Effect of group composition and pen size on behavior, productivity and immune response of growing pigs

APPLIED ANIMAL BEHAVIOUR SCIENCE

ELS EV I ER

Applied Animal Behaviour Science 40 ( 1994) 13-30

Effect of group composition and pen size on behavior, productivity and immune response of growing pigs A.S. Moore*, H.W. Gonyou 1, J. M. Stookey 2, D.G. McLaren 3 Department ofAnimal Sciences, Universityof lllinois, Urbana, IL 61801, USA (Accepted 6 December 1993)

Abstract

Levels of aggression, injuries, activity, performance and immune response were determined in 288 growing pigs in a 2 × 2 factorial experiment; the factors being group composition and pen size. Pigs were classified as small (SM) when allotted and then reclassified as medium (MED), large (LG) and extra-large (XL) at 3 week intervals. Static groups were initiated by 12 SM pigs and they remained together for 12 weeks. Dynamic groups consisted of three pigs of each size class. Pigs were introduced into dynamic groups as SM pigs and remained there for 12 weeks, progressing up through the size classes. At 3 week intervals the three XL pigs in dynamic groups were removed and replaced with three SM pigs. Pen sizes were 9.5 m 2 and 7.6 m 2. Pigs were weighed weekly and gains determined. Aggression during the 4 h period after regrouping was determined by 10 min of continuous observations at 20 min intervals. Ear and shoulder injuries were evaluated at 6 h, 24 h, 48 h and 144 h post-regrouping and each week thereafter. Intradermal response to phytohemagglutinin (PHA) as an indicator of in vivo cellular immunity was assessed in dynamic, static and control pigs (remaining in nursery pens and not regrouped), as was plasma cortisol concentration and neutrophil-to-lymphocyte ratio ( N / L ) . PHA was injected 1 h prior to regrouping and the response was measured at 8 h and 24 h post-regrouping. Weight gains and activity budgets over the entire trial did not differ between treatments (P> 0.10 ). However, SM and MED pigs in static groups and XL pigs in dynamic groups gained more than their contemporaries in the other grouping treatment ( P < 0.05). A greater proportion (P< 0.05) of pigs in dynamic groups (13.5%) were removed from test owing to poor health than in static groups *Correspondingauthor at Department of Agriculture, 5020 Illinois State University, Normal, IL 617905020, USA. ~Present address: Prairie Swine Centre, Saskatoon, Saskatchewan, Canada. 2present address: University of Saskatchewan, Saskatoon, Saskatchewan, Canada. 3Present address: Pig Improvement Company Inc., Franklin, KY 42135-0348, USA. 0168-1591/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDIO168-1591(93)OO476-H

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A.S. Moore et aL / Applied Animal Behaviour Science 40 (1994) 13-30

(6.3%). The SM pigs in dynamic groups spent less time fighting (72.3 s per pig) during the initial 2 h after regrouping than SM pigs in static groups ( 196.5 s per pig) (P < 0.05 ). In conclusion, levels of aggression following regrouping can be decreased by the use of dynamic grouping, however, this practice reduces the overall well-being of the pigs and should be avoided. Key words: Pigs; Aggression; Social groupings

1. Introduction

Movement and regrouping of pigs are common during the growing and finishing stages of production. Pigs are most often maintained in static (unaltered) groups composed of uniform individuals with regard to size. Research by Gonyou et al. ( 1983, 1986) and Rushen (1987) suggests that forming groups in this manner may exacerbate the levels of aggression as unfamiliar pigs encounter strangers of equal size and strength. The aggressive interactions that occur when unfamiliar pigs are regrouped often result in wounds to the combatants (McGlone, 1985; Gonyou et al., 1988 ). Regrouping has also been reported to depress performance (Teague and Grifo, 1961; Bryant and Ewbank, 1972 ) and to elevate plasma cortisol concentrations (Barnett et al., 1984; Blecha et al., 1985), which can affect immune function adversely (Westly and Kelley, 1984). Dynamic grouping is a method of group formation in which some animals are removed from the group while others are added at regular intervals. The composition of dynamic groups continuously changes, as opposed to static groups in which group membership remains stable over time. Dynamic grouping may be a useful tool for decreasing aggression if weight variation among members can be achieved through the manipulation of group composition (Rushen, 1987). Dynamic grouping could be used to achieve a large variation in weight among growing pigs by continuously replacing the portion of pigs in a group reaching market weight with pigs just coming out of the nursery. Another possible benefit is that dynamic groups contain pigs covering a wide range of weights, with no more than one-fourth of these pigs reaching market weight at the same time, thus the space requirement is lower for these groups. Nonetheless, dynamic groups are rarely formed in commercial situations to avoid frequent regrouping and possible social problems related to competition among pigs of widely differing weights. The present study was conducted in order to compare effects of two types of group composition, static groups of pigs similar in weight versus dynamic groups of pigs with large variations in weight, on the behavior, injuries, productivity and immunophysiology of growing pigs. In addition, the effect of reducing pen size, based on the requirements of dynamic groups, was assessed to determine how this would influence the pigs within static and dynamic groups.

