Solid/Slatted area) were considered dirty. Data was then analysed using Fisher's exact test, investigating the impact of Treatment (C or ES) on pen hygiene on pen level for each Farm and observation occasion separately.

Correlation between Pig and Pen hygiene
Spearman rank correlation was used to investigate the correlation between pig hygiene and solid/slatted oor hygiene. Pig hygiene was converted into a mean score per pen instead of the initial pig level value. Solid and slatted hygiene was kept as the initial values ranging from 0-4 for each occasion. Thus, the correlations were estimated based on pen level scores, for each scoring occasion, both on Farm and Treatment level.

Animals and housing
The study was conducted in ve commercial Swedish pig farms from November 2015 to June 2017. All pigs were progenies of crossbred sows (Landrace and Yorkshire sows; either Norwegian Landrace*Swedish Yorkshire or Norwegian Landrace*Topigs Large White, that were inseminated with Duroc (G1, G2, F2, F3, G5, F5) or Hampshire boars (F4). One batch each of growing pigs (10 to 30 kg) was investigated on three grower farms: one specialized piglet producing farm (G1), and two farrow-to-nish farms (G2, G4). One batch each of nishing pigs (30 kg -120 kg) was investigated on four farrow-to nish farms (F2, F3, F4, F5). On F2 and F4 the same batch of pigs was observed in the grower nishing pig stable. The experiment started as the pigs moved in to the grower/ nisher unit and nished as the pigs were moved from the grower unit to the rst pig was sent to slaughter, respectively. Detailed information about farms and housing is found in Table 1.
The pigs had daily straw provision and supervision according to normal farm routines. All pens consisted of both solid and slatted ooring ( Table 1). The pigs were mixed in pens with gilts and castrates and sorted by size; housing the heaviest and the lightest pigs together respectively. Intact litters was practices when possible. This study was part of a larger study investigating the impact of increased straw ration on pig behaviour and prevalence of tail lesions (see [5]).

Experimental design and treatments
The studied pens was divided in two treatments per farm: Control (C), receiving the farm normal straw ration; and Extra Straw (ES), receiving a double C-ration ( Table 1). The C-ration was determined and standardized before the experiment by measuring the daily straw ration provided by the herdsmen (Table  1). All pens of the same treatment were located in the same row of the stable unit, to ease for the animal caretaker. Except for straw ration, all pens in the stable unit were managed in the same way on farm level.
Any exceptions from the straw provision routine were recorded by the caretaker (Table 2). If there was blockage in the slatted ooring, and more than 50% of the slatted area was no longer visible, the daily straw provision could be paused until blockage was cleared.

Observations
The pig and pen hygiene was scored every second week, including the rst and last week of the experiment by two observers (taking turns scoring C or ES treatment, no intra-observer reliability was measured) ( Table 3). The recordings were conducted at least one hour after the daily cleaning and straw provision.

Pig hygiene
Pig hygiene was scored according to the Welfare Quality® protocol applied to growing and nishing pigs manure on the body (Welfare Quality 2009). All pigs were individually assessed for manure on the body, on the side that was visible towards the observer according to a three point scale; 1 if a maximum of 20% of the pig was covered in manure; 2: >20-50% of manure coverage; 3: >50 of manure coverage.

Pen hygiene
The solid and slatted oor of the pen were each divided in to four separate parts ( Figure 1). The solid oor part was considered dirty when at least 50% of the area was covered by faeces, mired straw or was wet. The slatted oor area was considered as blocked, when the slats was covered and no space between the slats was visible for at least 50% of the assessed area. For each past assessed as dirty/blocked, the pen was scored with one point. Subsequently the scores were added to receive the nal hygiene score of the solid and slatted area respectively. A pen could have a maximum of four points (all parts ≥50% dirty/blocked) and a minimum of 0 points (all parts <50% dirty/blocked) per solid/slatted area.

Statistical analysis
All data was recorded in Microsoft Excel 2016 and analysed through SAS 9.4 (SAS Institute Inc., Cary, NC, USA). The descriptive statistics were calculated through means and frequencies on both farm and age category (grower/ nisher) level. Pig and pen hygiene data was ordinal and data had repeated measurements on pen level.

