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J. Anim. Sci. 2003. 81:874-878
© 2003 American Society of Animal Science

Effect of group size on performance of growing-finishing pigs

S. A. Schmolke*,{dagger}, Y. Z. Li{dagger} and H. W. Gonyou1,*,{dagger}

* Department of Animal and Poultry Science, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5A8, Canada and and {dagger} Prairie Swine Centre Inc., Saskatoon, Saskatchewan S7H 5N9, Canada

1 Correspondence:
P.O. Box 21057, 2105 8th Street East (phone: 306-667-7443; fax: 306-955-2510; E-mail:
gonyou{at}sask.usask.ca).


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 
Six hundred forty growing-finishing pigs (initial BW = 23.2 ± 4.8 kg) were used in a 12-wk study (final BW = 95.5 ± 10.2 kg) to quantify the effects of group size (10, 20, 40, and 80 pigs/pen) on performance, tail biting, and use of widely distributed feed resources. One single-space wet/dry feeder was provided for every 10 pigs, and floor allowance was 0.76 m2/pig in all treatment groups. Weight gain and feed intake were measured every 2 wk. At weighing, a tail-biting injury score was given to each pig. Blood samples were collected and analyzed for neutrophil:lymphocyte ratio before regrouping at the beginning of the experiment, 24 to 48 h after regrouping, and on the last day of each trial. The use of feeders by individual pigs was assessed by behavioral observations. Average daily gain for the entire 12-wk trial did not differ among group sizes (861, 873, 854, and 845 g/d for groups of 10, 20, 40, and 80, respectively; P > 0.10). During the first 2 wk, ADG was lower for pigs in groups of 40 (554 g/d) than pigs in groups of 10 (632 g/d; P < 0.05), but not pigs in groups of 20 or 80 (602 and 605 g/d, respectively). Average daily feed intake, feed efficiency, and variability in final BW within a pen also did not differ among group sizes. Tail-biting injury scores increased throughout the study, but did not differ among group sizes. Similar proportions of pigs were removed from the trial for health reasons, primarily due to tail biting, in all treatments. Individual pigs in each group size ate from most, if not all, of the feeders in the pen. There was no evidence of spatial subgrouping within the larger groups. The results suggest that housing growing-finishing pigs in groups of up to 80 pigs is not detrimental to productivity and health if space allowance is adequate and feed resources are evenly distributed.

Key Words: Finishing • Group Size • Growth • Performance • Pigs


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 
Because marginal profit increases with the size of pig operations (Martin and Kruja, 2000), the pork industry is shifting toward larger production units. On such operations, which have large numbers of similarly aged animals, housing pigs in large groups is a means of reducing housing costs and simplifying some aspects of management. Housing pigs in large groups while maintaining individual floor space allowance also offers more total space and increases the potential for animals to select their preferred microenvironment (Spoolder et al., 1999). However, there are concerns about housing pigs in large groups. English et al. (1988) suggested that large groups might be associated with poor performance, increased variation in BW, and a higher incidence of behavioral vices. The results of recent studies on group sizes in excess of 50 pigs/pen would indicate that concerns about poor performance might be unfounded (Turner et al., 1999), although short-term reductions in growth may occur (Spoolder et al., 1999; Wolter et al., 2001). However, some studies, particularly with nursery pigs, do report group size effects (Verdoes et al., 1998; Wolter et al., 2000).

It has been suggested that a source of high levels of stress in large groups is the complexity of the social hierarchy (Stricklin and Mench, 1987; Moore et al., 1996). One strategy that pigs could use to reduce the complexity would be to form social subgroups that avoid interacting with other subgroups (Stricklin and Mench, 1987; Gonyou, 2001). The result of such a strategy would be that the pigs remain in a limited area of the pen and avoid other sections.

This study was made to assess the feasibility of producing grower-finisher pigs in large groups by addressing several of the objections raised in the literature. Specifically, the objectives were to determine the effects of group size on productivity, variation in weight, the incidence of tail biting, and use of widely distributed feed resources.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 Implications
 Literature Cited
 
Animals and Management

Six hundred forty barrows and gilts (Camborough and Canabrid terminal line cross; Pig Improvement Canada, Acme, Alberta), previously housed in groups of 8 to 12 pigs/pen in an intensive commercial-type nursery, from a minimal-disease research unit, were used in the experiment. The University Committee of Animal Care and Supply of the University of Saskatchewan reviewed and approved the animal care protocol for the experiment to ensure adherence to the Canadian Council of Animal Care guidelines (Olfert et al., 1993). The experiment consisted of four blocks; each was 12 wk in duration. The average weight of the animals at the beginning of the study was 23.2 (± 4.8 SD) kg. The average final weight at the conclusion of the study was 95.5 (± 10.2 SD) kg. Within each block, there were four experimental treatments imposed in one grower/finisher room: 2 pens of 10 pigs, and 1 pen each of 20, 40, and 80 pigs with an equal number of each sex housed in each pen (Figure 1Go). The location of treatments was randomly allocated within each block to account for possible environmental differences within the room. For allotment, pigs were stratified by weight into five categories within sex. One pig from each category was randomly assigned to each of 16 allotment groups. These groups were then randomly assigned to treatments to achieve pens of similar mean weight and variation.