A.S. Moore et al. / Applied AnimaI Behaviour Science 40 (1994.) 13-30

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2. Animals, materials and methods This trial involved 288 crossbred feeder pigs, approximately 10 weeks of age, assigned to a 2 X 2 factorially designed experiment, the factors being group composition and pen size. Pigs were placed on test during one of eight starting dates (blocked over time) during a 21 week period. Thirty-six pigs were removed from their nursery pens and randomly allotted within each block to one of four treatments: ( 1 ) a static group of 12 pigs in a standard size pen (SS); (2) a static group of 12 pigs in a reduced size pen (SR); (3) three pigs added to each of two previously established dynamic groups of nine pigs in standard sized pens, totaling 12 pigs per pen (DS); (4) three pigs added to each of two previously established dynamic groups of nine pigs in reduced sized pens, totaling 12 pigs per pen (DR). In addition, four pigs in each block served as controls for immunophysiological studies. Control pigs remained alone or with one or two familiar pigs in their nursery pens and were not moved or regrouped.

2.1. Group composition and pen size Pigs were classified as small (SM) when allotted and then reclassified as medium (MED), large (LG) and extra-large ( XL ) at 3 week intervals. Static groups were initiated by 12 SM pigs which remained together for 12 weeks. Although SM, MED, LG and XL are referred to as size classes, classification was based entirely on time after allocation to the experiment. However, in no case did a pig of a smaller size class exceed the weight of a pig in a larger size class within the same pen. Composition of static groups remained unaltered except when it was necessary to remove an animal owing to poor health. Dynamic groups consisted of three pigs of each size class. Pigs were introduced into dynamic groups as SM pigs and remained there for 12 weeks, progressing up through each of the four size classes. At 3 week intervals the three XL pigs in dynamic groups were removed and replaced with three SM pigs. Mean weights at regrouping for SM, MED, LG and XL pigs were 29.2 kg, 41.8 kg, 56.1 kg and 73.7 kg, respectively. The two pen sizes utilized in this study were a standard size of 9.5 m 2 and a reduced pen size of 7.6 m 2. Pens were of similar shape and within the same room of the building. The total area of standard sized pens was based upon the current recommendation of 0.79 m 2 per animal for finishing pigs, 70 kg to market weight, maintained on partially slotted floors (Fritschen and Muehling, 1986). This allowance was utilized in order to accommodate the pigs' maximum floor space requirement until reaching market weight. Reduced pen size was determined according to the biological relationship between the live weight of a pig and its surface area (Baxter, 1985 ). Space requirements were calculated using the equation A = k M °'67, where A is the area (m 2), M is kg live weight and k is a constant. The value for k (0.039) was derived from the values used for the standard size pen using 90 kg as the estimated final weight for the pigs. The space requirement of dynamic groups was calculated by substituting the predicted maximum live weights for each of the three SM (45 kg), MED (60 kg), LG (75 kg) and XL (90

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A.S. Moore et al. / Applied Animal Behaviour Science 40 (1994) 13-30

kg) pigs in a dynamic group into the equation A = 0 . 0 3 9 M °67. Total pen size was calculated as the sum of the areas (m 2) for each of the 12 pigs in the group. The end result was a reduced pen size of 7.6 m 2, or 0.63 m 2 per pig, which represents a 20% decrease in total area as compared with the standard pen size.

2.2. Routine management of experimental animals Prior to the start of each block, all pigs were housed in flat-deck nursery pens, holding four to six pigs. Pigs were allotted to experimental pens in such a way that previous penmates did not remain together. At 08:00 h, all pigs, excluding the four controls, were removed from their nursery pens, weighed, evaluated for preexisting injuries and placed into their respective experimental pens, where they remained for the 12 week period. It took approximately 1 h for all pigs to be processed. Pigs were marked with colored paint for identification prior to allotment. All experimental pens were contained in a naturally ventilated building with partially slotted concrete floors. Pigs were fed a 16% crude protein cornsoybean meal diet via ad libitum access to a pair of two-hole self-feeders in each pen. Two nipple waterers were located within each pen. A 24 h continuous lighting regimen was maintained to facilitate observations. The experimental room was naturally ventilated and temperatures remained above the lower critical temperature of the smallest pigs.

2.3. Productivity and behavior Pigs were individually weighed on the day of allotment (Day 0) and weekly thereafter for 12 weeks. Weight gains for each week and cumulative gains during the periods when pigs were classified as SM, MED, LG and XL, as well as total gains for the 84 day trial were determined. The morbidty rate for each group was calculated as the number of pigs that had to be removed owing to illness or lameness before Day 84 of the trial. Once removed from the trial, no pigs were returned to the trial nor were any substitute pigs added. Each newly formed group was continuously observed for 4 h after regrouping. Pigs from a common size class within each pen served as the experimental unit for collection of behavioral data. Aggression in each newly formed group during the 4 h period after regrouping was determined by continuous observation for 10 min at 20 rain intervals by three trained observers. For analyses, data were divided into four 1 h segments of time. Aggression was quantified as number of fights and as total seconds of fighting. A fight was considered to begin when openmouthed contact occurred and to conclude when pigs lost contact with each other for at least 5 s. Pushing and brief intervals of non-contact were considered as fighting, provided they occurred between the beginning and end of a fight (Gonyou et al., 1988). The number of animals lying (not sitting) versus standing, as well as nearest

A.S. Moore et al. / Applied Animal Behaviour Science 40 (1994) 13-30

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neighbor associations while lying during the initial 4 h after regrouping, were determined by instantaneous sampling (Lehner, 1979 ) at 10 min intervals. Additional data on time budgets were collected on Day 19 after regrouping. All pigs in experimental pens were observed over a 24 h period starting at 06:00 h. Activity budgets for lying, standing and eating were determined via instantaneous sampling at 10 min intervals. Data were divided into eight 3 h segments of time for analysis.