Pig hygiene
Page 10/25 The low variability made analysis of variance on age level impossible. To enable analysis the data was rearranged into binomial traits; hygiene score 1 (i.e. maximum 20% soiled body surface) were considered clean while pigs with score 2-3 (>20% soiled body surface) were considered dirty. Data was then analysed using Fisher's exact test, investigating impact of Treatment (C or ES) on Pig hygiene on pig level for each Farm and observation occasion separately.

Pen hygiene
The low variability made analysis of variance on age level impossible. To enable analysis of variation, the data was rearranged into binomial traits on the solid/slatted oor separately. Pens with the score 0 (no soling/blockage of the oor) were considered clean while pens with score 1-4 (soling/blockage of 25-100 % of the Solid/Slatted area) were considered dirty. Data was then analysed using Fisher's exact test, investigating the impact of Treatment (C or ES) on pen hygiene on pen level for each Farm and observation occasion separately.

Correlation between Pig and Pen hygiene
Spearman rank correlation was used to investigate the correlation between pig hygiene and solid/slatted oor hygiene. Pig hygiene was converted into a mean score per pen instead of the initial pig level value.
Solid and slatted hygiene was kept as the initial values ranging from 0-4 for each occasion. Thus, the correlations were estimated based on pen level scores, for each scoring occasion, both on Farm and Treatment level.

Pig hygiene
Grower level In grower pens, 99.4% of the observed pigs were scored 1 (ranging from 1-2 in C and 1-3 in ES treatment); 99.1% in C and 99.5% in ES treatment. On farm G1 scores ranged from 1-2 in both treatments, on farm G2 scores ranged from 1-2 in C treatment and 1-3 in ES treatment, on G4 all pigs were scored 1 at all occasions. In total, 0.8% of the observations on G1 and 1.1% on G2 were considered dirty.
No signi cant treatment effect on pig hygiene was found on any farm on grower level (Table 4).

Finishing pig level
In nisher pens, 97.1% of the pigs observed were scored 1 (ranging from 1-3), 96.7% in C and 97.4% in ES treatment. On F2, F4 and F5 scores ranged from 1-3 in both treatments. On F3 scores ranged from 1-3 in C and 1-2 in ES treatment. In total, 0.9% of the observations on F2, 0.8% on F3, 8.1% on F4 and 3.4% on F5 were considered dirty.
Signi cant treatment effect on pig hygiene was found at three occasions on two of the farms. On F4, the highest straw ratio (ES) had cleaner pigs on occasion 4 (P <0.05). On F5, lowest straw rations (ES) pigs were cleaner on occasion 6 (P<0.001) while C pigs were cleaner on occasion 10 (P<0.001) ( Table 4).

Pen hygiene
Solid oor area Grower level. 91.6 % of the C and 96.3% of the ES observations were scored 0 ( Table 4). The mean score ranged from 0.0-0.2 on farm level. On Farm G1, G2 and G4, no signi cant effect of treatment was found on Solid oor hygiene (Table 5).
Finishing pig level. 89.4% of the C and 92.0% of the ES observations were scored 0 ( Table 4). The mean score was from 0.0-0.4 between the farms. F3 and F2 had no effect of treatment (  (Table 4). On G2, i.e. the lowest straw ration, the score ranged from 0-4 in C Treatment and 0-2 in ES Treatment. On G2, ES pens were cleaner compared to C pens on occasion 3 (P<0.001), but no treatment effects were seen on farms G1 or G4 (Table 5).

Correlation between Pig and Pen Hygiene
Grower pig level. On Farm level, correlations were found on farm G2, where solid hygiene was positively correlated with pig hygiene (P <0.05) and slatted hygiene (P < 0.001) ( Table 6).
On Treatment level solid and slatted hygiene was fully correlated on farm G1 on WIP (weeks in production) 5 (P <0.0001) ( Table 7). In C treatment on G2 Pig hygiene was positively correlated with slatted hygiene (P <0.005) and fully correlated with solid hygiene (P >0.001) and solid and slatted hygiene was positively correlated (P <0.05) on WIP 5. In the ES treatment Solid and slatted hygiene was positively correlated in WIP 5 (P <0.05).
On Treatment level, effects were found on F2, F4 and F5 but not F3. On F2 pig hygiene was positively correlated with slatted hygiene on WIP 5 (P <0.01) WIP 13 (P<0.05) in C treatment (Table 7). In ES treatment solid and slatted hygiene was fully correlated (P<0.001) on F2. On F3 solid and slatted hygiene was positively correlated in WIP 11(P<0.01) in C treatment. On F5 solid and slatted hygiene was positively correlated in WIP 2 (P<0.01) and WIP 6 (P<0.01) in C treatment.

