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Effects of proportion of pigs removed from a group and subsequent floor space on growth performance of finishing pigs¹

J. M. DeDecker^*, M. Ellis^*,2, B. F. Wolter^*,3, B. P. Corrigan^*,4, S. E. Curtis^*, E. N. Parr and D. M. Webel

^* Department of Animal Sciences, University of Illinois, Urbana 61801; and and United Feeds, Inc., Sheridan, IN 46069

	Abstract

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The effects of the proportion of pigs removed from an established group and subsequent floor space on growth performance during the final 19 d of the finishing period were evaluated using 28 pens of mixed-sex crossbred pigs (mean initial BW = 113.4 ± 0.57 kg; n = 1,456; approximately 52 pigs per pen). A randomized block design was used with four pig-removal treatments: 1) 0% of pigs removed [Control], 2) approximately 25% of pigs removed, 3) approximately 50% of pigs removed, and 4) approximately 50% of pigs removed and floor and feeder spaces/pig decreased to equal those of Control. A block consisted of four pens with the same number of pigs and sex ratio per pen and with similar initial BW. Pens within blocks were randomly allocated to treatment, and the heaviest animals were removed from Treatments 2, 3, and 4 at the start of the study. Group size and floor space/pig for Treatments 1, 2, 3, and 4 were 52 and 0.65 m², 39 and 0.87 m², 26 and 1.30 m², and 26 and 0.65 m², respectively. Each pen contained a six-place feeder that provided 212 cm of total trough space; however, only three-places were accessible to pigs on Treatment 4. Compared with Controls, removing 25 or 50% of pigs resulted in increased (P < 0.001) ADG by 20.6 and 21.0%, ADFI by 10.8 and 7.9%, and G:F by 7.7 and 14.3%, respectively. Average daily gain by pigs on Treatment 4 (50% removal rate and decreased floor and feeder spaces) was greater (P < 0.05) than that of the Controls, but lower (P < 0.05) than that of Treatment 3 pigs (50% removal rate, no adjustment in floor or feeder spaces). No differences were observed among treatments for either morbidity or mortality. These results indicate that removing 25 or 50% of the heaviest pigs from groups of finishing pigs increased growth rate of the remaining pigs, and that the improved performance was only partly due to increased floor and feeder spaces.

Key Words: Growth • Pigs • Removal • Stocking Rate

	Introduction

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Due to variations in BW within a group of finishing pigs and the economic effect of marketing pigs outside specified weight ranges required by a particular slaughter plant, it is common practice to remove animals for marketing from a finishing barn over a period of time. Normally, the heaviest pigs are removed first, thereby allowing more time for lighter pigs to reach the target market weight. Removing the heaviest pigs from a pen has been shown to increase the growth performance of the remaining animals (Bates and Newcomb, 1997; Woodworth et al., 2000; DeDecker et al., 2002). However, the effect of different removal rates on the subsequent performance by the remaining pigs has not been determined. In addition, the factors responsible for the increased growth response in the pigs remaining in the pen after the heavy pigs have been removed have not been identified. Removing pigs from an established pen results in increased access to resources, including floor and feeder spaces, for the remaining animals, and also changes to group social dynamics. The objective of this study was to evaluate effects of removal rate and subsequent floor and feeder spaces on the growth performance of the remaining pigs.

	Materials and Methods

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This experiment was conducted in a growing-finishing facility at the Burton Russell Swine Research Farm (United Feeds, Inc., Frankfort, IN). The experimental protocol was approved by the University of Illinois Institutional Animal Care and Use Committee.