2.4. Injuries Injuries to each pig were evaluated prior to allotment (Day 0) and on Days 1, 2, 3, 7, 21, 42, 63 and 84 after allotment, by assessing the number of scratches or bite marks on four zones of the body (Gonyou et al., 1988 ). The four areas were: left ear, right ear, left shoulder and right shoulder. Scores were assigned to each area as follows: 0, no wounds; l, one to three wounds; 2, four to six wounds; 3, greater than six wounds. Data were combined into two categories, namely ears and shoulders, for analysis.

2.5. Immunophysiology At approximately 16:00 h on the day before the start of a block (i.e. 16 h prior to regrouping), approximately 5 ml of blood were collected from 20 SM pigs via vena cava puncture. The 20 SM pigs that were bled included four SM pigs scheduled for introduction into each of the four group composition/pen size treatment combinations on the following day, as well as four SM non-regrouped controls. No pigs were handled for longer than 2 min in order to avoid excessive stress. Plasma was harvested and frozen for later analysis. The same procedures were followed as blood was collected from the same 20 SM pigs 24 h later (approximately 8 h after regrouping) and again from 16 SM pigs (excluding the four controls) at approximately 16:00 h on Day 20 following regrouping. Plasma cortisol concentrations were determined in duplicate, using a commercially available radioimmunoassay kit (ICN Biomedicals, Costa Mesa, CA, USA) which had been previously validated for use in pigs (Dantzer et al., 1987 ). Within 60 rain after collection and prior to centrifugation of whole blood, Wright's stain smears were made from each sample. As glucocorticoids levels increase, there is a tendency for numbers of neutrophils (N) to increase while lymphocytes (L) decrease, resulting in an increase in the neutrophil-to-lymphocyte ratio ( N / L ) , which can be used as a measure of stress (Widowski et al., 1989). The N / L was obtained from differential leukocyte counts of not less than 100 total N and L. At 08:00 h on the date a block started, the same 20 pigs that were bled the previous day were tested for their intradermal response to the mitogen phytohemagglutinin (PHA; Sigma Chemical Co., St. Louis, MO, USA) as an indicator of in vivo cellular immunity (Blecha et al., 1983 ). Prior to injection with PHA, the double skin-fold thickness on the inside of the right flank was measured using

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A.S. Moore et al. / Applied Animal Behaviour Science 40 (1994) 13-30

a constant-tension micrometer. Intradermal injections of 1 ml of sterile physiologic saline containing 250 #g PHA were administered to the medial aspect of the right flank. The injection site was circled using a permanent felt-tip marker and double skin-fold thickness measurements at this site were recorded at 6 and 24 h post-injection. 2.6. S t a t i s t i c a l a n a l y s e s

All parametric analyses were conducted using Statistical Analysis System General Linear Model (GLM) procedure (Statistical Analysis Systems Institute Inc. (SAS Institute Inc. ), 1985 ). Linear models assumed for live weight and gain included group (static or dynamic) and sex as classification variables, with initial weight (Day 0) as a covariate. Preliminary analyses revealed block and pen size, as well as all two-factor interactions involving group, to be non-significant ( P > 0.25 ); therefore these terms were not included in the final model. Repeated measures analysis of variance was used to test hypotheses concerning changes in live weight data recorded from the same pigs over time. The equality of variances between groups was tested within each day using Levene's test. Size class means within each pen served as the experimental unit for behavioral analyses. Linear models assumed for the duration and frequency of fights involving SM pigs included the fixed effects of group and hour, and their interaction. The models assumed for time spent lying, standing and eating included effects of group and size class, and their interaction. Preliminary analyses revealed that block, pen size and all two-factor interactions involving these two effects were non-significant ( P > 0.25 ), thus they were not included in the final models. Differences in the frequency of fights initiated by pigs of different size classes were tested using chi-square analysis (Zar, 1984). Goodman and Kruskal's gamma ( 1954) was utilized as a coefficient of contingency for the frequency of initiator/ recipient fight data observed between dyads in dynamic groups. The significance of Goodman and Kruskal's gamma was tested by substituting its numerator for the numerator of Kendall's Tau (Ghent, 1976, 1984). Preference for pigs in each size class to be nearest neighbors to another pig of the same class were tested using chi-square analysis (Zar, 1984). The linear model used for injury scores to the ears and shoulders, assumed to be normally distributed random variables, included block, sex, color of pig (black, white or mixed), group, pen size and the interaction between group and pen size, with injury score on Day 0 as a covariate. Data from Day 0 to Day 7 within each group were analyzed using univariate repeated measures analysis over time. Univariate (split-plot) analyses were conducted and sphericity of orthogonal components tested to determine if the probabilities from the ordinary F-tests were correct. Unadjusted F-tests from the univariate analyses were used for testing within-subject effects. The preliminary models originally assumed for cortisol, N / L and intradermal response to PHA included the fixed effects of block, sex, group, pen size, groupXpen size, with initial values of the dependent variable of interest (i.e.

