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Lying characteristics as determinants for space requirements in pigs
Applied Animal Behaviour Science 80 (2003) 19–30
Lying characteristics as determinants for space requirements in pigs E. Dinand Ekkela,*, Hans A.M. Spoolderb, Ina Hulseggec, Hans Hopsterc a
Ethology Group, Department of Animal Sciences, Wageningen Agricultural University, P.O. Box 338, NL-6700 AH, Wageningen, The Netherlands b Research Institute for Animal Husbandry, P.O. Box 2176, NL-8203 AD, Lelystad, The Netherlands c Institute for Animal Sciences and Health (ID-Lelystad), Division of Animal Sciences, P.O. Box 65, NL-8200 AB, Lelystad, The Netherlands Accepted 8 July 2002
Abstract An experiment was carried out to study the lying postures, space occupation and percentage of space-sharing (as a consequence of huddling behaviour) in pigs. Information about these lying characteristics is essential for the calculation of lying space requirements, e.g. for pen design legislative purposes. The study included two replicates of eight groups of eight pigs each. Pigs were housed at thermoneutral conditions in 8 m2 partly (40%) slatted pens from 25 to 100 kg live weight. Lights were on from 06:30 until 18:30 h. Feed and water were available ad libitum. Behaviour of the pigs was recorded for 48 h on video tape at approximately 30, 50, 80 and 100 kg live weight. General activity, lying posture (sternum, half recumbent, fully lateral recumbent) and space-sharing percentage were scored by 20 min scan sampling. The results confirmed that pigs of all weight categories lie down for a great part of the day: the average numbers of pigs that were lying were 83.8, 85, 86.3 and 87.5% at 30, 50, 80 and 100 kg live weight. Especially at night (18:30–06:30 h), the fully recumbent lying posture is predominant. The percentages of lying pigs that lied in the fully recumbent position in that period were 65.7, 66.4, 67.7 and 69.1 for 30, 50, 80 and 100 kg pigs, respectively. In our study, we observed that pigs spent little time in contact with conspecifics: space-sharing percentages were 20– 40%. There was a significant period effect for the space-sharing data; pigs showed more ‘social lying behaviour’, i.e. tended to huddle more during the night in comparison to the day since the spacesharing percentages were higher during the night (P < 0:01). Based on the observations carried out in this study, average space occupation figures were calculated for lying pigs: 0.30, 0.46, 0.64, 0.76 m2 for 30, 50, 80 and 100 kg pigs. The present study confirms the suggestion of Petherick (1983) that, as a starting point for discussions about space requirements for pigs, the floor area occupied by lying
* Corresponding author. Tel.: þ31-317-483968; fax: þ31-317-385006. E-mail address: [email protected] (E.D. Ekkel).
0168-1591/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 5 9 1 ( 0 2 ) 0 0 1 5 4 - 5
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pigs at thermoneutral conditions should on average be based on the estimated floor area for half recumbent pigs, i.e. area ¼ 0:033 bodyweight0.66. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Pigs; Lying behaviour; Space occupation; Space requirements
1. Introduction Questions concerning the minimum space requirements for intensively kept pigs in relation to their welfare have been an issue of interest for many decades. Recently, the discussion about these questions has intensified, as there is a growing concern among western societies about the welfare of intensively kept pigs. Consumers increasingly demand food to be produced in more welfare friendly ways. Furthermore, retailers and fast food restaurant chains also tend to include welfare standards for finishing pigs in food production assurance schemes for their suppliers. Politicians, both nationally and at the EU level, have responded by re-evaluation of existing legislation, in order to meet the growing concern about the welfare of pigs. Recently, the EU agricultural ministers formally agreed a revision of the Council Directive 91/630/EEC. In this revision (Council Directive 2001/ 88/EC), space allowance was not altered, but it was recommended that not later than 2008, the Commission will submit a report to the Council that covers (among other issues) the effects of housing pigs at different stocking densities on welfare and health. Considering the space requirements of animals, it is a fact that all animals occupy a certain amount of space at any one moment in time due to their physical size and geometry. Additional space is needed to allow them to carry out normal behaviours, such as feeding, drinking, elimination and resting in designated areas of their pen, and for interacting with conspecifics. It is important to stress that the pig by nature is a ‘clean’ animal; the pig has specific lying areas and dunging areas. This places special demands on the pigs spatial requirements. If the available space is not in accordance with the animals requirements, the activity is suppressed, displaced or performed in a pen area unsuitable for the behaviour. This can cause aberrant behaviour and physiological changes in the animal which, in turn, may be regarded as symptoms of a reduction of its welfare. The scientific literature concerning space requirements for finishing pigs is abundant. Many papers studying the effects of space allowance on production performance, health and behaviour (Edwards et al., 1988; NCR-89 Committee on Confinement Management of Swine, 1993; McGlone and Newby, 1994; Hyun et al., 1998; Spoolder et al., 2000; Turner et al., 2000) have been published. These papers contribute to questions related to space allowances on the welfare of pigs, but do not attempt to develop a theoretical framework for space requirements that regards the animal and its physical dimensions as the primary determinant of spatial definition. In contrast, Petherick and Baxter have published several papers (Petherick and Baxter, 1981; Petherick, 1983) in which they attempt to give such a framework. However, their work was only a start and they state that more information is needed. For instance, they present equations for pigs adopting a fully recumbent lying position and a sternum position, but they acknowledge that information about the (1) temporal distribution of behaviours and (2) the lying postures of the pigs is of significant importance for a final assessment of the spatial
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requirements of these pigs. In addition, their equations do not take (3) the ‘group dynamics’ of a group of pigs into account. The present study was set up to increase our knowledge on these three issues, as there is little information available about them. Its results aim to further develop the theoretical framework on spatial requirements of pigs.