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  		Figure 1. One layout of the experimental room used in the study. Black squares represent single-space wet/dry feeders. The location of each group size was randomly allocated for each block.

 
The animals were managed according to routine management practices in accordance with the Recommended Code of Practice (Agriculture Canada, 1993). Space allocation was determined by an allometric equation in order to provide adequate space for the pigs at the expected final weight (100 kg): A = kBW0.67, where A = area in m2, k = 0.035, and BW = final BW of a pig in kilograms. Each pig, regardless of treatment and not counting the space occupied by each feeder, was provided with 0.76 m2. This level of space allowance was chosen to ensure maximal growth of the pigs and to avoid potential group size x space allowance interactions. The level of space allowance exceeded that on many commercial farms, but was in accordance with the Recommended Code of Practice (Agriculture Canada, 1993). Feed and water were supplied to the animals by single-space (32-cm wide) wet/dry feeders (Better Feeders, Red Deer, Alberta). All animals had ad libitum access to barley-based pelleted diets. Two diets were fed during the experiment: a grower diet for the first 6 wk (3.24 Mcal of DE/kg, 19.0% CP, 4.1% crude fiber) and a finisher diet for the last 6 wk (3.08 Mcal of DE/kg, 16.2% CP, 5.6% crude fiber). One feeder was provided for every 10 pigs to ensure that resources were not limiting (Gonyou and Lou, 2000). Feeders were placed equidistantly along the center alley to achieve spatial separation and maintain equal mean distance to the feeders regardless of group size. All pens were made of spindle penning to allow visual contact with pigs in other pens. The floors were fully slatted concrete. To accommodate the videotaping involved in the behavioral portion of this study, a 24-h continuous lighting regimen was followed. Ventilation fans were thermostatically controlled to achieve a minimal ventilation rate and temperature as near as possible to the thermoneutral zone. Room temperature was monitored twice per day, once in the morning and once in the late afternoon. Animals were removed from the study due to poor health or if their welfare was compromised by injury.

Measurements

Performance. Pigs were weighed at the beginning of the experiment and at 2-wk intervals thereafter on an individual basis to obtain ADG. Feed intake was monitored per feeder per pen on a daily basis by weighing the amount of feed added. Biweekly, at the same time as weighing pigs, excess feed was removed from each individual feeder and weighed in order to calculate feed disappearance from each feeder and ADFI by each pen of pigs.

Behavior. Video cameras were mounted over the feeders and a time-lapse videocassette recorder (Panasonic AG6730, Matsushita Electric of Canada, Ltd., Mississauga, ON, Canada) was used to monitor each feeder for a 24-h period during wk 3, 6, and 9 of the study for two of the blocks. A total of 5 focal pigs from each pen of 10, and 10 pigs each from the pens of 20, 40, and 80 pigs were randomly selected and identified with a unique pattern on their backs using a grease marker. The videotapes were reviewed using continuous observations to determine the total duration of eating by each focal pig from each feeder.

Tail Biting. On a biweekly basis, as pigs were being weighed, a tail-bite damage score was given to each pig. The scoring system used was 1 = no marks on the tail, 2 = some scratches evident on the tail, 3 = open wound with some bleeding, 4 = severe wound and flesh missing from the tail, 5 = shortened tail stub and swollen. At the onset of any tail-bites, pigs were treated with a spray (Red-kote, H.W. Naylor Co. Inc., Morris, NY) on the affected area daily in an attempt to prevent any further progression of tail biting. In the interests of the animal’s welfare, pigs were removed from the pen when they achieved a score of 4 or 5.

Neutrophil:Lymphocyte Ratio. Blood (approximately 5 mL/sample) was collected via jugular puncture, smears were prepared, and cells were counted to determine neutrophil:lymphocyte ratio for three of the blocks. Samples were collected from five randomly selected pigs/pen in the groups of 10, and 10 pigs/pen in other groups. Blood samples were collected from the same pigs a day before regrouping, 24 to 48 h postregrouping, and on the last day of each trial.

