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Impact of early postweaning growth rate as affected by diet complexity and space allocation on subsequent growth performance of pigsin a wean-to-finish production system¹

B. F. Wolter^*, M. Ellis^*,2, B. P. Corrigan^*, J. M. DeDecker^*, 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

² Correspondence:
216 Animal Sciences Laboratory, 1207 W. Gregory Dr. (phone: 217-333-6455; fax: 217-333-7861, E-mail:
mellis7{at}uiuc.edu).

	Abstract

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The objective was to evaluate the effect of restricted early postweaning growth rate due to diet complexity, pen space, or both on subsequent growth to market in a wean-to-finish system. Pigs (n = 1,728) were used in a randomized block design with a 2 x 2 factorial arrangement of treatments: 1) diet complexity (Complex vs Simple) and 2) space allocation (Unrestricted vs Restricted). Treatments were imposed for the first 8 wk after weaning (period 1) and growth was measured from weaning (5.0 ± 0.01 kg body weight; 15 d of age) to the end of wk 23 postweaning. The Simple diet was based on corn-soybean meal with minimal inclusion of milk products, processed cereals, and animal protein-based ingredients compared to the Complex diet. Floor and feeder-trough spaces were 0.63 m² and 4 cm and 0.21 m² and 2 cm per pig for Unrestricted and Restricted space treatments, respectively. From the end of wk 8 to end of wk 23 (period 2), pigs on all treatments had the same floor and feeder spaces and were fed common diets. There was no interaction (P > 0.05) between diet and space treatments. In period 1, Simple diets resulted in similar average daily feed intake (ADFI; 639 vs 650 ± 5.4 g; P > 0.05), but lower average daily gain (ADG; 408 vs 424 ± 3.8 g; P < 0.01) and gain:feed ratio (0.64 vs 0.65 ± 0.002; P < 0.001), and lighter body weight (2.8%; P < 0.01) compared to the Complex diets. In period 2, growth was not affected (P > 0.05) by previous diet complexity, and pig body weight was similar (114.4 vs 114.4 ± 0.37 kg; P > 0.05) at the end of wk 23. In period 1, pigs with Restricted space had lower ADG (398 vs 434 ± 3.8 g; P < 0.001), ADFI (621 vs 668 ± 5.4 g; P < 0.001), and gain:feed ratio (0.64 vs 0.65 ± 0.002; P < 0.01), and were lighter at the end of wk 8 (6.5%; P < 0.001) than those with Unrestricted space. However, in period 2, pigs with Restricted space had higher (P < 0.01) ADG (3%), ADFI (2%), and gain:feed ratio (3%) than those with Unrestricted space, and body weight was similar (114.5 vs 114.3 ± 0.37 kg; P > 0.05) at end of wk 23. Carcass backfat and loin-eye depth at market body weight were influenced by neither diet nor space treatment. Using a simple diet program and restricted space allowance immediately postweaning resulted in a lower early growth rate, but had no impact on pig body weight or carcass measures at market.

Key Words: Diet • Floor Space • Growth • Pigs

	Introduction

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The relationship between early and late growth of pigs is of significant practical relevance, but is not clearly established. In a recent study, Wolter et al. (2002b) found that pigs exhibiting decreased early growth rate due to increased number of pigs per pen had a 4% increase in feed efficiency when provided adequate space in the subsequent grow-finish period. However, other research has found little impact of modest variation in early growth rate on subsequent growth performance (Brumm et al., 2001; Wolter and Ellis, 2001).

In commercial practice, diet complexity and space allocation are two important factors affecting the cost of pig production. Previous research has demonstrated that both feeding corn-soybean meal-based diets that minimize the inclusion of milk, processed cereals, and animal protein-based ingredients (i.e., simple compared to complex diets) (Himmelberg et al., 1985; Whang et al., 2000) and increasing the number of pigs per unit of floor area (Kornegay and Notter, 1984; Brumm et al., 2001) can restrict growth rate during the nursery period. Moreover, research with growing pigs has found that simultaneously imposing factors on pigs that reduce growth rate can result in an additive negative effect on growth performance (Hyun et al., 1998). However, pigs that experience a period of restricted growth rate and in a later period exhibit increased growth performance may still have similar overall performance to those that have unaltered growth all the way to market. There has been insufficient scientific evidence to either support or refute the potential for pigs to exhibit increased growth performance following a period of restricted growth under commercial production conditions. The objective of the research reported here was to investigate the effect of postweaning pig growth rate as affected by diet complexity, space allocation, or both on subsequent growth performance to market weight in a commercial wean-to-finish system.

	Materials and Methods

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Experimental Design.

