J. Anim. Sci. 2005. 83:1361-1369
© 2005 American Society of Animal Science
Effects of ractopamine on performance and composition of pigs phenotypically sorted into fat and lean groups1
K. J. Mimbs*,
T. D. Pringle*,2,
M. J. Azain*,
S. A. Meers* and
T. A. Armstrong
* Edgar L. Rhodes Center for Animal and Dairy Science, University of Georgia, Athens 30602-2771; and
and
Elanco Animal Health, a Division of Eli Lilly and Co., Indianapolis, IN 46240
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Abstract
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Crossbred barrows (n = 144; 80 kg) from four farrowing groups were phenotypically selected into fat (FAT) and lean (LEAN) pens using ultrasound. The difference in 10th-rib fat depth between the LEAN and FAT groups was 
0.5 cm. Within a farrowing group, pigs were assigned to pens (five pigs per pen and eight pens per phenotype) to equalize pen weight and fat depth. Pigs were fed a corn-soybean meal diet containing 19% CP, 1.0% added animal/vegetable fat, and 1.1% lysine (as-fed basis). Half the pens received 10 ppm (as-fed basis) of ractopamine (RAC) during the 28-d finishing phase. At 7-d intervals, live weight and feed disappearance were recorded to calculate ADG, ADFI, and G:F, and 10th-rib fat depth and LM area were ultrasonically measured to calculate fat-free lean and fat and muscle accretion rates. During the first 7 d on feed, LEAN pigs fed RAC gained less (P < 0.05) than FAT pigs fed RAC or LEAN and FAT pigs fed the control diet (RAC x phenotype; P = 0.02); however, RAC did not (P > 0.25) affect ADG after the second, third, and fourth weeks, or over the entire 28-d feeding period. Although wk-2 and -3 ADG were higher (P 
0.03) in LEAN than in FAT pigs, phenotype did not (P = 0.08) affect overall ADG. Dietary RAC decreased (P 0.05) ADFI over the 28-d feeding trial, as well as in wk 2, 3, and 4, but intake was not (P > 0.20) affected by phenotype. Neither RAC nor phenotype affected (P > 0.10) G:F after 7 d on trial; however, RAC improved (P 0.04) wk-3, wk-4, and overall G:F. Lean pigs were more efficient (P 0.05) in wk 2 and 3 and over the duration of the trial than FAT pigs. Ultrasound LM accretion (ULA) was not (P 0.10) affected by RAC; however, LEAN pigs had greater (P 0.02) ULA in wk 2 and 4 than FAT pigs. Although fat depth was lower (P < 0.01) in RAC-fed pigs than pigs fed the control diet, ultrasound fat accretion rate indicated that RAC-pigs deposited less (P = 0.04) fat only during wk 4. In addition, calculated fat-free lean (using ultrasound body fat, ULA, and BW) was increased (P < 0.05) in RAC pigs after 3 and 4 wk of supplementation. In conclusion, RAC enhanced the performance of finishing swine through decreased ADFI and increased G:F, whereas carcass lean was enhanced through decreases in carcass fat and increases in carcass muscling.
Key Words: Growth • Fat-Free Lean • Phenotype • Ractopamine • Swine
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Introduction
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Paylean, the trade name for the repartitioning agent ractopamine hydrochloride (RAC; Elanco Animal Health, Greenfield, IN), was approved by the FDA for use in finishing swine in 1999. Research has shown that RAC improves live performance in swine by increasing ADG (Stites et al., 1991
; He et al., 1993; Dunshea et al., 1998), decreasing ADFI (Crenshaw et al., 1987; Aalhus et al., 1990), improving G:F (Watkins et al., 1990; He et al., 1993; Dunshea et al., 1998), and simultaneously increasing carcass lean-to-fat ratio (Watkins et al., 1990; Bark et al., 1992).
Current management practices in the swine industry have resulted in considerable variation in lean growth potential and growth performance of common-sourced pigs. Yen et al. (1990) supplemented diets of obese and lean pigs with RAC, and reported improvements in ADFI and G:F between these diverse genotypes. However, there is little known about the response of pigs with varied lean growth potentials from within a genotype (i.e., phenotypically fat vs. lean pigs), particularly with regard to lean and fat accretion during the finishing period. Thus, our objective was to determine the effects of RAC supplementation on performance and ultrasound fat and muscle measurements of pigs varying in prefinishing 10th-rib fat depth.