2. Material and methods 2.1. Animals and housing Sixteen groups of eight pigs each (50% male, ‘Krusta’ (a commerical breed) (GYNL)) were used in two batches. The experiment was carried out at the Research Station ‘Sterksel’ of the Institute of Animal Husbandry in Lelystad, The Netherlands. Pigs were housed in one room with eight pens measuring 8 m2 each. Half the pens had a 60% solid sloped floor and 40% metal slats. The other pens had a 60% solid arched floor and 40% slatted floor (both in the front as well as in the in the back of the pen). These types of flooring are the most common ones used in The Netherlands. Pigs were blocked by weight and assigned to one of each floor types. The environmental temperature in the experimental room was set according to the requirements for thermoneutral conditions (Bruce and Clarke, 1979); 24 8C at the start of the experiment, 20 8C at the end of the experiment. Pigs entered the experimental room at an average age of 69:2 2:3 days and an average weight of 25:4 2:1 kg. Water and a standard commercial concentrate diet for finishing pigs were available ad libitum. The concentrate was offered in one feed hopper per pen. Light was on from 6:30 to 18:30 h. During the night, a dim light was kept on, in order to be able to observe the pigs on video. 2.2. Behavioural observations The behaviour of the pigs was videotaped for 48 h at an average live weight of approximately 30, 50, 80 and 100 kg. The following ethogram was used for the behavioural observations: Ethogram Posture
Definition
Lying
Lying on side or belly, body not supported by any of the legs, position not changed Standing, walking or running, body supported by three or more legs, position change possible Body supported by one or two front legs
Standing Sitting Lying behaviour Sternum Half recumbent Fully recumbent Space-sharing (%)
The animal is lying on the belly with at least two legs folded under the body The animal is lying half on the side and half on the belly The animal is lying on the side with all four legs stretched out See comments in the text
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Fig. 1. Diagram showing how space-sharing was assessed. The rectangle was based on the model of Petherick (1983) for fully recumbent lying pigs.
Space-sharing was estimated as follows: for each fully recumbent lying pig, a theoretical space-rectangle (see work of Petherick and Baxter, 1981) was drawn around it. The percentage of that theoretical rectangle that was not occupied by the pig, but by one of its pen mates was recorded as the percentage of space-sharing. In other words, it was the space of the Petherick and Baxter (1981) rectangle that was shared with a pen mate (See Fig. 1). Social lying behaviour was also observed when pigs were lying in a half recumbent position, but given the fact that this position was adopted to a much lesser extend as the fully recumbent position and that the area that can be shared with other pigs within that smaller ‘envelope’ is much smaller than that of fully recumbent lying pigs, we think space-sharing among half recumbent lying pigs would have minor consequences for the figures presented here and therefore, space-sharing for half recumbent pigs was not taken into account. 2.3. Calculations of the total area occupatioon Based on the observational data, we calculated the floor area that was occupied by pigs. To be able to do this, two assumptions had to be made. Firstly, it was assumed that the space occupied by half recumbent pigs was the average of pigs lying in a fully recumbent position and a sternum position. Petherick and Baxter (1981) developed space requirement equations on these two postures. They estimated the floor area required with the following equations: Fully recumbency area ðm2 Þ ¼ 0:047 W 0:66
(1)
Sternum area ðm2 Þ ¼ 0:019 W 0:66
(2)
where W ¼ live weight.
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We think it is reasonable to assume that the area occupied by pigs lying in half recumbence is between the area of pigs lying in a fully recumbent position (1) and pigs lying in a sternum position (2). Therefore, we adopted the following equation: Half recumbency area ðm2 Þ ¼ 0:033 W 0:66
(3)
For the four weight categories studied in the present experiment (30, 50, 80 and 100 kg), space occupation factors were adopted based on these equations. These space occupation factors are presented in Table 1. Secondly, we assumed that the area around a fully recumbent lying pig in the Petherick rectangle (i.e. the black are of Fig. 1.) that can be shared with pen mates is 40%. This assumption was based on estimations from videotapes. Hence, the percentage of spacesharing is a percentage of this ‘empty’ part of the rectangle. Pigs lying on top of each other were never observed. The floor area requirement per lying pig in the present experiment can thus be calculated as follows: ((number of pigs lying in fully recumbent position sof (space occupation factor, see Table 1) 0.4 soff percentage of space-sharing) þ (number of pigs lying in half recumbent position sofh) þ (number of pigs lying in sternum position sofs))/ number of lying pigs: Lying pigs : al ¼
ðnf sof f 0:4 sof f %s nf Þ þ ðnh sof h Þ þ ðns sof s Þ nl (4)
where al is the area for lying pigs, sof the space occupation factor, see Table 1., %s the percentage space-sharing, nf the number of pigs in full recumbency, nh the number of pigs in half recumbency, ns the number of pigs in the sternum position, nl the number of pigs lying. 2.4. Statistical analyses First, for each pen, average frequencies (%) for postural behaviour and lying behaviour and the average space-sharing (%) per scan sample moment (0.00; 0.20; 0.40, etc.) based on data of both sampling days were calculated. Then, for each variable, averages per weight category, floor type, pen and batch were determined. Since major behavioural differences Table 1 Space occupation factors (m2) per weight category, based on Eqs. (1)–(3) Weight category (kg)
30 50 80 100
Lying posture Fully recumbency
Half recumbency
Sternum
0.443 0.621 0.847 0.982
0.311 0.436 0.595 0.689
0.179 0.251 0.343 0.397
These factors are the areas that result from the original equations of Petherick and Baxter (1981).
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were seen between the day-time observations (6:30–18:30 h) and night-time observations (18:30–6:30 h), ‘period’ (day, night) was additionally included in the model. Data were then analysed separately for each weight category in Genstat (1993) 5.3. using the GLMMprocedure (McCullagh and Nelder, 1990). A binomial distribution and logit transformation were incorporated in the GLM and dispersion was estimated from Pearson w. The method of analysis is based on maximum quasi likelihood. After full (including all interaction terms) analyses, irrelevant (i.e. non-significant interaction terms) were excluded from the model. Finally, most data were analysed with the model: Yijk ¼ m þ floor typei þ batchj þ periodk þ batchj periodk þ eijk Exceptions are given in the result paragraph. Pairwise comparisons were calculated with the RPAIR procedure (Thissen and Goedhart, 1992).