Experimental Design
The experiment was conducted as a randomized complete block design with seven replicates. Four treatments were compared: 1) Control, 0% removed (52 pigs per pen); 2) approximately 25% removed (39 pigs remained per pen); 3) approximately 50% removed (26 pigs remained per pen); and 4) approximately 50% removed with reduced space (26 pigs remained per pen, and floor, water, and feeder spaces per pig were decreased to equal those of Control). Removal of pigs during the normal production process prior to the allotment of pens to study caused variations in group size for the population of pigs (46 to 54 pigs per pen). Blocks were formed from four pens with the same group size and sex ratio and similar BW, and pens were randomly allotted to treatment from within each block. The number of pigs retained in a pen postremoval remained constant for each treatment; therefore, the percentage of pigs removed from pens within a treatment varied (21 to 26.5%, 43.5 to 50%, and 43.5 to 50% for Treatments 2, 3, and 4, respectively). Pigs were selected for removal on the basis of BW while maintaining the sex ratio of the pen and were removed from Treatments 2, 3, and 4 approximately 18 h after weighing.

Animals
A total of 1,456 crossbred pigs (Ausgene Line 5 sires x Ausgene Line 13 dams) in 28 mixed-sex pens (barrows and gilts) were allotted to the experiment at 22 wk after weaning.

Diets and Housing
Pigs were given ad libitum access to feed and water throughout the study and were fed a corn–soybean meal-based diet formulated to meet or exceed NRC (1998) recommendations for the nutrient requirements of finishing pigs. The diet contained 12.51% CP, 0.67% lysine, 0.43% Ca, 0.39% P, and 3,546 kcal/kg of ME (as-fed basis).

The study was carried out in an insulated, tunnel-ventilated, wean-to-finish house comprising two similar rooms. Flooring was concrete slats, and pen partitions and gates consisted of horizontal steel rods. Pen dimensions (length x width) were 5.74 m x 6.10 m for Treatments 1, 2, and 3 and 2.87 m x 6.10 m for Treatment 4. Therefore, floor space per pig for Treatments 1, 2, 3, and 4 was 0.65, 0.87, 1.30, and 0.65 m², respectively. Each pen was equipped with a six-place feeder (jumbo wean-to-finish feeder, Farmweld, Teutopolis, IL) accessible from both sides, which provided a total of 212 cm of trough space, allowing 4.0, 5.4, and 8.0 cm/pig for Treatments 1, 2, and 3, respectively. In addition, there was a minimum of one hanging water drinker for every 13 pigs on all treatments. Pigs on Treatment 4 had access to only three feeder places supplying a total of 106 cm of trough space (4.0 cm/pig) and two hanging waters. Air temperature was regulated using thermostat-controlled heaters and fan ventilation with a target temperature of 18°C for the duration of the study.

Growth Measurements
All pigs were individually weighed at 22 wk after weaning (start of study), and at d 19 after removal (end of study), with pen weights taken at d 7 after removal. Pigs experiencing health problems or injuries that did not respond to therapeutic treatment were removed from the study; the date of and the animal’s BW at removal were recorded and used in calculating growth performance. The within-pen CV in live weight was calculated for each pen on each weigh day by expressing the standard deviation of live weight within the pen as a percentage of mean pen live weight.

Feed data were collected using a computerized feed-mixing (LOMAN, Systemtechovik, Bremerhaven, Germany), delivery (AZA Int., Medolago, Italy), and recording (Fancom B.V., Pannigen, The Netherlands) system. The mixer was fitted with a load cell that recorded the weight of feed dispensed to each feeder. The load cell was calibrated weekly. In addition, four different feeder locations were chosen at random every week, and feed was mixed, delivered, and weighed after delivery to validate the feed-recording system. The average difference between amounts delivered and recorded was –0.01% (SD = 0.61%). Feed remaining in each feeder was measured each weigh day to determine feed intake and G:F.

Carcass Measurements
Pigs were ultrasonically scanned at the beginning and end of the study with an Aloka model 500V B-mode scanner fitted with an Aloka 5011 probe (Corometrics Medical Systems, Wallingford, CT), with the image taken longitudinally and anterior to the last rib, 5 cm off the midline. From the ultrasound image, backfat and loin depth were measured automatically using the AUSkey System (Animal Ultrasound Services, Ithaca, NY) and carcass lean percent was predicted using an equation published by Liu and Stouffer (1995).