The experiment was conducted at the United Feeds, Burton Russell Swine Research Farm (United Feeds, Inc., Frankfort, IN) during the period from May through October. The experimental protocol was approved by the University of Illinois Laboratory Animal Care Advisory Committee.

The experiment was carried out as a randomized complete block design with a 2 x 2 factorial arrangement of treatments with eight replicates blocked by day of weaning. The study was divided into two periods: period 1 was from weaning to the end of wk 8 postweaning and period 2 was from the start of wk 9 to the end of wk 23 postweaning. Treatments were imposed during period 1 and consisted of: 1) diet complexity (Complex vs Simple) and 2) space allocation (Unrestricted vs Restricted). In period 2, pigs on all treatments were managed identically and provided the same diet regimen and space allocation.

Animals.

A total of 1,728 crossbred pigs (Ausgene Line 5 sires x Ausgene Line 13 dams) were weaned at 15 d of age (mean BW = 5.0 ± 0.01 kg) and randomly allotted to mixed-sex treatment pens of 54 pigs from outcome groups of four animals formed on the basis of sex and BW. The ratio of barrows to gilts was kept constant across treatments within replicate.

Diets and Housing.

Pigs had ad libitum access to feed and all diets were formulated to meet or exceed NRC (1998) nutrient requirements. At the start of the experiment, pens of pigs on all treatments within a replicate were provided the same quantity of a pelleted starter diet (United Feeds Inc., Sheridan, IN) based on BW (1 or 2 kg/pig for pen-average BW greater than or less than 5.2 kg, respectively). Following the starter diet, experimental dietary treatments were fed in meal form using a three-phase regimen in which phases I, II, and III were fed for wk 1 and 2, wk 3 and 4, and wk 5 through wk 8 postweaning, respectively. The Simple diet was based on a corn-soybean meal diet that minimized the inclusion of milk, processed cereals, and animal-protein-based ingredients compared to the Complex. During period 1 of the study, samples (0.2 kg) of diets were taken from each feeder once per week and pooled, and a subsample was obtained for chemical analysis. Details of the experimental diets are presented in Table 1. In the period subsequent to wk 8, a standard multiple-phase dietary regimen was fed to all pigs (Table 2). During this period, pigs were fed according to a feeding budget such that every pen of pigs was given the same quantity at each dietary phase.

Temperature inside the building was maintained using thermostat-controlled heating and fan-ventilation systems. The thermostat was set at 24°C for wk 1 and 2, and then lowered by 2°C/wk until it reached 18°C, where it remained. During wk 1 postweaning, supplementary heat was provided in each pen by one propane brooder suspended 120 cm above the floor.

Growth Measurement.

All pigs were weighed individually at the beginning, end of wk 8, and at the end of the experiment (wk 23). The coefficient of variation in individual pig BW was calculated for each pen. Pigs experiencing health problems or injuries that did not respond to therapeutic treatment were removed from the study; date of and BW at removal were recorded and used in calculating growth performance.

Feed data were collected using a computerized feed-mixing (L.O.M.A.N. Systemtechovik, Bremerhaven, Germany), delivery (AZA International, Medolago, Italy), and recording (Fancom B.V., Pannigen, The Netherlands) system. The mixer was fitted with a load cell (calibrated each week) that recorded the weight of the feed dispensed to each feeder. Four 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 actual amount delivered and amount recorded was -0.01 % (SD = 0.45%). Feed remaining in each feeder was measured on each pig weigh day to determine feed intake and gain:feed ratio.

Carcass Measurement.

Pigs were ultrasonically scanned at the end of the test period using 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 midline. From the ultrasound image, backfat and loin depth were measured automatically using the AUSKey system (Animal Ultrasound Services, Inc., Ithaca, NY), and percent carcass lean was predicted using an equation published by Liu and Stouffer (1995).

Statistical Analysis.

Pig performance data were analyzed as a randomized block design and pen was considered the experimental unit. All data were tested for normality using PROC UNIVARIATE procedures of SAS (SAS Inst., Inc., Cary, NC). Data for percent mortality and morbidity did not conform to a normal distribution (Steel and Torrie, 1980); consequently, the Friedman’s ² rank-based test (Steel and Torrie, 1980) was carried out using PROC RANKS procedures of SAS. Data meeting criteria for normality were analyzed using the GLM procedure of SAS. The model included effects of diet complexity, space allocation, room, day of weaning (block) 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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No interactions (P > 0.05) were found for any variables in either period; therefore, only treatment main effects are presented. Growth performance data are summarized in Tables 3 and 4, and carcass measures taken at market BW in Table 5.