3. Results 3.1. Lying behaviour Fig. 2 shows the data of the general postural behaviour scores for each of the four weight categories. For clarity purposes and because of the fact that no differences in lying behaviour between floor types were found (P ¼ 0:082), data were pooled for both floor types. The results show that pigs of all weight categories were lying for most of the day and that the number of pigs that were lying increased by weight (general average 83.8, 85, 86.3 and 87.5 for 30, 50, 80 and 100 kg pigs, respectively). As a consequence, the activity-peaks at 08:00 and 16:00 h became more pronounced. A significant interaction between batch and period was found for 30 and 80 kg pigs (P ¼ 0:001 and P ¼ 0:029, respectively). For 50 and 100 kg pigs, significant more pigs were lying during the night period in comparison with the day period (50 kg: 76.8 versus 92.3%, P < 0:001 and 100 kg: 83.1 versus 93%, P < 0:001 for night and day, respectively). 3.2. Lying posture A significant floor batch effect was found (P ¼ 0:036) for ‘percentage of pigs lying in half recumbent position’ at 30 kg. For all other variables and weight categories, no floor effect was found. Therefore, again, pooled data are presented in Fig. 3 for all lying posture data. The percentages of lying pigs that lied in the fully recumbent position at night (18:30– 06:30 h) were 65.7, 66.4, 67.7 and 69.1 for 30, 50, 80 and 100 kg pigs, respectively. In general, but especially at night, this position is predominant. Significant interactions between batch and period were found for all lying posture variables for 80 kg pigs (P < 0:05) and for the variables ‘percentage of pigs lying in sternum position’ and ‘percentage of pigs lying in fully recumbent position’ for 100 kg pigs (P < 0:01). Minor differences between batch 1 and batch 2 were found. The percentage of pigs of 30 kg that was lying in a sternum position was higher (20.5 versus 15.1, P < 0:001) and the percentage of pigs of 30 kg that was lying in a half recumbent position was lower
E.D. Ekkel et al. / Applied Animal Behaviour Science 80 (2003) 19–30 Fig. 2. General postures (%) from 0:00 to 4:00 h of pigs of the four weight categories: lying (---), standing (—) and sitting (). Data of pigs for each behavioural category. Data were calculated based on behavioural observations of all pens and both floor types in the present experiment. 25
26 E.D. Ekkel et al. / Applied Animal Behaviour Science 80 (2003) 19–30 Fig. 3. Lying postures (%) from 0:00 to 24:00 h of pigs of the four weight categories: sternum (---), half recumbent (—) and fully recumbent (). Data were calculated based on behavioural observations of all pens and both floor types in the present experiment.
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Table 2 Space-sharing (% S:D.) for each of the four weight categories per day/night period and per batch. Percentages indicate that part of the ‘empty space’ of a virtual rectangle-area around a fully recumbent lying pig (based on equations of Petherick and Baxter, 1981), that is occupied by another pig Weight category
Day period Batch 1
30 50 80 100
26.12 20.28 22.22 26.11
Night period Batch 2
9.04 8.34 6.05 6.69
25.12 24.41 28.57 28.39
Batch 3 9.09 5.39 8.70 8.85
42.81 31.45 24.74 31.19
Period P-value
Batch P-value
<0.001 <0.001 <0.01 <0.001
<0.001 <0.05
Batch 4 14.56 8.62 9.14 6.72
38.57 34.47 37.14 37.31
8.17 8.42 8.04 7.61
(18.3 versus 23.8, P < 0:001) in the first batch in comparison with the second batch. The same applies for 50 kg pigs: 22.8 versus 11%, P < 0:001 and 16.9 versus 25.6% P < 0:001. Also, differences between the day and night period were small or subjected to interaction significance (see earlier). The percentage of pigs of 30 kg that was lying in a sternum position was higher (23.9 versus 11.8, P < 0:001) and the percentage of 30 kg pigs that was lying in a fully recumbent position was lower (30 versus 60.3, P < 0:001) during the day period in comparison with the night period. The percentage of pigs of 50 kg that was lying in a sternum position and a half recumbent position was higher (23 versus 11, P < 0:001; 22.5 versus 20, P ¼ 0:004) and the percentage of 50 kg pigs was lying in a fully recumbent position was lower (31.3 versus 61.3, P < 0:001) during the day period in comparison with the night period. Finally, the percentage of 100 kg pigs that was lying in a half recumbent
Fig. 4. Space occupation areas (m2) for lying pigs from 0:00 to 24:00 h for each weight category 30, 50, 80 and 100 kg.
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position was higher (26.1 versus 20.9, P < 0:001) during the day period in comparison with the night period. Fig. 3 shows the data of the lying postures. 3.3. Space-sharing and calculated occupation areas per pig Table 2 shows the space-sharing percentages for each of the four weight categories. These percentages are 20–40%. There was a strong significant period effect; pigs showed more ‘social lying behaviour’, i.e. tended to huddle more during the night in comparison to the day as the space-sharing percentages were higher during the night. In addition, there was a significant effect of the batch for 80 and 100 kg pigs; in batch 2, space-sharing percentages were higher than those found in batch 1. Fig. 4 shows the results of the calculations of the areas occupied per lying pig throughout the day for each of the four weight categories. The average areas occupied by lying pigs were 0.30, 0.46, 0.64 and 0.76 m2 for 30, 50, 80 and 100 kg pigs, respectively.