Statistical Analyses
Pig performance data were analyzed as a randomized complete block design, with pen considered as the experimental unit. All data were tested for normality using the PROC UNIVARIATE procedures of SAS (SAS Inst., Inc., Cary, NC). Data for mortality and morbidity percent did not conform to a normal distribution; consequently, a ² rank-based test (Steel and Torrie, 1980) was carried out using the PROC RANKS procedures of SAS. Data meeting criteria for normality (Steel and Torrie, 1980) were analyzed using the GLM procedure of SAS. The model used for data analysis included effects of treatment, room, replicate nested within room, and two- and three-way interactions. Least squares means were evaluated using the PDIFF and STDERR options of SAS.

	Results and Discussion

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Growth Performance
No interactions were detected (P 0.19); therefore, only main effects will be discussed. Live weights before the start of test were similar for all treatments (Table 1). By design, there was a difference (P < 0.001) in pig weight at the start of the study, with Control pigs being the heaviest; the pigs on treatments with 50% of pigs removed being the lightest, and pigs on Treatment 2 intermediate and different from those on the other treatments. Number of pigs removed from the study due to morbidity and mortality did not differ (P = 0.77) among treatments (Table 1).

Interestingly, the greater floor and feeder space provided by increasing removal rate from 25 to 50% produced no additional improvement in growth response relative to the Controls. Obviously, as the proportion of the heaviest pigs removed from a pen increases, those remaining are lighter and are likely to be the slower-growing animals. In addition, the floor space for pigs in the 25% removal treatment was in excess of that reported to be necessary for maximum growth rate in finishing pigs (Petherick and Baxter, 1981; Fritschen and Muehling, 1986), and further increases in floor space resulting from removal rates above 25% would be unlikely to result in any additional improvement in growth performance. Additional research is needed to determine the effect of pig removal rates below 25% on subsequent growth performance.

In support of the results of the current study, previous research reported that ADG and ADFI were improved in pens of slaughter-weight pigs in which 30 (DeDecker et al., 2002) and 50% (Bates and Newcomb, 1997) of the heaviest pigs were removed. Moreover, Woodworth et al. (2000) reported an increase in ADG for groups of 25 and 28 pigs after the four heaviest pigs were removed. However, those three studies did not show any increase in G:F after pigs were removed, suggesting that the improved growth rate resulted mainly from increased feed intake. In contrast, in the present study, all removal treatments produced an improvement in overall G:F relative to Controls, suggesting that improvements in growth rate resulted from a combination of increased intake and improved efficiency of feed utilization. Feed efficiency generally decreases with increasing live weight, and this may have been a contributing factor in the improvement in G:F in Treatments 2, 3, and 4, where pigs were lighter than the Controls (Table 1).

The contributions of increased floor and/or feeder space to improved growth performance observed when 50% of pigs were removed from a pen can be evaluated by comparing Treatment 4 with Treatments 1 and 3 (Table 1). Over the 19-d test period, growth rate by pigs on Treatment 4 (50% of pigs removed, decreased floor space) was greater (P < 0.05) than that of Control pigs (Treatment l) but lower (P < 0.05) than pigs on Treatment 3 (50% of pigs removed, no adjustment of floor space). Pigs on Treatment 4 had a lower (P < 0.001) feed intake but similar (P = 0.19) G:F compared to those on Treatment3, suggesting that the difference in growth rate between these two treatments was due largely to the difference in feed intake. Compared with the Control treatment, Treatment 4 produced a similar feed intake but greater G:F (Table 1).