A.S. Moore et al. / Applied Animal Behaviour Science 40 (1994) 13-30

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- 16 h cortisol, - 16 h N / L or Day 0 PHA) included as a covariate. Final models assumed for cortisol, N / L and PHA included the significant effects from the preliminary model, of block, sex and group along with initial values for the dependent variable of interest as a covariate.

3. Results

3.1. Productivity The morbidity rate was higher (P< 0.05 ) among pigs in dynamic groups as compared with static groups of pigs, 13.5% versus 6.3%, respectively. It should be noted that the entire grower-finisher herd was experiencing health problems, owing to an unknown cause, during approximately 8 weeks of this trial, which influenced the morbidity rate among pigs in both groups. Mean live weight of dynamically grouped pigs was lower (P< 0.02 ) than pigs in static groups from Day 7 to Day 77 of the experiment (Table 1 ). However, no difference was detected (P> 0.10) between the mean live weight for dynamically grouped pigs (90.3 kg per pig) and that for static groups (91.8 kg per pig), when removed from test on Day 84. Results from multivariate repeated measures analysis indicated that there was a quadratic (P< 0.01 ) effect of time on mean live weight. The linear time trend did not differ between groups (P> 0.10). The quadratic effect of time was different for the two groups (P< 0.01 ). This effect can be explained by the lower live weights initially found during the first 6 weeks in dynamic groups after which the slope of the line changed as pigs began to have Table 1 Least squares means" for live weight (kg) by group over the course of the experiment Day

0b 7 14 21 28 35 42 49 56 63 70 77 84

Dynamic group (n = 93 )

Static group (n = 190)

Mean

SE

Mean

SE

29.6 32.3 36.2 40.5 44.2 48.9 53.8 58.8 65.2 71.3 77.1 83.5 90.3

0.91 0.18 0.24 0.29 0.36 0.44 0.52 0.58 0.67 0.71 0.73 0.78 0.82

29.1 33.0 37.6 42.6 47.0 51.8 57.3 62.6 68.4 74.8 80.2 85.8 91.8

0.78 0.12 0.16 0.20 0.24 0.29 0.35 0.39 0.45 0.47 0.49 0.52 0.55

"Model: weight = p + sex + group + fit (Day 0 weight) + e. bDay 0 means are true means.

P-value

0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.02 0.12

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A.S. Moore et al. / Applied Animal Behaviour Science 40 (1994) 13-30 P < .001

20-

[]

Dynamic

[]

Static

P > .!0

P < .003 P < .001

z < r5 u.l < I.U >

o d 0-21

d 21-42

d 42-63

d 63-84

(SM)

(MED)

(LG)

(XL)

PERIOD (Size class of pig)

Fig. 1. Effectof groupcompositionon averageweightgainby period.Verticalbars indicatestandard errors of the means. greater increases in live weight. Pen size had no detectable effect (P> 0.10) on the live weight of pigs in either static or dynamic groups. Within-group live weight variances were equal (P>0.10) from Day 0 to Day 35. Beginning on Day 42, there was greater (P< 0.05 ) weight variation among contemporaries in dynamic groups (s 2= 41.6 kg) than in static groups (s 2= 29.8 kg). The within-group weight variation was greater ( P < 0.01 ) in dynamically grouped pigs (s2= 64.5 kg) than for static groups (s2= 39.7 kg) on Day 84 when pigs were removed from test. Group means for weight gains by period are presented in Fig. 1. When classified as SM pigs (during the first 21 days on test), pigs in static groups gained more (P<0.001) than those in dynamic groups, 13.6 kg per pig versus 11.4 kg per pig, respectively. Likewise, when classified as MED pigs (from Day 21 to Day 42), static pigs out-gained dynamic pigs ( P < 0.003 ) by 1.5 kg per pig during this period. Gains were nearly equal (P> 0.10) between LG pigs in both groups. During the final 3 weeks of the test, XL pigs in dynamic groups gained 19.0 kg per pig, whereas XL pigs in static groups gained only 17.0 kg per pig (P<0.001). Total weight gains over the entire course of the trial did not differ (P>0.10) between static and dynamically grouped pigs, 62.8 kg per pig and 61.2 kg per pig, respectively. 3.2. B e h a v i o r

Dynamic SM pigs spent less time (P< 0.001 ) fighting, 50.2 s per pig, during the first hour after regrouping than did SM pigs in static groups, 134.8 s per pig (Table 2). Time spent fighting was reduced ( P < 0.05 ) by more than one-half in

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Table 2 Least squares means a for total time spent fighting (s per pig) by SM pigs during the initial 4 h following regrouping Time after regrouping (h)

1b 2¢ 3 4

Group composition Dynamic ( n = 2 0 pens)

Static ( n = 10 pens)

50.2 _ 11.50 22.1 +_ 11.50 43.5 + 11.50 16.5+11.50

134.8 + 16.27 61.7 + 16.27 22.0 + 16.27 13.5+16.27

"Least squares means ( _+SE); Model: seconds =/~ + group + hour + (group × hour) + e. ~Freatment means differ ( P < 0.001 ). ~Freatment means differ ( P < 0.05 ).

