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Application of broken-line analysis to assess floor space requirements of nursery and grower-finisher pigs expressed on an allometric basis¹

H. W. Gonyou^*,2, M. C. Brumm, E. Bush, J. Deen, S. A. Edwards^#, T. Fangman^||, J. J. McGlone^¶, M. Meunier-Salaun^**, R. B. Morrison, H. Spoolder, P. L. Sundberg and A. K. Johnson^,3

^* Prairie Swine Centre, Inc., Saskatoon, Saskatchewan, Canada S7H 5NP; and Department of Animal Science, University of Nebraska, Concord 68728; and USDA-APHIS-VS. Center for National Animal Health Surveillance, Fort Collins, CO 80526; and Department of Veterinary Population Medicine, College of Veterinary Medicine, University of Minnesota, St. Paul 55108; and ^# School of Agriculture, Food and Rural Development, University of Newcastle, Newcastle Upon Tyne, NE1 7RU England; and ^|| University of Missouri-Columbia, College of Veterinary Medicine, Columbia 65211; and ^¶ Pork Industry Institute, Texas Tech University, Lubbock 79409; and ^** INRA UMR SENAH, 35590 Saint-Gilles, France; and Division of Applied Research, Animal Sciences Group, Wageningen, UR, Lelystad, The Netherlands; and and National Pork Board, Des Moines, IA 50306

	Abstract

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Few issues in swine production are as complex as floor space allowances. One method for pork producers to calculate floor space allowance (A) is to convert BW into a 2-dimensional concept yielding an expression of A = k * BW^0.667. Data on ADG, ADFI, and G:F were obtained from published peer-reviewed studies. Five data sets were created: A = grower-finisher pigs, fully slatted floors, and consistent group size; B = grower-finisher pigs and fully slatted floors (group size did not need to be consistent); C = grower-finisher pigs, partially slatted floors, and consistent group size; D = grower-finisher pigs, partially slatted floors (group size did not need to be consistent); and E = nursery pigs, fully slatted or woven wire floors (group size did not need to be consistent). Each data set was analyzed using a broken-line analysis and a linear regression. For the broken-line analyses, the critical k value, below which a decrease in ADG occurred, varied from 0.0317 to 0.0348. In all cases the effect of space allowance on ADG was significant (P < 0.05). Using the linear analyses based on data with k values of <0.030, the critical k values for the 4 grower-finisher data sets did not differ from those obtained using the broken-line analysis (0.0358 vs. 0.0336, respectively; P > 0.10); however, none of the linear regressions explained a significant proportion of the variation in ADG. The slopes for the nonplateau portion of the broken-line analyses based on percent values varied among data sets. For every 0.001 decrease in k (approximately 3% of the critical k value), ADG decreased by 0.56 to 1.41%, with an average value of 0.98% for the 5%-based analyses. The use of an allometric approach to express space allowance and broken-line analysis to establish space requirements seem to be useful tools for pig production. The critical k value at which crowding becomes detrimental to the growth of the pig is similar in full- and partial-slat systems and in nursery and grower-finisher stages. The critical point for crowding determined in these analyses approximated current recommendations to ensure the welfare of pigs.

Key Words: analysis • growth • model • performance • pig • space requirement

	INTRODUCTION

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Floor space allowances are important in pork production from performance, economic, and animal welfare perspectives. Individual pig productivity decreases as crowding increases (Gonyou and Stricklin, 1998), signifying a welfare concern, but production per unit of area may increase, improving the economics (Edwards et al., 1988; Powell and Brumm, 1992). Thus, for this aspect of management, animal welfare and the economics of production are inversely related.

The most common means to express space allowance is as space per animal (e.g., m²/pig), but it is recognized that this should change as animals grow. Expressing space allowance as a weight density (e.g., kg/m²) also fails over a wide range of BW because it relates weight, a 3-dimensional concept (density x volume), to a 2-dimensional concept, area. A third means is to convert BW into a 2-dimensional concept yielding an expression in the form of A = k * BW^0.667, in which A represents floor space allowance and k represents a space allowance coefficient. The use of this allometric approach to determining space requirements was proposed by Petherick (1983a), and Baxter (1984), supported by Hurnik and Lewis (1991), and applied by Edwards et al. (1988) and Gonyou and Stricklin (1998). The allometric expression can be applied over a wide range of weights, and it allows comparison of studies that used different weight end points.

The response of pigs to crowding has been characterized using linear and curvilinear regression analyses (Kornegay and Notter, 1984). Another means of analysis, the broken-line method (Robbins, 1986), assumes that productivity will increase with increased space up to a critical value, above which a plateau occurs. Our objective was to analyze previously reported data for pigs using broken-line and other analyses. The allometric approach should be more applicable over a wide range of weights than previous analyses, and the broken-line approach should identify a specific point at which the negative effects of crowding begin.