4. Discussion The present study aims to built on the theoretical framework concerning the space occupation of pigs published by Petherick and Baxter (1981) and Petherick (1983). In many studies, space allowances or requirements have been assessed in experiments where the performance of the pig is the predominant measure (Edwards et al., 1988; NCR-89 Committee on Confinement Management of Swine, 1993; McGlone and Newby, 1994). This suggests that there is no need to house pigs at an area larger than the area below which production decreases. From a welfare perspective however, this is questionable. Our study regards the physical dimensions of the animal and its behaviour as the primary determinants of spatial requirements; we observed the lying behaviour of pigs and their lying postures. Based on these observations, we calculated the area occupied by the pigs. In addition, we took into account the consequences of social lying behaviour (such as huddling) for the space occupation of a group of pigs. The behavioural observations show that the pigs lie down for many hours a day. It has been reported that the amount of time young pigs are resting is between 40 and 60% (Kuipers and Whatson, 1979; Blackshaw, 1981). At the start of our study, pigs had an average age of 10 weeks, which is much older than the piglets in those studies. Our results correspond more to the study of Ruckebush (1972), who showed that pigs spend about 80% of the time resting; during the night, more than 90% of the pigs was resting in our study. Thus, the amount of space allocated for a group of finishing pigs from 25 kg onwards should take into account that these pigs prefer to lie down together for most of the day. It should, in fact, facilitate that all pigs can rest at any one time. The present study also shows that, when pigs are lying, the fully recumbent position is predominant. On average, more than 60% of the lying animals appears to lie down in a fully recumbent position. This percentage was even higher during the night period and increases with age. In general, the climatic environment affects the posture adopted by an animal. At high temperatures, pigs will lie down in a fully recumbent position with their limbs extended, in order to be able to transfer as much heat as possible to the
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environment (Close et al., 1981). At low environmental temperatures, pigs will adopt a body posture which minimises their contact with the floor (Baldwin and Ingram, 1967; Close et al., 1981). However, in our study pigs were housed at thermoneutral conditions (Bruce and Clarke, 1979). As a consequence, there was no need for the pigs to adjust their posture to the environmental temperature. Petherick (1983) presumed that the floor area occupied by resting pigs at these temperatures can on average be based on a half recumbent lying posture of the pigs. Adopting this theory, the average lying area occupied by pigs at thermoneutral conditions can be calculated with the equation A (m2 ) ¼ 0:033 W 0:66 . The floor occupied by lying pigs in our study, in which we used the observed lying postures adopted by the pigs at different weight categories, equals an average equation of Area ¼ 0:033 W 0:66 . Thus, although pigs were lying predominantly in a fully recumbent position, the space they occupy is on average that of half recumbent lying pigs. This can be explained by the fact that we took space-sharing of the pigs lying in a fully recumbent position, as a consequence of social lying behaviour, into account. Our data support the presumption of Petherick (1983): for thermoneutrally kept pigs, taking the equation for half recumbent lying pigs (A (m2) ¼ 0.033W0.66) as a starting point for lying space occupation calculations for groups of pigs is reasonable. Many recommended space allowances, such as the one recommended for commercial practices by Edwards et al., 1988 (A (m2) ¼ 0.030W 0.66) and the minimum space requirements laid down in EU Council Directive 2001/88/EC are far below this equation as these recommendations/requirements concern the total area. As an example, based on our study, the space occupied by lying pigs of 100 kg corresponds to 0.76 m2, whereas the recommendation of Edwards et al. (1988) is 0.65 m2 total space, which is also the minimum requirement for 85–110 kg pigs of EU Council Directive 2001/88/EC. Our results suggest that the amount of space allocated for a group of finishing pigs from 25 kg onwards should take into account that these pigs prefer to lie together for most of the day. In addition, we showed that when pigs are given a reasonable amount of space, they predominantly prefer to lie down and rest in a fully recumbent lying posture, with their limbs extended and only some contact with conspecifics. The present study does of course not answer the question whether pigs actually need an amount space that goes beyond recommendations generally based on production performance studies. It showed how much space pigs will use when given the opportunity, not that less space is detrimental to their welfare. Additional validation experiments, studying the welfare relevancy of those behavioural expressions should be carried out in order to answer this question. When successful, minimum space requirements for intensively kept finishing pigs can be calculated on an ‘animal needs’ basis, rather than a production performance basis.
Acknowledgements The authors thank Johan v.d. Burgwal and Joop te Brake for carrying out the video observations. We also gratefully acknowledge the help of Anita Hoofs and the pig staff of the Research Station ‘Sterksel’.
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References Baldwin, B.A., Ingram, D.L., 1967. Behavioural thermoregulation in pigs. Physiol. Behav. 2, 15–21. Blackshaw, J.K., 1981. Environmental effect of lying behaviour and use of trough space in weaned pigs. Appl. Anim. Ethol. 7, 281–286. Bruce, J.M., Clarke, J.J., 1979. Models of heat production and critical temperature for growing pigs. Anim. Prod. 28, 353–369. Close, W., Heavens, R.P., Brown, D., 1981. The effects of ambient temperature and air movement on heat loss from the pig. Anim. Prod. 32, 75–84. Edwards, S.A., Armsby, A.W., Spechter, H.H., 1988. Effects of floor area allowances on performance of growing pigs kept on fully slatted floors. Anim. Prod. 46, 453–459. Genstat 5 Committee, 1993. Genstat 5, Release 3. Reference Manual (R.W. Payne, Chairman; P.W. Lane, Secretary). Clarendon Press, Oxford. Hyun, Y., Ellis, M., Johnson, R.W., 1998. Effects of feeder type, space allowance, and mixing on the growth perfromance and feed intake pattern of growing pigs. J. Anim. Sci. 76, 2771–2778. Kuipers, M., Whatson, T.S., 1979. Sleep in piglets: an observational study. Appl. Anim. Ethol. 5, 145–151. McGlone, J.J., Newby, B.E., 1994. Space requirements for finishing pigs in confinement: behavior and performance while group size and space vary. Appl. Anim. Behav. Sci. 39, 331–338. McCullagh, P., Nelder, J.A., 1990. Generalized Linear Models, 2nd. ed. Chapman & Hall. London. NCR-89 Committee on Confinement Management of Swine, 1993. Space requirements of barrows and gilts penned together from 54 to 113 kilograms. J. Anim. Sci. 71, 1088–1091. Petherick, J.C., 1983. A biological basis for the design of space in livestock housing. In: Baxter, S.H., Baxter, M.R., MacCormack, J.A.C. (Eds.), Farm Animal Housing and Welfare. pp. 103–120. Petherick, J.C., Baxter, S.H., 1981. Modelling the static spatial requirements of livestock. In: MacCormack, J.A.D. (Ed.), Proceedings of the CIGR Section II Seminar on Modelling, Design and Evaluation of Agricultural Buildings. Aberdeen, August 1981, Scottish Farm Buildings Investigation Unit, Bucksburn, Aberdeen, UK, pp. 75–82. Ruckebush, Y., 1972. The relevance of drowsiness in the circadian cycle of farm animals. Anim. Behav. 20, 637–643. Spoolder, H.A.M., Edwards, S.A., Corning, S., 2000. Legislative methods for specifying stocking density and consequences for the welfare of finishing pigs. Livestock Prod. Sci. 64, 167–173. Thissen, J.T.N.M., Goedhart, P.W., 1992. Procedure RPAIR. In: Goedhart, P.W., Thissen, J.T.N.M. (Eds.), Genstat Procedure Library Manual and GLW-DLO Procedure Library Manual Release 3 [1]. Report LWA94-16, DLO-Groep Landbouwwiskunde, Wageningen, The Netherlands. Turner, S.P., Ewen, M., Rooke, J.A., Edwards, S.A., 2000. The effects of space allowance on performance, aggression and immune competence of growing pigs housed on straw deep-litter at different group sizes. Livestock Prod. Sci. 66, 47–55.