both groups during the second hour after regrouping, however the amount of time SM pigs spent fighting remained greater (P< 0.05 ) in static groups. Beyond the initial 2 h after regrouping there were no differences (P>0.10) in time spent fighting by SM individuals in either of the two groups. Small pigs in dynamic groups were involved in a greater number of fights per pig ( P < 0.05 ) during each of the 4 h following regrouping. The greatest number of fights ( P < 0.05 ) occurred during the first hour in each group. During this time SM pigs in dynamic groups averaged 20.9 fights per pig whereas those in static groups were involved in 7.5 fights per pig (P< 0.001 ). The frequency of fights decreased after the first hour in much the same manner as time spent fighting. Figure 2 summarizes the distribution of all fights occurring in dynamic groups during the initial 4 h following regrouping, categorized by the size class of the initiator and recipient of each encounter. The percentages of fights initiated by each size class were 5.2, 78.9, 87.8 and 94.8 for SM, MED, LG and XL, respectively. A highly significant (P<0.0001) gamma coefficient of 0.469, which is sensitive to the directional ordering of the table, indicates that as pigs progressed up through the larger size classes they were more likely to be the initiator of a fight and less likely to be the recipient of an attack. Of all fights occurring during this trial, 90% were initiated by the pig from the larger size class and only 2.4% were initiated by the smaller of the two pigs involved (P<0.001). Larger pigs won most encounters and SM pigs in dynamic pens were clearly subordinate to the other three size classes. During the initial 4 h after regrouping, all pigs in dynamic groups exhibited a preference for lying nearest to a member of the same size class. Based upon the probability levels associated with each size class, there appears to be a stronger tendency for this preference among SM and MED pigs (P<0.001) than it was for LG (P<0.05) or XL(P<0.01 ) animals. Time spent lying did not differ (P> 0.10) between pigs in static and dynamic groups, nor were there any differences among the different size classes of pigs within each group. With regard to time spent eating, there were no differences (P> 0.10) between static and dynamic groups. Within dynamic groups there were

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A.S. Moore et al. / Applied Animal Behaviour Science 40 (1994) 13-30 RECIPIENT OF AGGRESSION SM

MED

LG

XL

SM

63

MED

Totals

4

3

3

73

359

16

II

4

390

LG

414

39

24

12

489

XL

483

45

30

12

570

INITIATOR OF FIGHT

Totals

1522

Fig. 2. Distribution of the total number of fights during the initial 4 h after regrouping in dynamic pens. Frequencies are arranged with reference to the size class of the pigs involved and the initiator/ recipient of each fight. Frequencies in the highlighted diagonals refer to fights involving pigs in the same size class. Frequencies below and to the left of the highlighted diagonals represent fights that were initiated by the larger of the two combatants. Values to the right-hand side of the highlighted diagonals indicate the number of fights that were initiated by the smaller of the two pigs. As pigs progressed through the larger sized classes they were more likely to be the initiator of a fight and less likely to be the recipient of agression (P < 0.0001 ).

several periods of the day when LG and XL pigs spent more time eating. Pigs in the XL class spent more time eating ( P < 0 . 0 5 ) from 09:00 to 12:00 h than did the other three classes of pigs. Also from 18:00 to 21:00 h, XL pigs spent more time eating ( P < 0.05 ) than MED and LG pigs.

3.3. Injuries Mean injury scores of the ears and shoulders for each group are presented in Table 3. Mean injury score of the ears was greater (P<0.01) in dynamically grouped pigs on Days 1, 2, 3 and 7 than in static pigs. However, by Day 63 pigs in static groups had more ( P < 0.05 ) injuries on their ears. This difference remained significant ( P < 0.01 ) when pigs were removed from test on Day 84. The amount of wounding to the shoulders was greater ( P < 0.05) among pigs in dynamic groups on Days 2, 3 and 7 after regrouping. However, this trend reversed itself and on Days 63 and 84 pigs in static groups had more wounds ( P < 0.05 ) on their shoulders than pigs in dynamic groups. Results obtained from univariate repeated measures analysis detected a significant main effect of time ( P < 0.001 ) on mean injury score of the ears as well as the shoulders of SM pigs over Day 0 through Day 7 post-regrouping. There were significant linear and quadratic components of time ( P < 0.001 ) which did not differ between groups ( P > 0.10). These components of the slope for time reflect the abrupt linear increase in the levels of injuries owing to the fighting amongst pigs 24-48 h after regrouping, after which time the wounds begin to heal and

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Table 3 Least squares means a ( + SE) for injury scores b on the ears and shoulders by treatment group and days on test Day

0 1 2 3 7 21 42 63 84

Ears

Shoulders

Dynamic ¢

Static d

Dynamic

Static

0.65 + 0.06 2.05f+ 0.10 2.10f+0.10 1.86f_+ 0.10 1.14 f _ 0 . 0 7 0.51 +0.07 0.42 + 0.06 0.25 e + 0.06 0.01f+ 0.06

0.66 + 0.05 1.62 _+0.08 1.72+0.08 1.47 + 0.07 0.79 + 0.05 0.57+0.06 0.47 + 0.06 0.40 + 0.05 0.19+0.05

0.84 +_0.09 1.97 _+0.11 2.22~+0.11 1.90 ~+ 0.11 1.15f+ 0.08 0.43+0.07 0.13 + 0.06 0.16f+ 0.06 0.11~+0.05

0.89 + 0.08 1.80 + 0.09 1.98+0.08 1.69 + 0.08 0.77 + 0.07 0.51 +0.06 0.32 + 0.05 0.33 + 0.05 0.20+0.05

aModel: ears and shoulders=/t+block+ sex+ color of p i g + g r o u p + s i z e + (group×size) +fl,. (injury score on Day 0) + E. blnjury scores: 0, no wounds; 1, one to three wounds; 2, four to six wouds; 3, more than six wounds. Cn=84. dn = 166. ~Treatment means in the same location and day differ ( P < 0.05). fTreatment means in the same location and day differ (P < 0.01 ).