	MATERIALS AND METHODS

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Data on ADG, ADFI, and G:F were obtained from published peer-reviewed studies on pigs fed ad libitum. Literature was gathered from both electronic databases [primarily Agricola (USDA-National Agricultural Library, Beltsville, MD), Current Contents (ISI, Philadelphia, PA), and CABI (CAB International, Wallingford, UK)] and from sources known to the authors. The period covered by the electronic databases in this review was from 1970 to 2001. Earlier literature was gathered by authors from nonelectronic sources. Papers by authors that were in press also were included. Only papers for which sufficient information was presented to calculate k, in which k = area, m²/BW, kg^0.667, were included. The minimum k for each space allowance treatment, typically based on the final BW when all pigs remained in the pen, was used in the analysis. Studies that did not provide this information, typically those that removed individual pigs as they reached market weight, were excluded. For studies in which relatively constant k values were maintained by adjusting pen size (e.g., Edwards et al., 1988), the treatment k was used. Each study was required to have at least one treatment with a minimum k that was >0.030 and at least one treatment <0.030, a level used by Spoolder et al. (2000), which approximates the allowances stipulated by the European Community (2001). Because very few papers provided data for the final growing period, when crowding would be greatest, overall trial data were used to determine ADG, ADFI, and G:F. For factorial studies, interaction effects were not considered and the main effect of space allowance was used.

Papers were classified according to size of pig, floor type, and whether group size (pigs per pen) was consistent across crowding treatments. Sufficient numbers of papers were identified to allow analysis of 5 data sets, each with a minimum of 6 treatment groups. These sets were: A = grower-finisher pigs, fully slatted floors, and consistent group size; B = grower-finisher pigs and fully slatted floors (group size did not need to be consistent); C = grower-finisher pigs, partially slatted floors, and consistent group size; D = grower-finisher pigs, partially slatted floors (group size did not need to be consistent); and E = nursery pigs and fully slatted or woven wire floors (group size did not need to be consistent). The papers used in each analysis are listed in Table 1. For grower-finisher studies, the initial BW ranged from 20 to 70 kg, and final BW ranged from 85 to 135 kg. Nursery studies had initial and final BW of approximately 6 and 20 kg, respectively. The minimum k values were typically approximately 0.025 but ranged from 0.017 to 0.030.

Each data set was analyzed using 2 methods. A broken-line analysis (Robbins, 1986) was conducted using the NLIN procedure of SAS (SAS Inst., Inc., Cary, NC). This analysis resulted in a critical k value, above which the production measurement plateaued at the control value (100% or 0) and below which the data were fitted to a linear regression line. The slope of this regression line, as well as the total r² (both sloped and flat lines), were also obtained. The second analysis was a linear regression (PROC REG of SAS), using all data with k values <0.030, that is, results from treatment groups expected to be affected by crowding. The slope, critical k at which values equal the control, and the r² were obtained. The critical k values obtained by broken-line and linear analyses were compared by paired t-test.

	RESULTS

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The results of the analysis of the 5 datasets for ADG are presented in Table 2. The broken-line analysis of grower-finisher pigs on fully slatted floors (set B) using percentage data is illustrated in Figure 1. Using broken-line analysis (nonlinear), the critical k value, below which gain was decreased as space allowance was further restricted, varied from 0.0317 to 0.0348 over all data sets, which represents a range of 9.2% of the average value. The variation accounted for by these analyses ranged from 58 to 90%. In all cases, the effect of space allowance on ADG was significant (P < 0.05). The average critical k value obtained using percentage data did not differ from that obtained using absolute data (0.0331 vs. 0.0341; P > 0.10).

View larger version (17K): [in this window] [in a new window]   Figure 1. Broken-line analysis of ADG for grower-finisher pigs on fully slatted floors (data set B; see Table 1). The allometric expression of space allowance is k where k = area in m2/BW in kg0.667. The ADG is expressed as a percentage of that in the most spacious treatment within each experiment; r2 = 0.90, P < 0.001.

The slopes for the nonplateau portion of the broken-line analysis based on percent values varied among data sets. For every 0.001 decrease in k (approximately 3% of the critical k value), ADG decreased by 0.56 to 1.41%, with an average of 0.98% for the 5%-based analyses. Slopes in the linear analyses did not differ (P > 0.10) from those of the broken-line analysis.

The results of the analysis of the 5 datasets for ADFI are presented in Table 3. Using broken-line analysis the critical k value, below which feed intake was decreased as space allowance was further restricted, varied from 0.0335 to 0.0358. The variation among treatment means accounted for by these analyses ranged from 38 to 68%. Only the broken-line analyses on grower-finisher pigs were significant. The broken-line analyses on nursery pigs, and all linear analyses, did not explain a significant portion of the variation. The critical k values obtained for ADFI using the percent and absolute data sets did not differ, nor did those obtained by linear analysis differ from those derived from broken-line method. The critical k values obtained by broken-line analysis for ADFI were similar to those obtained for ADG.