Pigs fed Simple vs Complex diets were lighter (2.8%; P < 0.01) and had greater (P = 0.05) variation in BW within a pen at the end of wk 8 after weaning (Table 3). Diet complexity had its greatest impact on pig growth performance during the early period after weaning (Table 4). During the first 2 wk after weaning (i.e., dietary phase I), pigs fed the Simple diet had lower ADG (19.2%; P < 0.001), ADFI (14.2%; P < 0.001) and gain:feed ratio (6.0%; P < 0.001) than those fed the Complex diet. From wk 3 through 4 (i.e., dietary phase II), pigs fed the Simple diet had a lower ADFI (3.8%; P < 0.05) and tended to have a lower ADG (3.4%; P = 0.08), but similar (P > 0.05) gain:feed ratio than those fed the Complex diet. Diet treatment did not impact (P > 0.05) ADG or ADFI of pigs from wk 5 through 8 (i.e., dietary phase III), but pigs fed the Simple diet had a lower (P < 0.05) gain:feed ratio than those fed the Complex diet. Diet complexity did not affect (P > 0.05) rates of pig removal due to injury, illness, or death.

Previous research has also shown that feeding pigs simple as opposed to complex diets during the early postweaning period results in lower BW gain (Himmelberg et al., 1985; Dritz et al., 1996; Whang et al., 2000). Similar to the current study, the lower growth rate observed in pigs fed simple diets in previous studies resulted from both lower feed intake and lower feed efficiency (Okai et al. 1976; Graham et al., 1981; Dritz et al., 1996). Moreover, Stairs et al. (1991) and Dritz et al. (1996) found that the increased feed intake associated with increased diet complexity was most pronounced in the immediate postweaning period. Similarly, in the current study, the impact of diet program on pig growth decreased with increasing time postweaning, and therefore the impact of diet complexity on BW at the end of wk 8 postweaning, albeit significant, was relatively small.

There is little if any scientific information on the impact of diet complexity on pig mortality and morbidity. Results of the current study suggest little impact, but these results may well vary across diet programs and production conditions, and therefore require further validation.

In the subsequent period, from wk 9 to 23, when pigs were fed the same diet regimen, growth performance was not affected (P > 0.05) by previous dietary treatment (Table 4). In addition, the slight advantage in BW for Complex-treatment pigs compared to Simple-diet pigs at the end of wk 8 was not maintained, and pig BW was similar (P > 0.05) for both diet treatments at the end of wk 23 (Table 3). However, variation in pig BW within a pen at the end of wk 23 tended to be greater (P = 0.10) for pigs fed Simple than those fed Complex diets during the nursery period. Previous research also has indicated that differences in early postweaning growth resulting from feeding simple vs complex diets had little impact on growth performance in the subsequent growing-finishing period (Hancock et al., 1994; Whang et al., 2000). Some previous research has found that the advantage in BW at the end of the nursery period was maintained to market with a concomitant decrease in the time required to reach market for pigs that had higher growth rates during the nursery period (Hancock et al., 1994; Chiba, 1995; Dritz et al., 1996). In contrast, Whang et al. (2000) reported that pigs fed simple vs complex diets were lighter at the end of the nursery period, but had similar BW at the fixed time of market (152 d postweaning). Inconsistencies among studies in the effect of early diet complexity on overall pig growth performance may be associated with differences in the levels of nutrients supplied in the diets fed after the nursery period (Chiba, 1995). Nevertheless, results of the current study suggest that the use of complex diets in efforts to achieve maximal pig growth rate during the early postweaning period may not be necessary to optimize overall performance from weaning to market BW. However, the trend for increased BW variation within a group at market observed for pigs fed simple diets would have an economic impact in commercial practice, and therefore, the impact of diet complexity on pig BW variation within a group warrants further investigation.

In the current study, carcass measures at market BW were similar (P > 0.05) among diet treatments (Table 5). Dritz et al. (1996) found carcass measures of pigs at market were not affected by the complexity of the diet fed in the nursery. Martin et al. (1974) and Gaines et al. (2002) reported that backfat and longissimus area at market BW were unaffected by nutritional treatment during early growth. Therefore, it appears that feeding a nursery diet that does not support maximal early rate of growth may have little influence on carcass measures at market BW.

Effect of Space Allocation.

Pigs on Restricted-space treatment were lighter (6.5%; P < 0.001) than those on the Unrestricted treatment at the end of wk 8 postweaning (Table 3). However, variation in pig BW within a pen at wk 8 was not influenced (P > 0.05) by space treatment. Pigs with Restricted vs Unrestricted access to space had lower ADFI (4.4%; P < 0.01), but similar (P > 0.05) ADG and gain:feed ratio during the initial 2 wk postweaning (Table 4). From wk 3 to 4, pigs on the Restricted-space treatment had lower ADFI (5.0%; P < 0.01) and ADG (4.7%; P < 0.01), but similar (P > 0.05) gain:feed ratio than those on the Unrestricted treatment. Access to feeder-trough space and to floor space had the greatest impact on pig performance from wk 5 to 8, during which period pigs on Restricted had lower (P < 0.001) ADFI (7.6%), ADG (9.7%), and gain:feed ratio (2.4%) than those on the Unrestricted-space treatment.