Discussion
Our study did not indicate that pen provided with straw had poor hygiene since the absolute majority of observations were (>90%) were clean. This experiment was conducted to gain further knowledge on straw impact on pig and pen hygiene in order to facilitate straw usage in commercial pig production as the compatibility of straw into current production systems, with slatted oors and mechanic manure handling, has been questioned [9,10].
The effect of the straw amount on pig behaviour could be found in Wallgren et al [5]. Whenever there was a Treatment effect on pig hygiene, ES treatment resulted in cleaner pigs, compared C treatment. The effect on pen hygiene was more variable although most pens were considered clean and implies that straw has different effects on different farms. Increased straw rations might help differentiating between lying and dunging areas, leading to less soiling of the lying area, on farms where straw provision lower. On farms with larger C rations, negative effect was mainly seen on the slatted oor possibly indicating that if larger amounts of straw do end up on the slats it might not pass through the slats and hence reducing slatted oor hygiene. However, it should be noted that the overall hygiene scores were very good on all farms, suggesting a satisfactory pen hygiene for all treatments.
The scoring of pig and pen hygiene was conducted at least 1 hour after the daily supervision, when pens were cleaned and provided with straw, which might have affected the results. However, as reported from the same experiment [5], the treatment had no effect on manual cleaning. It could be argued that the pig hygiene is a more long term effect of the pen hygiene as pigs will get soiled lying in a soiled pen. The pig hygiene will thus sustain also if the pen has been temporarily cleaned. Therefore, the cleanliness of the pigs supports the conclusion that the pen hygiene was good. On few occasions, positive correlations were found between solid oor hygiene and pig hygiene, indicating that pigs needed to lie down on a dirty solid area. Correlations between slatted oor hygiene and pig hygiene were less common. Signi cant correlations between solid and slatted oor pen hygiene were more common, although only found on few occasions. The equivocal results are probably dependant on the low variability and relative cleanliness of the pens, where the possible poor hygiene on the slatted oor is not enough to change the behaviour of the pig to soil also the solid oor and vice versa. Dirty pigs could also be as result of wallowing, i.e. the covering of the body in mud-like substances [12]. The behavioural background of wallowing is not fully understood but is thought to be part of e.g. thermoregulation, grooming, sexual and social behaviour [12].
Wallowing in excreta, the only type of wallowing offered here, has however been advocated as an abnormal behaviour and suggested to happen only when there is no other suitable wallowing substrate available [13,14]. Further, wallowing in excreta would only serve as thermoregulatory as pigs normally would avoid lying in excreta [13,14]. Wallowing in excreta is thus rather an indicator of improper environment and not primarily a hygiene issue.
This study was done on commercial farms, and daily straw provision and reporting was performed by the herdsmen. The low variability in the data made sophisticated modelling of the effect of increased straw di cult, while the descriptive data and simple tests still has great value describing the practicalities of using straw in partly slatted pens. Studies conducted on-farm differ compared to studies conducted on research farms where research is the main activity. The accuracy of studies made in commercial settings may therefore be discussed in terms of e.g. self-reporting and consistency of applied treatment. In order to increase the reliability the experiment was conducted in several farms. Conducting the experiment in commercial setting aimed to increase the applicability of the results. Daily straw provisions to pigs in partly slatted pens did not result in poor pig or pen hygiene.

Conclusions
The amount of straw provided had no effect on the hygiene scoring of the pen oor (solid or slatted) or pen hygiene. It could therefore be concluded that it is possible to provide pigs with straw without risking large problems with hygiene. Pens with presence of poorer hygiene were commonly not affected by the amount of straw provided, why hygiene issues was probably related to other management issues rather than the provision of straw. Availability of data and materials The datasets used and analysed during the current study are available from the corresponding author on reasonable request.

Competing interests
The authors declare that they have no competing interests. Funding application: SG. All authors read and approved the manuscript.     Division of pens into units of assessment