disappear over the next 5 days. Repeated measures analyses over each 7 day period following regrouping for dynamically grouped MED, LG and XL pigs on Day 21, Day 42 and Day 63, respectively, revealed that there were no time effects ( P > 0.10) during any of these periods. This indicates that as SM pigs were added to dynamic groups there were no increases in the levels of injuries in MED, LG or XL pigs. The only significant effect of pen size on injury scores occurred on Day 1 when pigs housed in reduced pens had higher injury scores associated with their ears ( P < 0.05). Injuries to the ears and shoulders at all other times did not differ ( P > 0.10) between pen sizes.

3.4. Immunophysiology Pen size had no effect ( P > 0.25 ) on mean cortisol concentration, therefore data for the two pen sizes within each group composition were pooled and these results are presented in Table 4. Levels of plasma cortisol did not differ ( P > 0.10) between pigs in static and dynamic groups during any of the three sampling periods. Eight hours after regrouping, pigs in both static and dynamic groups had higher cortisol concentrations ( P < 0 . 0 5 ) as compared with non-regrouped controls. There was a moderate negative correlation between plasma cortisol concentration at 8 h post-regrouping and weight gain from 0 to 21 days ( r = - 0 . 3 5 ;

P<0.01). There was no significant effect ( P > 0.25 ) of pen size on mean N/L, therefore

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A.S. Moore et al. / Applied Animal Behaviour Science 40 (1994) 13-30

Table 4 Least squares m e a n s a for p l a s m a cortisol c o n c e n t r a t i o n ( n g m l - ~+ SE) Period

n 16 h before regrouping 8 h post-regrouping 20 days post-regrouping

Control

14 29.68+3.31 27.41¢+5.69 -

Regrouped Dynamic

Static

Both b

36 26.26+2.02 4 3 . 6 5 d + 3.49 29.42 + 3.94

36 27.25+1.99 51.55d+ 3.44 32.42 + 4.39

72 26.76+1.66 46.98d+ 2.47 30.43 + 3.13

aModel: cortisol = # + block + sex + group + fit ( - 16 h cortisol) + ~. bBoth, d y n a m i c + static. ¢'dMeans in the s a m e row t h a t do n o t h a v e a c o m m o n superscript letter differ ( P < 0.05 ).

Table 5 Least s q u a r e s m e a n s a for N / L ( + SE) period

n 16 h before regrouping 8 h post-regrouping 20 days post-regrouping

Control

14 0.49 _+0.07 0.68 e _+0.20 -

Regrouped Dynamic

Static

Both b

36 0.75 + 0.06 1.23 d _+0.12 0.72 +_0.10

36 0.61 ± 0.06 1.44 d__ 0.11 0.83 __0.12

72 0.68 + 0.05 1.36 d + 0.09 0.79 + 0.08

aModel: N / L = / ~ + b l o c k + s e x + g r o u p + i l l ( - 16 h N / L ) +E. bBoth, d y n a m i c + static. ¢'dMeans in the s a m e row that do n o t h a v e a c o m m o n superscript letter differ ( P < 0.05 ).

Table 6 Least s q u a r e s m e a n s " for response to P H A double skin-fold t h i c k n e s s test ( m m + SE) Period

n 0 h post-regrouping 8 h post-regrouping 24 h post-regrouping

Control

11 2.74 + 0.09 5.74 ¢ + 0.17 6.13 + 0.24

Regrouped Dynamic

Static

Both b

28 2.70 + 0.07 5.32 d + 0.12 6.03 + 0.15

28 2.72 + 0.07 5.15 d + 0.12 5.85 + 0.13

56 2.71+0.05 5.23 d + 0.08 5.94 + 0.09

aModel: P H A = # + block + sex + group + fl~ (0 h P H A ) + ~. bBoth, d y n a m i c + static. ¢'dMeans in t h e s a m e row t h a t do n o t h a v e a c o m m o n superscript letter differ ( P < 0.05 ).

data within each group composition were pooled and these results are presented in Table 5. Mean N / L did not differ (P> 0.10) between individuals housed in static or dynamic groups during any of the three sampling periods. Pigs which were regrouped had higher N / L (P< 0.05) than non-regrouped controls when

A.S. Moore et al. / Applied Animal Behaviour Science 40 (1994) 13-30

25

measured 8 h post-regrouping. There was a positive correlation (r= 0.27; P < 0.05 ) between N / L and plasma cortisol at 8 h post-regrouping. Pen size did not have an effect (P> 0.25 ) on intradermal response to PHA, therefore data within each group composition were combined (Table 6). Response to PHA did not differ (P> 0.10) between pigs in static and dynamic groups during any of the three periods when double skin-fold thickness was measured. At 8 h post-regrouping, non-regrouped controls showed a greater response to PHA (P< 0.05 ) than pigs in either static or dynamic groups. However, this difference was only detected at 8 h post-regrouping and by 24 h there was no difference between treatments (P> 0.10).