Lying characteristics as determinants for space requirements in pigs E. Dinand Ekkela,*, Hans A.M. Spoolderb, Ina Hulseggec, Hans Hopsterc a
Ethology Group, Department of Animal Sciences, Wageningen Agricultural University, P.O. Box 338, NL-6700 AH, Wageningen, The Netherlands b Research Institute for Animal Husbandry, P.O. Box 2176, NL-8203 AD, Lelystad, The Netherlands c Institute for Animal Sciences and Health (ID-Lelystad), Division of Animal Sciences, P.O. Box 65, NL-8200 AB, Lelystad, The Netherlands Accepted 8 July 2002
Abstract An experiment was carried out to study the lying postures, space occupation and percentage of space-sharing (as a consequence of huddling behaviour) in pigs. Information about these lying characteristics is essential for the calculation of lying space requirements, e.g. for pen design legislative purposes. The study included two replicates of eight groups of eight pigs each. Pigs were housed at thermoneutral conditions in 8 m2 partly (40%) slatted pens from 25 to 100 kg live weight. Lights were on from 06:30 until 18:30 h. Feed and water were available ad libitum. Behaviour of the pigs was recorded for 48 h on video tape at approximately 30, 50, 80 and 100 kg live weight. General activity, lying posture (sternum, half recumbent, fully lateral recumbent) and space-sharing percentage were scored by 20 min scan sampling. The results confirmed that pigs of all weight categories lie down for a great part of the day: the average numbers of pigs that were lying were 83.8, 85, 86.3 and 87.5% at 30, 50, 80 and 100 kg live weight. Especially at night (18:30–06:30 h), the fully recumbent lying posture is predominant. The percentages of lying pigs that lied in the fully recumbent position in that period were 65.7, 66.4, 67.7 and 69.1 for 30, 50, 80 and 100 kg pigs, respectively. In our study, we observed that pigs spent little time in contact with conspecifics: space-sharing percentages were 20– 40%. There was a significant period effect for the space-sharing data; pigs showed more ‘social lying behaviour’, i.e. tended to huddle more during the night in comparison to the day since the spacesharing percentages were higher during the night (P < 0:01). Based on the observations carried out in this study, average space occupation figures were calculated for lying pigs: 0.30, 0.46, 0.64, 0.76 m2 for 30, 50, 80 and 100 kg pigs. The present study confirms the suggestion of Petherick (1983) that, as a starting point for discussions about space requirements for pigs, the floor area occupied by lying
* Corresponding author. Tel.: þ31-317-483968; fax: þ31-317-385006. E-mail address: [email protected] (E.D. Ekkel).
0168-1591/03/$ – see front matter # 2003 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 1 5 9 1 ( 0 2 ) 0 0 1 5 4 - 5
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pigs at thermoneutral conditions should on average be based on the estimated floor area for half recumbent pigs, i.e. area ¼ 0:033 bodyweight0.66. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Pigs; Lying behaviour; Space occupation; Space requirements
1. Introduction Questions concerning the minimum space requirements for intensively kept pigs in relation to their welfare have been an issue of interest for many decades. Recently, the discussion about these questions has intensified, as there is a growing concern among western societies about the welfare of intensively kept pigs. Consumers increasingly demand food to be produced in more welfare friendly ways. Furthermore, retailers and fast food restaurant chains also tend to include welfare standards for finishing pigs in food production assurance schemes for their suppliers. Politicians, both nationally and at the EU level, have responded by re-evaluation of existing legislation, in order to meet the growing concern about the welfare of pigs. Recently, the EU agricultural ministers formally agreed a revision of the Council Directive 91/630/EEC. In this revision (Council Directive 2001/ 88/EC), space allowance was not altered, but it was recommended that not later than 2008, the Commission will submit a report to the Council that covers (among other issues) the effects of housing pigs at different stocking densities on welfare and health. Considering the space requirements of animals, it is a fact that all animals occupy a certain amount of space at any one moment in time due to their physical size and geometry. Additional space is needed to allow them to carry out normal behaviours, such as feeding, drinking, elimination and resting in designated areas of their pen, and for interacting with conspecifics. It is important to stress that the pig by nature is a ‘clean’ animal; the pig has specific lying areas and dunging areas. This places special demands on the pigs spatial requirements. If the available space is not in accordance with the animals requirements, the activity is suppressed, displaced or performed in a pen area unsuitable for the behaviour. This can cause aberrant behaviour and physiological changes in the animal which, in turn, may be regarded as symptoms of a reduction of its welfare. The scientific literature concerning space requirements for finishing pigs is abundant. Many papers studying the effects of space allowance on production performance, health and behaviour (Edwards et al., 1988; NCR-89 Committee on Confinement Management of Swine, 1993; McGlone and Newby, 1994; Hyun et al., 1998; Spoolder et al., 2000; Turner et al., 2000) have been published. These papers contribute to questions related to space allowances on the welfare of pigs, but do not attempt to develop a theoretical framework for space requirements that regards the animal and its physical dimensions as the primary determinant of spatial definition. In contrast, Petherick and Baxter have published several papers (Petherick and Baxter, 1981; Petherick, 1983) in which they attempt to give such a framework. However, their work was only a start and they state that more information is needed. For instance, they present equations for pigs adopting a fully recumbent lying position and a sternum position, but they acknowledge that information about the (1) temporal distribution of behaviours and (2) the lying postures of the pigs is of significant importance for a final assessment of the spatial
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requirements of these pigs. In addition, their equations do not take (3) the ‘group dynamics’ of a group of pigs into account. The present study was set up to increase our knowledge on these three issues, as there is little information available about them. Its results aim to further develop the theoretical framework on spatial requirements of pigs.