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Materials and Methods
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The crossbred barrows (n = 144; with an initial BW of 80 kg) used in the present study originated from four farrowing groups (Table 1) at The University of Georgia Swine Center. Pigs in the first three farrowing groups were from Yorkshire x Landrace x Hampshire females mated to Duroc boars; however, because of a change in genetics, pigs in the last farrowing groups were from PIC C-42 females mated to PIC 426 boars (Pig Improvement Co., Franklin, KY). Pigs were measured for 10th-rib fat depth using ultrasound and sorted into lean (LEAN) or fat (FAT) phenotypes. The 10th-rib fat depth difference between the FAT and LEAN groups was 0.5 cm. Within a farrowing group and phenotype, pigs (three pigs per pen for Replicate 1 and five pigs per pen for Replicates 2, 3, and 4) were assigned to pens (eight pens per farrowing group) to equalize pen weight and backfat depth. Pigs received a corn-soybean meal basal diet containing 19% CP and 1.1% lysine (Table 2). Half the pens received 10 ppm RAC, which was added to the diet at the expense of ground corn. All pigs had ad libitum access to feed and water throughout the 28-d experimental feeding period.
Feed intake, live weights, and ultrasound images for 10th-rib fat depth and LM area measures were collected at 7-d intervals. Images were collected using an Aloka 500-V ultrasound unit (Corometrics Medical Systems, Wallingford, CT) with a 17.2-cm, 3.5-MHz linear probe, and were interpreted with Beef Information Manager software (Version 3.5; Critical Vision Inc., Atlanta, GA). Accretion rates for 10th-rib backfat (UBF) and LM area (ULA) were calculated by determining the change in the measurement in a given time period and dividing the change by the number of days in that time period (i.e., overall UBF accretion = [UBF at d 28 – UBF at d 0]/28 d; and wk 1 UBF accretion = [UBF at d 7 – UBF at d 0]/7 d). In addition, the 10th-rib fat depth and LM areas were used, in combination with live weight, to calculate the expected percentages of fat-free lean (FFL) of the pigs (NPPC, 1999). Kilograms of FFL were calculated using the following equation: 0.9972 –(2.6397 x 10th-rib fat depth, cm) + (0.3485 x LM area, cm2) + (0.3312 x live weight, kg). Percentage of FFL was then calculated by dividing kilograms of FFL by predicted carcass weight, assuming a dressing percent of 74% (NPPC, 1999).
Data were analyzed using GLM procedures of SAS (SAS Inst., Inc., Cary, NC) for a 2 x 2 factorial design with the main effects of RAC (0 vs. 10 ppm) and phenotype (LEAN vs. FAT). Replicate was not a significant source of variation in this study; however, replicate and replicate interactions were included in the model to remove replicate-associated variation. Pen was used as the experimental unit for the analysis. Least squares means were generated and separated using the LSD procedures when there was a significant (P 0.05) F-test for a main effect or interaction.
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Results
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Growth Performance
Mean BW were not (P = 0.13) different between pigs fed 0 or 10 ppm RAC at any time during the study (Table 3). Pigs of the FAT phenotype were heavier (P < 0.01) than those of the LEAN phenotype at the beginning of the experiment, and this BW advantage was maintained through the first 14 d of the study; however, BW did not differ (P 0.50) between phenotypes during the third and fourth weeks and across the 28-d feeding period.
There were no main effects (P > 0.25) on weekly or overall ADG (Table 3
). Lean pigs fed RAC had lower (P < 0.05) ADG during the first week on trial than FAT pigs fed RAC or either phenotype fed 0 ppm RAC (RAC x phenotype; P = 0.02; Figure 1). During the first week on the trial, ADFI was not (P = 0.15) affected by dietary inclusion of RAC; however, ADFI was decreased (P < 0.05) after the second, third, and fourth weeks, as well as across the entire 28-d trial in RAC-fed pigs. Gain:feed was not (P > 0.60) affected by RAC after 7 or 14 d on trial, although by the end of the third and fourth week on feed, G:F was improved (P < 0.05) by the addition of the 10 ppm RAC in the finishing diet. Over the course of the 28-d finishing period, pigs fed 10 ppm RAC were more (P < 0.01) efficient than pigs fed 0 ppm RAC.
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Figure 1. The interactive effect (P = 0.02) of dietary ractopamine (RAC) inclusion level (0 vs. 10 ppm) and phenotype (lean vs. fat) on ADG during the first week on feed. Bars without a common letter differ at P < 0.05.