Statistical Analysis

The experimental unit used to compare group sizes was the pen. When data on individual animals were available, sex was treated as a subplot within pen. The coefficients of variation for BW and feeder usage were determined within each pen. The basic ANOVA model was performed using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). The model included the effects of block, group size, and sex, with block (group size) as the error term for group size (main plot), and group size and sex nested within block as the error for sex (subplot). Results are reported as least squares means. A Fisher protected LSD means separation test was used to compare group sizes. The number of pigs removed from the study for health and welfare reasons was analyzed using {chi}2 analysis to determine if there were any significant differences among group sizes.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 Implications
 Literature Cited
 
Group size did not influence overall performance, although it should be noted that the level of replication was low (4 pens/treatment). The average initial and final BW and their coefficients of variation were similar among group sizes (Table 1Go). Although not differing at the beginning of the study, the average final weight for barrows was higher than for gilts (98.6 vs. 93.3, ± 0.87 kg; P < 0.05). The variations in final weight were similar between sexes.


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  		Table 1. Initial and final body weight, coefficients of variation, average daily gain, average daily feed intake, and efficiency of growing-finishing pigs housed in different group sizes
 
Average daily gain for the entire 12-wk trial was not affected by group size (Table 1Go). However, a significant difference was observed during the initial 2-wk period for ADG, which was lower in groups of 40 than in groups of 10 pigs/pen (554 vs. 632 ± 15 g/d; P < 0.05), whereas groups of 20 and 80 pigs/pen were intermediate (602 and 605 g/d, respectively). Average daily feed intake was similar among group sizes over the entire study (Table 1Go), and for each 2-wk interval. Feed efficiency (gain:feed) was also similar among treatments over the entire study (Table 1Go) and during each 2-wk interval.

There was considerable variation in feed consumption from individual feeders within each pen (Table 2Go). This variation was similar across all treatments involving two or more feeders/pen. Each focal pig in the group size of 20 was observed to use both of the possible feeders provided. Within groups of 40, 90% of the pigs ate from every feeder, and this was the case for 80% of the pigs in groups of 80. A pig was recorded as preferring a feeder if it was observed to be at a feeder for more than 50, 25, or 12.5% of its total feeding time for groups of 20, 40, and 80 pigs/pen, respectively. All focal pigs from the groups of 20 preferred one of the two feeders, whereas 80% of the focal pigs from groups of 40 preferred two of the four feeders, and 55% of the focal pigs in the groups of 80 preferred three feeders. These distributions would not differ from random expectations.


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  		Table 2. Variation in feeder usage by pigs in different group sizes
 
The proportion of pigs removed from the experiment for health or welfare reasons did not differ among group sizes (7.5, 6.2, 5.0, and 5.6% for pens of 10, 20, 40, and 80 pigs, respectively; P = 0.90). The main reason for pigs to be removed from the study was because their tail was severely bitten. The average tail-biting damage scores were similar among group sizes throughout the 12-wk study, but increased from an average of 1.09 (± 0.04) after wk 2 to 1.48 (± 0.22) at the end of the study.

The neutrophil:lymphocyte ratios before regrouping, in the first 2 d after regrouping, and at the end of the trial were similar for all group sizes (Table 3Go). The barrows had slightly higher neutrophil:lymphocyte ratios than the gilts, and the difference between sex was significant before regrouping (0.721 vs. 0.545 ± 0.047, respectively; P < 0.05). Although the neutrophil:lymphocyte ratios were not different among group sizes for any of the sampling days, the neutrophil:lymphocyte ratios were higher during the first 2 d after regrouping than before regrouping and the last day of the trial (Table 3Go).


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  		Table 3. The effect of group size on neutrophil:lymphocyte ratios of growing-finishing pigs
 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
Implications
 Literature Cited
 
One concern about large group sizes is the potential for reduced growth rate. Based on their review of the literature, Kornegay and Notter (1984) developed equations that predicted an 11 to 21% decrease in growth rate of pigs in groups of 80 compared to groups of 10 pigs. The differences in growth rate among the group sizes in this study were not statistically different and were of a much smaller magnitude. Perhaps the difference between their results and ours is that studies included in their review did not have group sizes larger than 40 pigs per pen, and many involved much smaller groups. Other studies would confirm that increasing group size in groups of less than 10 pigs does reduce growth rate (Gonyou et al., 1992; Gonyou and Stricklin, 1998), but neither of these studies reached the minimal size comparison of this experiment. Therefore, the prediction equation proposed by Kornegay and Notter (1984) to calculate the effect of altering group size on ADG may not be appropriate for large groups of pigs as seen in this study. Other studies examining the effects of increasing group size on productivity have reported results similar to ours. Spoolder et al. (1999; groups of 20 to 80), Turner et al. (1999; groups of 20 vs. 60), and Wolter et al. (2001; groups of 25 to 100) reported similar growth performance among group sizes when the entire study was considered.