2. Material and methods 2.1. Animals and housing Sixteen groups of eight pigs each (50% male, ‘Krusta’ (a commerical breed) (GYNL)) were used in two batches. The experiment was carried out at the Research Station ‘Sterksel’ of the Institute of Animal Husbandry in Lelystad, The Netherlands. Pigs were housed in one room with eight pens measuring 8 m2 each. Half the pens had a 60% solid sloped floor and 40% metal slats. The other pens had a 60% solid arched floor and 40% slatted floor (both in the front as well as in the in the back of the pen). These types of flooring are the most common ones used in The Netherlands. Pigs were blocked by weight and assigned to one of each floor types. The environmental temperature in the experimental room was set according to the requirements for thermoneutral conditions (Bruce and Clarke, 1979); 24 8C at the start of the experiment, 20 8C at the end of the experiment. Pigs entered the experimental room at an average age of 69:2 2:3 days and an average weight of 25:4 2:1 kg. Water and a standard commercial concentrate diet for finishing pigs were available ad libitum. The concentrate was offered in one feed hopper per pen. Light was on from 6:30 to 18:30 h. During the night, a dim light was kept on, in order to be able to observe the pigs on video. 2.2. Behavioural observations The behaviour of the pigs was videotaped for 48 h at an average live weight of approximately 30, 50, 80 and 100 kg. The following ethogram was used for the behavioural observations: Ethogram Posture
Definition
Lying
Lying on side or belly, body not supported by any of the legs, position not changed Standing, walking or running, body supported by three or more legs, position change possible Body supported by one or two front legs
Standing Sitting Lying behaviour Sternum Half recumbent Fully recumbent Space-sharing (%)
The animal is lying on the belly with at least two legs folded under the body The animal is lying half on the side and half on the belly The animal is lying on the side with all four legs stretched out See comments in the text
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Fig. 1. Diagram showing how space-sharing was assessed. The rectangle was based on the model of Petherick (1983) for fully recumbent lying pigs.
Space-sharing was estimated as follows: for each fully recumbent lying pig, a theoretical space-rectangle (see work of Petherick and Baxter, 1981) was drawn around it. The percentage of that theoretical rectangle that was not occupied by the pig, but by one of its pen mates was recorded as the percentage of space-sharing. In other words, it was the space of the Petherick and Baxter (1981) rectangle that was shared with a pen mate (See Fig. 1). Social lying behaviour was also observed when pigs were lying in a half recumbent position, but given the fact that this position was adopted to a much lesser extend as the fully recumbent position and that the area that can be shared with other pigs within that smaller ‘envelope’ is much smaller than that of fully recumbent lying pigs, we think space-sharing among half recumbent lying pigs would have minor consequences for the figures presented here and therefore, space-sharing for half recumbent pigs was not taken into account. 2.3. Calculations of the total area occupatioon Based on the observational data, we calculated the floor area that was occupied by pigs. To be able to do this, two assumptions had to be made. Firstly, it was assumed that the space occupied by half recumbent pigs was the average of pigs lying in a fully recumbent position and a sternum position. Petherick and Baxter (1981) developed space requirement equations on these two postures. They estimated the floor area required with the following equations: Fully recumbency area ðm2 Þ ¼ 0:047 W 0:66
(1)
Sternum area ðm2 Þ ¼ 0:019 W 0:66
(2)
where W ¼ live weight.
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We think it is reasonable to assume that the area occupied by pigs lying in half recumbence is between the area of pigs lying in a fully recumbent position (1) and pigs lying in a sternum position (2). Therefore, we adopted the following equation: Half recumbency area ðm2 Þ ¼ 0:033 W 0:66
(3)
For the four weight categories studied in the present experiment (30, 50, 80 and 100 kg), space occupation factors were adopted based on these equations. These space occupation factors are presented in Table 1. Secondly, we assumed that the area around a fully recumbent lying pig in the Petherick rectangle (i.e. the black are of Fig. 1.) that can be shared with pen mates is 40%. This assumption was based on estimations from videotapes. Hence, the percentage of spacesharing is a percentage of this ‘empty’ part of the rectangle. Pigs lying on top of each other were never observed. The floor area requirement per lying pig in the present experiment can thus be calculated as follows: ((number of pigs lying in fully recumbent position sof (space occupation factor, see Table 1) 0.4 soff percentage of space-sharing) þ (number of pigs lying in half recumbent position sofh) þ (number of pigs lying in sternum position sofs))/ number of lying pigs: Lying pigs : al ¼
ðnf sof f 0:4 sof f %s nf Þ þ ðnh sof h Þ þ ðns sof s Þ nl (4)
where al is the area for lying pigs, sof the space occupation factor, see Table 1., %s the percentage space-sharing, nf the number of pigs in full recumbency, nh the number of pigs in half recumbency, ns the number of pigs in the sternum position, nl the number of pigs lying. 2.4. Statistical analyses First, for each pen, average frequencies (%) for postural behaviour and lying behaviour and the average space-sharing (%) per scan sample moment (0.00; 0.20; 0.40, etc.) based on data of both sampling days were calculated. Then, for each variable, averages per weight category, floor type, pen and batch were determined. Since major behavioural differences Table 1 Space occupation factors (m2) per weight category, based on Eqs. (1)–(3) Weight category (kg)
30 50 80 100
Lying posture Fully recumbency
Half recumbency
Sternum
0.443 0.621 0.847 0.982
0.311 0.436 0.595 0.689
0.179 0.251 0.343 0.397
These factors are the areas that result from the original equations of Petherick and Baxter (1981).
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were seen between the day-time observations (6:30–18:30 h) and night-time observations (18:30–6:30 h), ‘period’ (day, night) was additionally included in the model. Data were then analysed separately for each weight category in Genstat (1993) 5.3. using the GLMMprocedure (McCullagh and Nelder, 1990). A binomial distribution and logit transformation were incorporated in the GLM and dispersion was estimated from Pearson w. The method of analysis is based on maximum quasi likelihood. After full (including all interaction terms) analyses, irrelevant (i.e. non-significant interaction terms) were excluded from the model. Finally, most data were analysed with the model: Yijk ¼ m þ floor typei þ batchj þ periodk þ batchj periodk þ eijk Exceptions are given in the result paragraph. Pairwise comparisons were calculated with the RPAIR procedure (Thissen and Goedhart, 1992).