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Average daily gain during the second (P = 0.03) and third (P = 0.02) weeks on feed was greater in LEAN than FAT pigs; however, phenotype had no (P > 0.20) effect on ADG during the first and fourth weeks, or across the entire finishing period (Table 3). Average daily gain of FAT pigs decreased (P < 0.05) over time, whereas LEAN pigs maintained their early gains through the third week of the study. As expected, ADFI increased (P < 0.01) over the course of the study; however, weekly and overall ADFI were not (P 0.20) affected by phenotype (Table 3). Feed efficiency at the end of the second and third weeks, as well as overall G:F, was higher (P 0.05) in LEAN vs. FAT pigs; however, G:F was not (P > 0.40) different between phenotypes at other measurement times.
Fat, Muscle, and Fat-Free Lean Accretion
As expected, ultrasound fat thickness increased (P < 0.01) over time in pigs fed 0 and 10 ppm RAC (Figure 2). Although an attempt was made to minimize fat depth differences during assignment of pigs to treatments, RAC pigs initially (0 d) tended to be fatter (P = 0.08) than those not fed RAC. By the end of the trial, however, pigs fed 10 ppm RAC were leaner (P < 0.01) than those fed 0 ppm RAC. Additionally, RAC had no effect (P 0.10) on wk 1, 2, or 3 UBF accretion rates (Table 4); however, during the fourth week of the trial, RAC pigs deposited less (P = 0.05) fat than those fed diets with 0 ppm RAC (Table 4). It is interesting to note that as the trial progressed, the effects of RAC became more evident, such that UBF accretion in the fourth week and across the 28-d trial were lower (P 0.05) with RAC feeding (Table 4). Moreover, FAT pigs fed the diet devoid of RAC and pigs fed the RAC diet maintained their level of fatness (RAC x phenotype; P = 0.04; Figure 3). As expected, UBF depth was lower (P 0.01) in LEAN pigs on d 0, 7, 14, 21, and 28 compared with FAT pigs (Figure 2). Neither weekly nor overall UBF accretion was affected (P > 0.50) by phenotype (Table 4).
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Table 4. The effects of ractopamine feeding and phenotype on 10th-rib fat depth and longissimus muscle area accretiona
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Figure 3. The interactive effect (P = 0.05) of dietary ractopamine (RAC) inclusion level (0 vs. 10 ppm) and phenotype (lean vs. fat) on 10th-rib fat depth accretion rate during the second week on feed. Bars without a common letter differ at P < 0.05.
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Ultrasound LM area in RAC pigs was larger (P < 0.05) on d 21 and 28 compared with pigs fed diets without RAC (Figure 4); however, RAC supplementation had no effect (P 0.10) on ULA accretion (Table 4). Lean pigs tended to have larger (P < 0.10) ULA at d 0 and 14 and, by the end of the trial (d 28), ULA were larger (P < 0.05) compared with the FAT pigs (Figure 4). In addition, during the second (P = 0.07) and fourth (P < 0.01) weeks of the trial, ULA was higher in LEAN than FAT pigs; however, overall ULA was not (P = 0.59) affected by phenotype.
Carcass FFL percents, calculated from NPPC equations, were significantly affected by both diet and phenotype (Figure 5). Improvements in FFL after the third (P < 0.05) and fourth (P < 0.01) weeks of the trial were noted in the RAC-fed pigs compared with those consuming diets without RAC. In addition, LEAN pigs had higher (P < 0.01) calculated FFL than FAT pigs over the entire course of the feeding trial.
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Figure 5. The main effects of dietary ractopamine (RAC) inclusion level (0 vs. 10 ppm; A) and phenotype (lean vs. fat; B) on percentage of fat-free lean across the 28-d feeding period. Within a specific time on feed, means with a single asterisk (*) differ at P < 0.05, whereas means with a double asterisk differ at P 0.01.
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Discussion
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Growth Performance
Average daily gain was not affected by RAC in this study, which is consistent with reports by Aalhus et al. (1990) and Yen et al. (1990). However, a number of other studies (He et al., 1993; Crome et al., 1996; Xiao et al., 1999) have reported that RAC increased ADG.