Analyses of the efficiency data (G:F) did not yield any significant relationships with space allowance (data not shown).

	DISCUSSION

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Allometry refers to the relationships among physical measurements of an object and how these change as the size (volume) of that object changes. Linear measures, such as length or height, are proportional to volume^0.333, whereas measures of area (surface or cross-sectional) are proportional to volume^0.667 (Petherick, 1983a). The use of the allometric relationship between volume and surface area, in this case represented by BW and floor area requirement, allowed us to seek a general description of space needs that applies over a range of BW. Allometric relationships should remain constant over a range of BW if the shape and density of the object remain the same. Although the shape of pigs may vary somewhat with size, and differ with genotype (McGlone et al., 2004), these differences are likely to have a much smaller effect on linear or area measures than does changing BW. Thus, Petherick (1983b) found linear measures were closely related to BW^0.333, and therefore area, representing the product of 2 linear measures, should vary proportionately to BW^0.667. Additional assumptions made when using an allometric equation to estimate floor space requirements are that space requirements are directly related to behavior and that behavior, in terms of postures and time budgets, is consistent over the weight range considered. Pigs <50 kg of BW will overlie each other, resulting in a decreased area needed for lying (Boon, 1981), which suggests that nursery pigs may require less space, expressed on an allometric basis, than larger pigs. This did not seem to be the case in our analysis; the nursery requirement for growth was within the range obtained for grower-finisher pigs. The postures used by grower-finisher pigs (standing, sitting, and lateral and ventral lying) are similar for all BW, but we recognize the potential for some differences in time budgets. The proportion of time spent eating decreases as pigs grow (Hyun et al., 1997), and this may be reflected in an increase in time spent lying. The studies included in these analyses involved housing systems in which most of the floor area would be used for static space requirements. More extensive systems, in which large areas would be used for foraging or manure accumulation (e.g., bedded systems), might require additional dynamic space. The similarity in critical k values obtained in all our analyses supports the establishment of space requirements using allometric relationships, at least within the housing conditions represented in the studies included.

In using the broken-line analysis, our hypothesis was that the response to increased space allowance would be minimal above a critical degree of crowding, and that as space allowance decreased below that level, a linear depression would be evident in growth and feed intake. Previous attempts to describe growth response to crowding have used either linear or curvilinear models over the entire range of space allowances studied (Kornegay and Notter, 1984). The results of our analyses indicate that our hypothesis reflects the actual response of the pigs.

We attempted to confirm our approach by using a linear analysis of only the crowded data, that is, data that were below the critical space allowance. Including data above the critical space allowance, as was done in the broken-line analysis, could shift the intercept of the maximum growth or intake portion of the response higher than would actually be the case. To avoid such an overestimation of the critical k, we restricted our linear analysis to data with k values <0.030 (i.e., below the expected critical space allowance). Although the linear analysis resulted in greater variability in the estimated critical k values, these values were not significantly different from those obtained using the broken-line analysis. The one data set in which k values and slopes seemed to differ using the 2 methods was data set D (partial slat floors and varying group size). In this case, the nonlinear approach yielded results more similar to the analyses of other data sets using either linear or nonlinear methods.

One shortcoming in the data we used was that most studies reported the growth response of pigs over the entire study period, including the early stages when our hypothesis would suggest crowding would not occur, as well as the final period of intense crowding. Inclusion of the noncrowded period would tend to dilute the effect of crowding during the later stages of each trial and decrease the slope of the line below the critical space allowance. To better define the critical point of crowding and the response below that point, it would be helpful if future papers on space allowance would publish growth rates obtained at several stages of the study.

The European Community (2001) specifies space allowances for several BW ranges of pigs that approximate k values of 0.028 for grower-finisher pigs. The Canadian Code of Practice recommends a k of 0.035 for pigs on fully slatted floors (AAFC, 1993). The Swine Care Handbook (NPB, 2003) recommends a space allowance equivalent to a k of 0.034 at 68 kg. The problem of providing a specific floor space allowance is represented by the Swine Care Handbook’s recommendation of a single value for a market-weight pig. This value could represent a k value from 0.033 to 0.029 for BW of 110 to 130 kg, respectively. Our analysis for fully slatted floors resulted in an estimate for the critical value of k = 0.034, approximating that for the Canadian and US recommendations and approximately 15% greater than the European Community (2001) value.