3. Results 3.1. Lying behaviour Fig. 2 shows the data of the general postural behaviour scores for each of the four weight categories. For clarity purposes and because of the fact that no differences in lying behaviour between floor types were found (P ¼ 0:082), data were pooled for both floor types. The results show that pigs of all weight categories were lying for most of the day and that the number of pigs that were lying increased by weight (general average 83.8, 85, 86.3 and 87.5 for 30, 50, 80 and 100 kg pigs, respectively). As a consequence, the activity-peaks at 08:00 and 16:00 h became more pronounced. A significant interaction between batch and period was found for 30 and 80 kg pigs (P ¼ 0:001 and P ¼ 0:029, respectively). For 50 and 100 kg pigs, significant more pigs were lying during the night period in comparison with the day period (50 kg: 76.8 versus 92.3%, P < 0:001 and 100 kg: 83.1 versus 93%, P < 0:001 for night and day, respectively). 3.2. Lying posture A significant floor batch effect was found (P ¼ 0:036) for ‘percentage of pigs lying in half recumbent position’ at 30 kg. For all other variables and weight categories, no floor effect was found. Therefore, again, pooled data are presented in Fig. 3 for all lying posture data. The percentages of lying pigs that lied in the fully recumbent position at night (18:30– 06:30 h) were 65.7, 66.4, 67.7 and 69.1 for 30, 50, 80 and 100 kg pigs, respectively. In general, but especially at night, this position is predominant. Significant interactions between batch and period were found for all lying posture variables for 80 kg pigs (P < 0:05) and for the variables ‘percentage of pigs lying in sternum position’ and ‘percentage of pigs lying in fully recumbent position’ for 100 kg pigs (P < 0:01). Minor differences between batch 1 and batch 2 were found. The percentage of pigs of 30 kg that was lying in a sternum position was higher (20.5 versus 15.1, P < 0:001) and the percentage of pigs of 30 kg that was lying in a half recumbent position was lower
E.D. Ekkel et al. / Applied Animal Behaviour Science 80 (2003) 19–30 Fig. 2. General postures (%) from 0:00 to 4:00 h of pigs of the four weight categories: lying (---), standing (—) and sitting (). Data of pigs for each behavioural category. Data were calculated based on behavioural observations of all pens and both floor types in the present experiment. 25
26 E.D. Ekkel et al. / Applied Animal Behaviour Science 80 (2003) 19–30 Fig. 3. Lying postures (%) from 0:00 to 24:00 h of pigs of the four weight categories: sternum (---), half recumbent (—) and fully recumbent (). Data were calculated based on behavioural observations of all pens and both floor types in the present experiment.
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Table 2 Space-sharing (% S:D.) for each of the four weight categories per day/night period and per batch. Percentages indicate that part of the ‘empty space’ of a virtual rectangle-area around a fully recumbent lying pig (based on equations of Petherick and Baxter, 1981), that is occupied by another pig Weight category
Day period Batch 1
30 50 80 100
26.12 20.28 22.22 26.11
Night period Batch 2
9.04 8.34 6.05 6.69
25.12 24.41 28.57 28.39
Batch 3 9.09 5.39 8.70 8.85
42.81 31.45 24.74 31.19
Period P-value
Batch P-value
<0.001 <0.001 <0.01 <0.001
<0.001 <0.05
Batch 4 14.56 8.62 9.14 6.72
38.57 34.47 37.14 37.31
8.17 8.42 8.04 7.61
(18.3 versus 23.8, P < 0:001) in the first batch in comparison with the second batch. The same applies for 50 kg pigs: 22.8 versus 11%, P < 0:001 and 16.9 versus 25.6% P < 0:001. Also, differences between the day and night period were small or subjected to interaction significance (see earlier). The percentage of pigs of 30 kg that was lying in a sternum position was higher (23.9 versus 11.8, P < 0:001) and the percentage of 30 kg pigs that was lying in a fully recumbent position was lower (30 versus 60.3, P < 0:001) during the day period in comparison with the night period. The percentage of pigs of 50 kg that was lying in a sternum position and a half recumbent position was higher (23 versus 11, P < 0:001; 22.5 versus 20, P ¼ 0:004) and the percentage of 50 kg pigs was lying in a fully recumbent position was lower (31.3 versus 61.3, P < 0:001) during the day period in comparison with the night period. Finally, the percentage of 100 kg pigs that was lying in a half recumbent
Fig. 4. Space occupation areas (m2) for lying pigs from 0:00 to 24:00 h for each weight category 30, 50, 80 and 100 kg.
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position was higher (26.1 versus 20.9, P < 0:001) during the day period in comparison with the night period. Fig. 3 shows the data of the lying postures. 3.3. Space-sharing and calculated occupation areas per pig Table 2 shows the space-sharing percentages for each of the four weight categories. These percentages are 20–40%. There was a strong significant period effect; pigs showed more ‘social lying behaviour’, i.e. tended to huddle more during the night in comparison to the day as the space-sharing percentages were higher during the night. In addition, there was a significant effect of the batch for 80 and 100 kg pigs; in batch 2, space-sharing percentages were higher than those found in batch 1. Fig. 4 shows the results of the calculations of the areas occupied per lying pig throughout the day for each of the four weight categories. The average areas occupied by lying pigs were 0.30, 0.46, 0.64 and 0.76 m2 for 30, 50, 80 and 100 kg pigs, respectively.