Despite the fact that there were no significant treatment differences on the overall growth performance of the pigs, the ADG in groups of 40 pigs was 12% lower than pigs in groups of 10 during the initial 2 wk. The fact that this was the only significant difference in gain detected in a large number of comparisons would suggest that the result might be spurious. However, other studies have also reported poorer growth in larger group sizes during the nursery phase (Wolter et al., 2000), during studies of short duration (Petherick et al., 1989), or during the early stages of longer studies (Spoolder et al., 1999). Spoolder et al. (1999) reported a 5% reduction in growth rate in groups of 40 to 80 pigs up to 65 kg, but not during the finishing period, compared with pigs in groups of 20. In a study of wean-to-finish pigs, Wolter et al. (2001) noted that growth rates were poorer in groups of 50 and 100 than in groups of 25 between the weights of 6 and 35 kg. But they did not find any differences in final BW and overall growth rate among the group sizes. Although the neutrophil:lymphocyte ratio, which generally rises with increased cortisol levels (Widowski et al., 1989), never differed among group sizes, the highest levels were found in larger groups shortly after regrouping. It would appear that any stressors associated with large group sizes are short lived.

It has been suggested that as group size increases, variation in performance also increases (English et al., 1988). If the social stress is significantly greater in pens of larger group sizes, one could expect a breakdown in social stability and hence an increase in the variability of animals (Stricklin and Mench, 1987). We did not observe an increase in variation in performance as group size increased, and the literature appears to support these results. Vermeer and Hoofs (1994), Spoolder et al. (1999), and Wolter et al. (2001) all reported similar variation in final BW among their various group size treatments. Interestingly, in the study by Petherick et al. (1989), pigs housed in groups of 36 had greater initial variation in BW than those in groups of 6 or 18 pigs, but by the end of the study the variation in BW was similar among group sizes.

Gonyou and Stricklin (1998) suggested that some group size studies might be confounded by the location of resources, such as feed and water. Our pen design placed the feeders equidistantly along one wall of the pen, maintaining a constant distance across group sizes from the farthest location in the pen to a combination feed and water source. Had the resources been clumped in one location, the distance from the farthest point to the wet/dry feeders would have increased with group size and perhaps limited intake. One of the dangers of spreading resources throughout the pen is that animals may choose one area of the pen for feeding and underutilize the feeders located elsewhere. Although the variation in feed intake among feeders within a pen in this study appears quite large (CV of over 20%), it was consistent across group sizes. It would appear that wide dispersal of resources in large pens, as in our study, need not result in ineffective use of some of those resource points.

The feeder locations used in the study allowed us to examine the social subgrouping hypothesis (Gonyou, 2001). This hypothesis states that as group size increases beyond the point that pigs can maintain a definitive social order, the animals will form several social subgroups that would avoid interacting with those outside their own subgroup. The hypothesis predicts that the pigs would limit themselves spatially in order to avoid other subgroups. This did not appear to be the case in our study. The vast majority of pigs in all group sizes were observed to feed from most or all of the feeders even though they were spread over a large portion of the pen. The hypothesis would also predict that pigs would form a strong preference for only a few feeders, those within their area, and that this would become more evident as group size increased. In fact, across group sizes, pigs had a preference for the number of feeders predicted by random usage (half of those available).

Several authors suggest that tail biting is more severe as group size increases (Bryant and Ewbank, 1974; English et al., 1988). Our results indicate that the same proportion of pigs was removed from the study regardless of group size. Similarly, the different group sizes had similar tail-biting damage scores. Larger group sizes did not induce higher levels of tail biting in this study. However, these results on only a few pens do not preclude the possibility that once tail biting reaches a critical level in a large group that social facilitation would lead to an epidemic of the behavior. Based on our results, fears concerning high levels of tail biting in large groups are unfounded.


   Implications
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Implications
Literature Cited
 
Commonly stated reasons for avoiding the production of growing-finishing pigs in large groups seem to be poorly founded. In groups of up to 80 pigs/pen, with adequate space for maximal growth, we found no evidence of reduced gain, increased variation in final weight, or increased tail biting. Pigs in large groups showed no evidence of forming subgroups and restricting themselves to parts of the pen. Feeders and drinkers spread throughout the pen were used by most pigs and were used as evenly in large groups as in small.