Both feeder-trough and floor spaces were decreased for the pigs with Restricted space and, therefore, it is not possible to identify which factor(s) may have been responsible for the lower growth rate observed. However, previous research results suggest both factors may have impacted growth performance (Wolter et al., 2002a, b). With respect to floor space, previous results suggest that both ADG and ADFI decrease with decreasing floor-space allowance, and a similar, but smaller, effect has been noted for feed efficiency (Kornegay and Notter, 1984). In an earlier study conducted in the same facility, the 2-cm/pig feeder-trough space allowance used in the current study limited pig growth rate after wk 6 postweaning (Wolter et al., 2002b). In contrast to the simple diet treatment, the effect of restricted space on pig growth increased as number of days postweaning increased, and space restriction had greater impact on BW at the end of wk 8 postweaning.

Lack of interaction between diet complexity and space allocation suggests an additive effect of these two factors on pig performance. This is illustrated by the interaction means for BW at the end of wk 8 postweaning: 29.8, 28.9, 27.8, and 27.1 kg for Complex-Unrestricted, Simple-Unrestricted, Complex-Restricted, and Simple-Restricted treatment subclasses, respectively. These data suggest that the effect of diet complexity was similar in both Unrestricted and Restricted pigs. Previous studies that have attempted to overcome the decrease in pig performance due to decreased space by employing various dietary approaches, including higher amino acid concentrations (Kornegay et al., 1993a; Ferguson et al., 2001), higher vitamin and trace mineral concentrations (Yen and Pond, 1987; Kornegay et al., 1993b), and addition of antibiotics (Yen and Pond, 1987) have largely been unsuccessful.

Restricted space for the first 8 wk after weaning resulted in increased (P < 0.01) rate of pig removal from the study due to death, poor health, or injury during that period (Table 3). In contrast, recent studies investigating the effects of decreased feeder-trough and floor-space allowance by increasing the number of pigs per pen found no increase in the number of pigs that died or were removed for injuries or poor performance (Brumm et al., 2001; Wolter et al., 2002b). Therefore, the impact of space allowance on the rate of pig mortality and morbidity remains unclear and may depend on other environmental factors that require further study.

Subsequent to wk 8 through the end of the study (wk 23 postweaning), during which period pigs on all treatments were allowed the same feeder-trough and floor space, pigs on the Restricted-space treatment had greater ADFI (2.1%; P < 0.01), ADG (3.0%; P < 0.01), and gain:feed ratio (2.4%; P < 0.01) than those on the Unrestricted diet. As a result, pigs on both treatments had similar (P > 0.05) BW at the end of study (Table 3). In addition, ultrasonically measured backfat and loin depth were similar (P > 0.05) among treatments at market BW (Table 5). These results concur with previous research, which suggested that pigs exhibiting lower early growth rate due to increased number of pigs per pen had increased feed efficiency when provided adequate space in the subsequent grow-finishing period (Wolter et al., 2002b). The degree to which pigs may increase growth rate after a period of restriction can be influenced by the severity and duration of restriction (Prince et al., 1983), as well as the stage of growth at which the restriction occurs (Mersmann et al., 1987). Obviously, pigs on the Restricted vs Unrestricted floor-space treatment experienced a change in environment as well as an increase in floor space when space allowance was increased at 8 wk postweaning. However, the contribution, if any, of this change in environment to the improved growth performance of Restricted pigs is not clear.

The improvement in feed efficiency observed in this study would have an impact on the economics of finishing-pig production, and therefore, a further understanding of the impacts of degree and length of restriction in pigs at various stages of growth is warranted.

	Implications

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The results suggest that pigs that experience a moderate decrease in early growth rate due to either feeding simple diets or restricting space allowance (and thereby are lighter at week 8 postweaning), can achieve increased growth performance in the subsequent period without impacting carcasses at market. Nevertheless, the impact of floor space on animal well-being needs to be considered. Therefore, management strategies that maximize pig growth rate during the early period postweaning may not be necessary to achieve an optimal overall growth performance from weaning to market. The increased rate of pig removal observed for pigs experiencing a restricted space allowance is practically significant and warrants further investigation.

	Footnotes

 
¹ The authors gratefully acknowledge the Illinois Council on Food and Agricultural Research and the College of Agricultural, Consumer, and Environmental Sciences, University of Illinois, for financial support of this research and United Feeds, Inc., Sheridan, IN, for cooperation and use of facilities.

Received for publication May 20, 2002. Accepted for publication September 2, 2002.

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