4. Discussion The present study aims to built on the theoretical framework concerning the space occupation of pigs published by Petherick and Baxter (1981) and Petherick (1983). In many studies, space allowances or requirements have been assessed in experiments where the performance of the pig is the predominant measure (Edwards et al., 1988; NCR-89 Committee on Confinement Management of Swine, 1993; McGlone and Newby, 1994). This suggests that there is no need to house pigs at an area larger than the area below which production decreases. From a welfare perspective however, this is questionable. Our study regards the physical dimensions of the animal and its behaviour as the primary determinants of spatial requirements; we observed the lying behaviour of pigs and their lying postures. Based on these observations, we calculated the area occupied by the pigs. In addition, we took into account the consequences of social lying behaviour (such as huddling) for the space occupation of a group of pigs. The behavioural observations show that the pigs lie down for many hours a day. It has been reported that the amount of time young pigs are resting is between 40 and 60% (Kuipers and Whatson, 1979; Blackshaw, 1981). At the start of our study, pigs had an average age of 10 weeks, which is much older than the piglets in those studies. Our results correspond more to the study of Ruckebush (1972), who showed that pigs spend about 80% of the time resting; during the night, more than 90% of the pigs was resting in our study. Thus, the amount of space allocated for a group of finishing pigs from 25 kg onwards should take into account that these pigs prefer to lie down together for most of the day. It should, in fact, facilitate that all pigs can rest at any one time. The present study also shows that, when pigs are lying, the fully recumbent position is predominant. On average, more than 60% of the lying animals appears to lie down in a fully recumbent position. This percentage was even higher during the night period and increases with age. In general, the climatic environment affects the posture adopted by an animal. At high temperatures, pigs will lie down in a fully recumbent position with their limbs extended, in order to be able to transfer as much heat as possible to the
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environment (Close et al., 1981). At low environmental temperatures, pigs will adopt a body posture which minimises their contact with the floor (Baldwin and Ingram, 1967; Close et al., 1981). However, in our study pigs were housed at thermoneutral conditions (Bruce and Clarke, 1979). As a consequence, there was no need for the pigs to adjust their posture to the environmental temperature. Petherick (1983) presumed that the floor area occupied by resting pigs at these temperatures can on average be based on a half recumbent lying posture of the pigs. Adopting this theory, the average lying area occupied by pigs at thermoneutral conditions can be calculated with the equation A (m2 ) ¼ 0:033 W 0:66 . The floor occupied by lying pigs in our study, in which we used the observed lying postures adopted by the pigs at different weight categories, equals an average equation of Area ¼ 0:033 W 0:66 . Thus, although pigs were lying predominantly in a fully recumbent position, the space they occupy is on average that of half recumbent lying pigs. This can be explained by the fact that we took space-sharing of the pigs lying in a fully recumbent position, as a consequence of social lying behaviour, into account. Our data support the presumption of Petherick (1983): for thermoneutrally kept pigs, taking the equation for half recumbent lying pigs (A (m2) ¼ 0.033W0.66) as a starting point for lying space occupation calculations for groups of pigs is reasonable. Many recommended space allowances, such as the one recommended for commercial practices by Edwards et al., 1988 (A (m2) ¼ 0.030W 0.66) and the minimum space requirements laid down in EU Council Directive 2001/88/EC are far below this equation as these recommendations/requirements concern the total area. As an example, based on our study, the space occupied by lying pigs of 100 kg corresponds to 0.76 m2, whereas the recommendation of Edwards et al. (1988) is 0.65 m2 total space, which is also the minimum requirement for 85–110 kg pigs of EU Council Directive 2001/88/EC. Our results suggest that the amount of space allocated for a group of finishing pigs from 25 kg onwards should take into account that these pigs prefer to lie together for most of the day. In addition, we showed that when pigs are given a reasonable amount of space, they predominantly prefer to lie down and rest in a fully recumbent lying posture, with their limbs extended and only some contact with conspecifics. The present study does of course not answer the question whether pigs actually need an amount space that goes beyond recommendations generally based on production performance studies. It showed how much space pigs will use when given the opportunity, not that less space is detrimental to their welfare. Additional validation experiments, studying the welfare relevancy of those behavioural expressions should be carried out in order to answer this question. When successful, minimum space requirements for intensively kept finishing pigs can be calculated on an ‘animal needs’ basis, rather than a production performance basis.
Acknowledgements The authors thank Johan v.d. Burgwal and Joop te Brake for carrying out the video observations. We also gratefully acknowledge the help of Anita Hoofs and the pig staff of the Research Station ‘Sterksel’.
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References Baldwin, B.A., Ingram, D.L., 1967. Behavioural thermoregulation in pigs. Physiol. Behav. 2, 15–21. Blackshaw, J.K., 1981. Environmental effect of lying behaviour and use of trough space in weaned pigs. Appl. Anim. Ethol. 7, 281–286. Bruce, J.M., Clarke, J.J., 1979. Models of heat production and critical temperature for growing pigs. Anim. Prod. 28, 353–369. Close, W., Heavens, R.P., Brown, D., 1981. The effects of ambient temperature and air movement on heat loss from the pig. Anim. Prod. 32, 75–84. Edwards, S.A., Armsby, A.W., Spechter, H.H., 1988. Effects of floor area allowances on performance of growing pigs kept on fully slatted floors. Anim. Prod. 46, 453–459. Genstat 5 Committee, 1993. Genstat 5, Release 3. Reference Manual (R.W. Payne, Chairman; P.W. Lane, Secretary). Clarendon Press, Oxford. Hyun, Y., Ellis, M., Johnson, R.W., 1998. Effects of feeder type, space allowance, and mixing on the growth perfromance and feed intake pattern of growing pigs. J. Anim. Sci. 76, 2771–2778. Kuipers, M., Whatson, T.S., 1979. Sleep in piglets: an observational study. Appl. Anim. Ethol. 5, 145–151. McGlone, J.J., Newby, B.E., 1994. Space requirements for finishing pigs in confinement: behavior and performance while group size and space vary. Appl. Anim. Behav. Sci. 39, 331–338. McCullagh, P., Nelder, J.A., 1990. Generalized Linear Models, 2nd. ed. Chapman & Hall. London. NCR-89 Committee on Confinement Management of Swine, 1993. Space requirements of barrows and gilts penned together from 54 to 113 kilograms. J. Anim. Sci. 71, 1088–1091. Petherick, J.C., 1983. A biological basis for the design of space in livestock housing. In: Baxter, S.H., Baxter, M.R., MacCormack, J.A.C. (Eds.), Farm Animal Housing and Welfare. pp. 103–120. Petherick, J.C., Baxter, S.H., 1981. Modelling the static spatial requirements of livestock. In: MacCormack, J.A.D. (Ed.), Proceedings of the CIGR Section II Seminar on Modelling, Design and Evaluation of Agricultural Buildings. Aberdeen, August 1981, Scottish Farm Buildings Investigation Unit, Bucksburn, Aberdeen, UK, pp. 75–82. Ruckebush, Y., 1972. The relevance of drowsiness in the circadian cycle of farm animals. Anim. Behav. 20, 637–643. Spoolder, H.A.M., Edwards, S.A., Corning, S., 2000. Legislative methods for specifying stocking density and consequences for the welfare of finishing pigs. Livestock Prod. Sci. 64, 167–173. Thissen, J.T.N.M., Goedhart, P.W., 1992. Procedure RPAIR. In: Goedhart, P.W., Thissen, J.T.N.M. (Eds.), Genstat Procedure Library Manual and GLW-DLO Procedure Library Manual Release 3 [1]. Report LWA94-16, DLO-Groep Landbouwwiskunde, Wageningen, The Netherlands. Turner, S.P., Ewen, M., Rooke, J.A., Edwards, S.A., 2000. The effects of space allowance on performance, aggression and immune competence of growing pigs housed on straw deep-litter at different group sizes. Livestock Prod. Sci. 66, 47–55.