Received for publication June 21, 2002. Accepted for publication December 17, 2002.


   Literature Cited
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
 Implications
 Literature Cited
 


Agriculture Canada. 1993. Publication 1898/E. Recommended Code of Practice for the Care and Handling of Pigs. Communications Branch, Agriculture Canada, Ottawa, ON, Canada.

Bryant, M. J., and R. Ewbank. 1974. Effects of stocking rate upon the performance, general activity and ingestive behaviour of groups of growing pigs. Br. Vet. J. 130:139–149.[Medline]

English P. R., V. R. Fowler, S. Baxter, and B. Smith. 1988. The growing-finishing pig: the basis of efficient systems. Page 331–373 in The Growing and Finishing Pig: Improving Efficiency. Farming Press Ltd., Ipswich, U.K.

Gonyou, H. W. 2001. The social behaviour of pigs. Page 147–176 in Social Behaviour in Farm Animals. L. J. Keeling and H. W. Gonyou, ed. CABI Publ. Wallingford, U.K.

Gonyou, H. W., R. P. Chapple, and G. R. Frank. 1992. Productivity, time budgets and social aspects of eating in pigs penned in groups of five or individually. Appl. Anim. Behav. Sci. 34:291–301.

Gonyou, H. W., and Z. Lou. 2000. Effects of eating space and availability of water in feeders on productivity and eating behavior of grower/finisher pigs. J. Anim. Sci. 78:865–870.[Abstract/Free Full Text]

Gonyou, H. W., and W. R. Stricklin. 1998. Effects of floor area allowance and group size on the productivity of growing/finishing pigs. J. Anim. Sci. 76:1326–1330.[Abstract/Free Full Text]

Kornegay, E. T., and D. R. Notter. 1984. Effects of floor space and number of pigs per pen on performance. Pig News Info. 5:23–33.

Martin, L., and Z. Kruja. 2000. The Western Canada Advantage. Pages 17–36 in Advances in Pork Production. Vol. 11. Banff Pork Seminar 2000.

Moore, C. M., J. Z. Zhou, W. R. Stricklin, and H. W. Gonyou. 1996. The influence of group size and floor area space on social organization of growing-finishing pigs. Page 34 in Proc. 30th Int. Cong. of Int. Soc. Appl. Ethol., Guelph, Canada.

Olfert, E. D., B. M. Cross, and A. A. McWilliam. 1993. CCAC-Guide to the Care and Use of Experimental Animals. Vol. 1. 2nd ed. Canada Council on Animal Care, Ottawa, ON, Canada.

Petherick, J. C., A. W. Beattie, and D. A. V. Bodero. 1989. The effect of group size on the performance of growing pigs. Anim. Prod. 49:497–502.

Spoolder, H. A. M., S. A. Edwards, and S. Corning. 1999. Effects of group size and feeder space allowance on welfare in finishing pigs. Anim. Sci. 69:481–489.

Stricklin, W. R., and J. A. Mench. 1987. Social organization. Pages 307–322 in Farm Animal Behavior. E. O. Price, ed. Vet. Clin. N. Amer. Food Anim. Pract.

Turner, S. P., S. A. Edwards, and V. C. Bland. 1999. The influence of drinker allocation and group size on the drinking behaviour, welfare and production of growing pigs. Anim. Sci. 68:617–624.

Vermeer, H., and A. Hoofs. 1994. Gespeende biggen in kleine of grote groepen (Rearing of weaned piglets in small or large groups). Praktijkonderzoek-Varkenshouderij 8:12–13.

Verdoes, N., H. M. Vermeer, and A. J. A. M. van Zeeland. 1998. Housing types, performance and ammonia emission in large groups of weaned piglets. Paper 94–4070 in Proc. Amer. Soc. Agric. Eng., St. Joseph, MI.

Widowski, T. M., S. E. Curtis, and C. N. Graves. 1989. The neutrophil:lymphocyte ratio in pigs fed cortisol. Can. J. Anim. Sci. 69:501–504.

Wolter, B. F., M., Ellis, S. E. Curtis, N. R. Augspurger, D. N. Hamilton, E. N. Parr, and D. M. Webel. 2001. Effect of group size on pig performance in a wean-to-finish production system. J. Anim. Sci. 79:1067–1073.[Abstract/Free Full Text]

Wolter, B. F., M. Ellis, S. E. Curtis, E. N. Parr, and D. M. Webel. 2000. Group size and floor-space allowance can affect weanling-pig performance. J. Anim. Sci. 78:2062–2067.[Abstract/Free Full Text]


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