4. Discussion One of the arguments against continuous flow grower units is that disease transmission between age groups is more likely than in 'all in, all out' systems. In this study it could be argued that disease transfer within pens would be more likely in dynamic rather than static groups. Both grouping types were exposed to similar between pen risks, because they were in the same room. The higher morbidity rate in dynamic groups supports this argument, although these results could also be explained by a greater susceptibility to a common risk owing to stressors imposed by the different social systems. There is the possibility that differences between treatments used in this study would be dependent upon the health status of the herd in which they are imposed. Reduced or inadequate space allowance and regrouping in combination with restricted floor space have been shown to decrease live weight gains among growing-finishing pigs (see reviews by Petherick, 1983; Kornegay and Notter, 1984). The lack of effect of pen size on productivity, and all other measures in this study with the exception of injuries to the ears on the first day, can be attributed to the fact that the space restriction in reduced pens (20% compared with standard) was not sufficient to significantly affect the pigs. Previous research has shown that regrouping may significantly depress the productivity of finishing pigs (Tan et al., 1991 ). However, ~tudies utilizing growingfinishing pigs have shown either small and temporary effects (McGlone and Curtis, 1985 ) or no adverse effects at all on rate of gain (Friend et al., 1983; Clark et al., 1985; Greer, 1987). Likewise, results from studies examining within-group weight variation at the time of group formation and the influence this variation has on weight gains after mixing, have also shown only temporary effects (Tindsley and Lean, 1984; Gonyou et al., 1986; McGlone et al., 1987 ). Two factors make this study unique. Firstly, no other studies have practiced dynamic grouping where pigs were allotted to a previously established group. Secondly, differences in weight between pigs in our dynamic groups represented a much greater range in weight than has been utilized in previous studies. Nevertheless, productivity results obtained here are comparable with the previously mentioned research. Specifically, the cumulative weight gain of pigs in our dy-

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namic groups was no different from that of pigs housed in static groups. In addition, the observed reduction in performance of dynamically grouped pigs was only a temporary situation, albeit a 6 week one. The fact that dynamically grouped pigs in our study had depressed weight gains for the initial 6 weeks after regrouping, indicates that this grouping method had a much more profound effect on the pigs than any previous research done on groups having large variations in weight (Gonyou et al., 1983, 1986; Tindsley and Lean, 1984; McGlone et al., 1987 ). During the period from Day 0 to Day 21, when pigs were classified as SM, pigs allotted to both groups were mixed with an equal number of unfamiliar animals. If performance during this period were to be affected solely by the fact that SM pigs were regrouped with unfamiliar animals, then weight gains should have been equally affected in both groups. The almost immediate and prolonged reduction in performance among SM pigs in our dynamic groups suggests that these pigs were experiencing some additional form (s) of social stress beyond that of regrouping every third week, which were not present in static groups. Perhaps a better explanation for the observed differences in weight gains involves the idea of the "preformed weight hierarchy" set forth by Tindsley and Lean (1984). These authors reported that the "preformed weight hierarchy" in groups with large within-pen weight variations persisted throughout the trial. This phenomenon may be especially relevant in our study, where there was an even larger range of weight at regrouping. Large differences in weight can be a decisive factor in determining social rank (McBride et al., 1964). In our study, the extremely large difference in weight between SM pigs and other members of the dynamic group most likely resulted in SM pigs being the lowest ranking animals in the pen. Dominant pigs may hinder the lower ranking pigs' access to self-feeders even when ad libitum access to the feeders is provided (Hansen and Hagelso, 1980). Time course changes in rate of gain also lend support to this hypothesis. If a strict weight hierarchy were present in dynamic groups which allowed dominant animals priority of access to the self-feeder, then gains would be expected to increase as pigs progressed through the size classes, eventually becoming the dominant class, and this was exactly the case in our study. The increased gains recorded among XL pigs in dynamic pens was most likely the result of compensatory gains by these animals which had previously experienced depressed performance as SM and MED pigs. Upon entering the XL category, dynamic pigs were more than 3.0 kg lighter than pigs in static groups. Food intake is a major component of weight gain and is believed to be more functionally associated with body-weight than age (Kanis and Koops, 1990). The XL pigs in dynamic groups most likely were able to control the feeders and increase their feed intake, thus compensating for their earlier losses in productivity. Time budgets for eating lend some support to this hypothesis as there were several periods of the day when XL pigs spent more time eating than SM or MED individuals within dynamic pens. However, there were no differences between the time spent eating by XL pigs in static and dynamic pens. It is possible that the lighter weight pigs in dynamic