Some researchers have observed significant decreases in ADFI in RAC-fed pigs (Crenshaw et al., 1987; Yen et al., 1990; Bark et al., 1992), whereas others have reported no effect of RAC on ADFI (Stites et al., 1991; He et al., 1993; Xiao et al., 1999). In the present study, dietary inclusion of RAC decreased ADFI, and when coupled with similar gains, feeding RAC improved G:F. One of the most common observations with feeding pigs RAC is an improvement in feed efficiency, similar in magnitude to the improvement noted in the current study (Stites et al., 1991; Bark et al., 1992; He et al., 1993). Nonetheless, Crenshaw et al. (1987) and Aalhus et al. (1990) reported that efficiency was not affected by feeding 10 ppm RAC. Previously published live performance results are variable in terms of RAC-stimulated changes, and this variability may be due to a number of factors, such as length of supplementation (28 to >35 d), dietary CP level (13 to 20%), and the level of dietary RAC inclusion (5 to 20 ppm). In addition, pigs with average levels of ultrasound 10th-rib fat depth were removed from the farrowing groups used in the current study, and this experimental design may have affected the performance responses noted in RAC-fed pigs compared with previously published results.
Growth rates of the LEAN and FAT pigs were not different in this study, which agrees with the findings of Neely et al. (1979). The average backfat difference between the FAT and LEAN phenotypes used by Neely et al. (1979) was 0.2 cm, whereas the difference between FAT and LEAN groups used in the present study was 0.5 cm, which is a greater than 50% difference in selection criteria between the studies. Conversely, Friesen et al. (1994) reported that pigs with lower lean gain potential had lower ADG than their leaner counterparts.
Feed intake was not affected by phenotype, which is consistent with other studies comparing ADFI of differing phenotypes (Neely et al., 1979; Freisen et al., 1994; McNeel et al., 2000). With respect to efficiency of gain, Neely et al. (1979) reported that G:F was not different between FAT and LEAN pigs, whereas Freisen et al. (1994) observed that pigs with higher lean gain potentials had higher G:F, which concurs with the findings of this study. These discrepancies in the literature may be due to genotype differences in the pigs studied, and may not be comparable to the present results because pigs of similar genotype were sorted phenotypically into fat and lean groups.
Fat, Muscle, and Fat-Free Lean Accretion
Composition of lean gain, measured by ultrasound, was improved by RAC in this study through decreases in fat accretion (Figure 2) coupled with larger LM areas (Figure 4). These changes resulted in improvements in calculated percentage of FFL (Figure 5) in the RAC-supplemented pigs after 21 and 28 d on feed. This should result in economic advantages for dietary inclusion of RAC because the RAC-fed pigs had a greater than 1.0% improvement in FFL after 28 d of supplementation compared with pigs fed the finishing diet that did not contain RAC. Other studies have reported increased muscle accretion (Bark et al., 1992; Sainz et al., 1993; Dunshea et al., 1998) and decreased fat accretion (Bark et al., 1992; He et al., 1993) in pigs fed RAC; however, accretion in those studies was determined by actual carcass measurements as opposed to ultrasound measurements. In addition, most of those studies reported greater changes in muscle accretion than in fat accretion.
Although ultrasound has been shown to be an accurate tool for assessing live animal composition, measurement errors exist (Houghton and Turlington, 1992). It has been reported that the most difficult ultrasound trait to accurately measure is LM area (Smith et al., 1992). In pigs, Moeller and Christian (1998) reported that the error associated with estimating LM area was an underestimation of large LM areas and overestimation of small LM areas, whereas Perkins et al. (1992) suggested that ultrasound was slightly more precise for beef cattle with smaller LM areas. Results comparing ultrasound measurements to carcass measurements in a sub-sample (n = 56) of the population used in this study revealed that the ultrasound data was acceptably accurate (R2 0.80 for both 10th-rib fat depth and LM area).
Other possible explanations for the differences found in this study compared with the literature may include the fact that pigs in the leanest and fattest subgroups were selected for use. The elimination of pigs with average fat depths from this study was a unique experimental approach compared with other studies. Moreover, the level of RAC included in the finishing diet was less than that used by other researchers, and less than the maximum inclusion level allowed by FDA.
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Implications
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Results of this study indicate that including ractopamine hydrochloride in swine finishing diets should be economically advantageous as a result of improved feed efficiency, as well as increased carcass value through altered composition of gain. Furthermore, the lack of interactions between ractopamine and phenotype for most major traits suggests that ractopamine is equally efficacious in improving performance and composition of both lean and fat pigs within a genotype.
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Footnotes
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1 The authors thank T. Glaze for his assistance with collection and interpretation of ultrasound data, and Elanco Animal Health for financial support of this study. 
2 Correspondence—phone: 706-542-0997; fax: 706-542-0399; e-mail: dpringle{at}uga.edu.
Received for publication March 1, 2004.
Accepted for publication March 7, 2005.
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Abstract
Introduction