The Canadian Code of Practice (AAFC, 1993) recommends an increase in space allowance if pigs are on partially rather than fully slatted floors. Our results based on partially slatted studies (data sets C and D), compared with those based on fully slatted floors (data sets A and B), would not support this differentiation in floor types and recommendations. The 2 papers used in these analyses that included both fully and partially slatted floors recommended similar space allowances for both floor types (Gehlbach et al., 1966; Jensen et al., 1973). Our analyses resulted in greater slopes for the growth and intake responses in the crowded range of the data for pigs on partially slatted floors, suggesting that although the critical point of crowding may be similar, the effect of further crowding is more detrimental compared with fully slatted systems. One could speculate that further crowding leads to a more rapid deterioration of conditions in a partially slatted system.

As indicated previously, nursery pigs may be expected to have lower static space requirements because of their willingness to overlie pen mates while huddling; however, their floor space requirements would seem to be similar to those of larger pigs. It may be that overlying at thermoneutral temperatures is indicative of crowding, so space requirements do not differ. An alternative is that the decrease in requirements for lying space due to overlying is offset by an increased requirement for activity space. It should be noted that conventional recommendations for space allowance for nursery and market pigs are not directly comparable on an allometric basis. We restricted our analysis to grower-finisher data collected before pigs were marketed, and we used the average BW of the pigs in the pen at that time. Similar conditions apply to nursery recommendations, in that the BW targeted is the average BW of the pigs at the time they leave the nursery, en masse. For market pigs, most recommendations refer to the space allowance for pigs marketed at a specific BW. In fact, the average BW of the pigs in the pens would not reach that point, but would probably be 1 to 2 SD below market set points when the first pigs are removed from the pen. Thus, allometric recommendations for market pigs, if based on market weights, would be lower than that for nursery pigs even if the actual allometric requirement is the same.

Two commonly postulated effects on space requirements could not be examined in our analyses. It is generally believed that space requirements increase in hotter temperatures (English et al., 1988: AAFC, 1993). Although the studies used represent research conducted in several regions of the United States (Midwest, southeast, and south central) as well as Canada and the UK, it was not possible to assess climatic effects.

It also has been suggested that space requirements for pigs in large groups may be less than in small groups due to the sharing of free (nonstatic) space (McGlone and Newby, 1994). The results of Wolter et al. (2000) provided some support for this hypothesis, in that large and small groups performed similarly under slightly different crowded conditions. A recent factorial study in which small and large crowded groups were provided identical floor space allowances failed to detect a differential response to crowding in small and large groups (Street and Gonyou, 2005). Our analyses included data sets (B and D) in which space allowance in some studies was varied by adjusting group size, but these results did not differ from the data sets restricted to same-sized groups (A and C), and a group size effect could not be assessed.

Productivity, indicated by growth rate in grower-finisher pigs, is often included with physiology and health status as measures to assess the welfare status of animals. These analyses identified the point of crowding at which productivity is decreased, suggesting a point of reduced welfare. Because physiological responses and health status of crowded animals are rarely reported (see Leek et al., 2004, for an exception), it was not possible to determine the point of crowding at which such indices of welfare would be affected. Until physiological and health data are obtained, the results of these analyses provide a reasonable approximation of welfare requirements.

Numerous studies had to be excluded from our analyses because the information provided was insufficient to calculate k values. As indicated previously, our analyses were somewhat compromised by the fact most studies only reported data for the entire study, rather than intervals that could be associated with changing k values as the pigs grew. To facilitate similar analyses in the future, we recommend that studies include a minimum of 2 levels of k, both above and below the critical values we obtained, and that results be presented for numerous intervals within each study.

	IMPLICATIONS

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION IMPLICATIONS LITERATURE CITED

 
The use of an allometric approach to express space allowance and broken-line analysis to establish space requirements seem to be useful tools for pig production. The critical k value at which crowding becomes detrimental to the growth of the pig is similar in full and partial slat systems, and in nursery and grower-finisher stages. Results available in the published literature may underestimate the effects of crowding, in that overall study data are reported, including both crowded and noncrowded portions of the trials. The critical point for crowding determined in these analyses approximate current recommendations to ensure the welfare of pigs.

	Footnotes

 
¹ The workshop leading to this publication was funded by the US Pork Checkoff from the National Pork Board, Des Moines, IA.

³ Current address: Dept. of Anim. Sci., Iowa State Univ., Ames 50011-3150.

² Corresponding author: gonyou{at}sask.usask.ca

Received for publication March 14, 2005. Accepted for publication August 26, 2005.

	LITERATURE CITED

Top Abstract INTRODUCTION MATERIALS AND METHODS RESULTS DISCUSSION IMPLICATIONS LITERATURE CITED

 


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This Article Abstract Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via HighWire Citing Articles via Google Scholar Google Scholar Articles by Gonyou, H. W. Articles by Johnson, A. K. Search for Related Content PubMed PubMed Citation Articles by Gonyou, H. W. Articles by Johnson, A. K.