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groups were able to convert feed to gain more efficiently, however no records of individual intakes were available for analysis. An alternative explanation for the group composition/pig size interaction effect on weight gain is that space allowance, or space allowance and social factors together, limited growth in SM pigs in dynamic pens and that of XL pigs in static pens. SM pigs in dynamic pens experienced greater space restrictions than SM pigs in static pens, owing to the presence of LG and XL pigs rather than only SM animals, respectively. Access to space by SM pigs in dynamic pens could have been further reduced by the competitive advantage that LG and XL pigs may have held over their smaller penmates. On the other hand, XL pigs in static pens were exposed to more crowded conditions and had to compete for space with larger (XL) pigs, than did XL pigs in the dynamic groups. It is likely that both social status and space allowance contributed to the observed differences in growth. Gonyou et al. (1986) also reported that the effects of within-pen weight variation differ with the pigs' relative weight. In contrast to our study, the smallest pigs within variable weight pens out-gained those in uniform groups. While the larger weight range among pigs in our study may account for this difference, another explanation may be that SM pigs allotted to our dynamic groups faced stronger resistance when attempting to assimilate into the previously established dynamic group. Groups of uniform individuals are often formed with the goal of emptying all pigs from the pen at once when they reach market weight. Tindsley and Lean (1984) found no difference between the weight variation within groups of uniform and variable weight pens at the end of their 10 week trial. This is in contrast to our results which showed the within-pen weight variances of SM pigs from both groups to be equal at the start of the 12 week trial, but the variance within dynamic groups increased and was greater within dynamically grouped pigs from Day 42 to Day 84. This greater variation in weights affects the spread of time over which pigs are sold and delays in pen emptying. Dynamic groups are managed in such a way that pens are never completely emptied at one time. Nonetheless, from a management standpoint, it remains important that a large enough number of uniform pigs are available at the same time for transport to market. Thus, from these results it appears that static groups are advantageous over dynamic grouping with respect to maintaining a uniformity of weight among penmates. Another noteworthy effect of group composition in this study was its influence upon the duration and frequency of fights. The total time spent fighting during the first 2 h post-regrouping by SM pigs in dynamic groups was reduced by about 60% from that of SM pigs in static groups. This is similar to the findings of Rushen ( 1987 ) who reported a 40% decrease in the total duration of fighting when there was a large variation in weight within the group. All other research examining the effect of within-pen weight variation on time spent fighting have failed to show any differences between treatments (Gonyou et al., 1983, 1986; Tindsley and Lean, 1984). Once again, the large difference in weight among the four size classes

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in the dynamic groups was probably responsible for this effect. Large weight or size differences between two unfamiliar animals may facilitate the pigs' adeptness at determining the relative fighting ability of the other, thereby reducing the time needed to decide the outcome of the encounter (Rushen and Pajor, 1987 ). Although one cannot discount the possibility that being placed into an already established group of nine pigs was also a major contributing factor. The nearly threefold increase in the number of fights involving SM pigs in dynamic groups during the 4 h following regrouping, indicates that the reduction in time spent fighting is due to fights which are much shorter in length than those recorded in static groups. Injury scores also suggest that total duration of fighting may be a poor indicator of the intensity of fights. Most of the fights recorded in dynamic groups were directed against SM pigs and lasted only a very short while. Many of these fights consisted of nothing more than a single bite directed towards the SM pig. It is likely that placing strange pigs into a previously established group increases aggression beyond that normally observed when pigs are mixed in a neutral area. Resident pigs are more likely to initiate the first aggressive encounter (Tan and Shackleton, 1990). Therefore, the distribution of fights observed in our study may be influenced, to a greater extent, by the fact that MED, LG and XL pigs had already resided in the pen, rather than the fact that they were from a larger size class. Fights between two SM pigs in dynamic pens were very infrequent, however those that did occur were similar in length to fights involving SM pigs in static pens. Rushen ( 1987 ) suggested that the decreased fighting in groups with large variation in weight, results from shorter fights between pigs of different weight, which is similar to the findings of our research. Immediately upon regrouping, SM pigs in dynamic groups usually congregated together in a corner of the pen, avoiding the resident animals. The three previously unacquainted SM pigs showed a clear preference for lying next to one another during the 4 h period following regrouping. This 'forced friendship' remained intact throughout the remainder of the trial as MED, LG and XL pigs continued to exhibit a preference for lying nearest to a member of the same size class. This indicates that the common experience shared by the newly introduced pigs is enough to stimulate the formation of a stable subgroup. Similar behavior following regrouping has been observed in chickens (Guhl, 1953 ) and beef cattle (Mench et al., 1990). As previously mentioned, there were no differences between pigs in static and dynamic groups with regard to mean cortisol concentration, N / L or response to PHA. However, when compared with non-regrouped controls, pigs that were regrouped had increased levels of cortisol 8 h post-regrouping, indicating that regrouping was an acute stressor. Blecha and Kelley (1981 ) reported a similar change in cortisol 1 day after regrouping. The increase in N / L 8 h post-regrouping lends support to the conclusion that regrouping is an acute stressor as N / L can serve as a useful indicator of stress (Widowski et at., 1989 ). Furthermore, in the present study, regrouping was found to have a detrimental effect on response to PHA when measured 8 h after mixing. The only other study to address the effect of regrouping on PHA failed to detect any significant difference between

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regrouped and non-regrouped animals (Blecha et al., 1985). However, in their study, PHA was injected 1 h after regrouping and measurements were taken 24 h post-injection. The results from our 24 h post-injection measurements also failed to detect any significant difference. Although several of the variables measured did not differ between dynamic and static treatments, there was evidence that the dynamic treatment was more stressful. As indicated above, although aggressive interactions in dynamic pens were shorter, they resulted in the SM pigs being at the bottom of the social hierarchy. As a result, growth was reduced for several weeks. In addition, despite the fact that immunophysiological measures did not differ, more pigs in dynamic pens were affected by the health challenges present in the herd during the experiment. In conclusion, time spent fighting following regrouping can be decreased by the use of dynamic grouping. However, this practice appears to reduce the overall well-being of pigs and therefore should be avoided.

5. Acknowledgments The contributions and instructive advice of Drs S.E. Curtis and A.W. Ghent are gratefully acknowledged. Appreciation is expressed to the University of Illinois Swine Research Center staff for their assistance.

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