Thus, these results suggest that the increased growth rate of pigs remaining in pens after their heaviest contemporaries have been removed is only partly due to increased access to resources such as floor and feeder space. In addition to decreasing group size and increasing floor and feeder space per pig, removing pigs from an established group is also likely to change the social dynamics and behavior of the group. However, the effect of these factors on growth performance has not been established, and it was not possible to assess behavioral changes under the commercial research setting used in this study. Mixing pigs has increased fighting behavior and decreased growth performance in a number of studies (Stookey and Gonyou, 1994; Hyun et al., 1998); however, there has been limited research on the effect of removing pigs from an established group on the subsequent behavior of the remaining pigs. Augspurger et al. (2000) found that removing pigs from a group altered the feeding behavior of the remaining pigs, such that it was similar to that of an undisturbed group with the same number of pigs. Scroggs et al. (2002) evaluated aggression and immune response in groups pre- and postremoval, and found that postremoval aggression and measures of immune response were similar for groups with pigs removed compared to undisturbed groups of the same size. Ewbank and Meese (1971) found no increase in fighting after the removal of one pig from a pen of eight, and reported that when the dominant animal was removed, the second-highest-ranking animal became the new dominant pig of the group. Nonetheless, the studies of Ewbank and Meese (1971), Augspurger et al. (2000), and Scroggs et al. (2002) were carried out with relatively small groups of three and nine pigs. Additional research is needed to determine the effects of removing pigs from larger groups on social dynamics in commercial settings.

Body Weight Variation
At the start of study, after pig removal was carried out, the within pen CV for BW was greater (P < 0.05) for the Control and 25% removed treatments compared with Treatment 3 (50% removed; Table 1). At the end of study, the CV remained higher (P < 0.01) for Control pigs compared with Treatment3 (50% removed) pigs, with those on Treatment 2 and 4 being intermediate and not different (P 0.09) from the others (Table 1).

Thus, one of the treatments that involved removing 50% of the heaviest animals (Treatment 3) produced a decrease in BW variation in the remaining pigs, both immediately after removal and also 19-d later, at the end of study. In addition, there was a numerical trend (P = 0.09) for BW variation to be decreased relative to Controls in Treatment 4, which also involved removal of 50% of the pigs. Wolter et al. (2002) showed that sorting pigs into groups on the basis of weight decreased variation when sorting at 30 kg BW but not after a period of growth when the pigs reached 110 kg BW. It is possible that removing the heaviest pigs from an established group could have a similar effect on BW variation as sorting whole pens by weight. This possibility is supported by results of the study by DeDecker et al. (2002), who reported a decrease in BW variation immediately after removal of the 25% heaviest pigs, but not at 21-d after removal. In contrast, in the current study, the decrease in BW variation at removal was maintained to the end of the study at 19-d after removal in one of the 50% removal treatments. In addition, in comparing BW variation of all pigs marketed per treatment (i.e., BW of pigs removed for market at start of test and BW of remaining pigs at end of test), the total BW variation at market was greater (P < 0.05) for the Controls compared with Treatments 2, 3, and 4 (see Table 3). Changes in BW variation after removing pigs from a group is likely to depend both on the proportion of pigs removed and on the time of measurement post-removal.

	Footnotes

 
¹ This research was funded by the State of Illinois through the Illinois Council on Food and Agricultural Research (C-FAR) and the College of Agricultural, Consumer, and Environmental Sciences, Univ. of Illinois. The authors also gratefully acknowledge United Feeds, Inc., Sheridan, IN, for cooperation and use of facilities.

³ Current address: The Maschhoffs, Inc. 7475 State Route 127 Carlyle, IL 62231.

⁴ Current address: Vita Plus Corp., 1508 West Badger Rd. P.O. Box 259126, Madison, WI 53725-9126.

² Correspondence: 216 Anim. Sci. Lab., 1207 W. Gregory Dr. (phone: 217-333-6455; fax: 217-333-7861; e-mail: mellis7{at}uiuc.edu).

Received for publication January 15, 2004. Accepted for publication November 5, 2004.

	Literature Cited

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Ewbank, R., and G. B. Meese. 1971. Aggressive behavior in groups of domesticated pigs on removal and return of individuals. Anim. Prod. 13:685–693.

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