Time-response relationship of ractopamine feeding on growth performance, plasma urea nitrogen concentration, and carcass traits of finishing pigs

and V. S. MiyadaV. V. Almeida, A. J. C. Nuñez, A. P. Schinckel, C. Andrade, J. C. C. Balieiro, M. Sbardella

urea nitrogen concentration, and carcass traits of finishing pigsTime-response relationship of ractopamine feeding on growth performance, plasma

doi: 10.2527/jas.2012-5372 originally published online January 10, 20132013, 91:811-818.J ANIM SCI 

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Time-response relationship of ractopamine feeding on growth performance, plasma urea nitrogen concentration, and carcass traits of fi nishing pigs1

V. V. Almeida,*2 A. J. C. Nuñez,† A. P. Schinckel,‡ C. Andrade,* J. C. C. Balieiro,§ M. Sbardella,* and V. S. Miyada*

*Department of Animal Science, University of São Paulo, Piracicaba, 13418-900, Brazil; †Department of Animal Science, University of São Paulo, Pirassununga, 13635-900, Brazil; ‡Department of Animal Sciences, Purdue University, West

Lafayette, IN 47907; and §Department of Basic Science, University of São Paulo, Pirassununga, 13635-900, Brazil

ABSTRACT: Ractopamine hydrochloride (RAC) improves swine production effi ciency by redirecting nutrients to favor muscle accretion rather than fat deposition. In the present study, the time-dependent effect of RAC feeding on performance, plasma urea N (PUN) concentrations, and carcass traits of fi nishing pigs were evaluated. In a 28-d growth study, 80 barrows (average initial BW = 69.4 ± 7.9 kg) were assigned to 1 of 5 treatments in a randomized complete block design with 8 replicate pens per treatment and 2 pigs per pen. The pigs were fed a corn–soybean meal-based diet with no added RAC (control) or 10 mg of RAC/kg fed for 7, 14, 21, or 28 d before slaughter. All diets were formulated to contain 0.88% standardized ileal digestible Lys (1.0% total Lys) and 3.23 Mcal of ME/kg. Individual pig BW and pen feed disappearance were recorded weekly to determine BW changes, ADG, ADFI, and G:F. Anterior vena cava blood samples were taken on d 28 for determination of PUN concentrations. After 28 d on trial, the pigs were slaughtered and carcass measurements made at 24 h

postmortem. Overall, providing pigs with different RAC feeding durations did not affect the fi nal BW and ADFI but resulted in a tendency (P = 0.09) for a linear increase in ADG and a linear improvement (P = 0.003) in G:F. No effect of RAC feeding was found for weekly ADFI. Weekly improvements (P < 0.05) in ADG and G:F were observed over the fi rst 21 d of RAC feeding. However, the growth response declined (P < 0.05) in wk 4 of RAC treatment. The concentrations of PUN exhibited a quadratic decrease (P = 0.004) as the RAC feeding duration increased. Although RAC feeding did not affect any backfat measurements and carcass length, increasing the RAC feeding duration linearly increased HCW (P = 0.01), dressing percentage (P = 0.03), LM depth (P = 0.001), LM area (P < 0.001), muscle-to-fat ratio (P = 0.004), and predicted carcass lean percentage (P = 0.02). These results indicate that a greater growth rate was achieved within the fi rst 21 d of RAC feeding whereas the magnitude of carcass response was directly dependent on the duration of RAC feeding.

Key words: carcass leanness, growth, pigs, ractopamine, urea

© 2013 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2013.91:811–818 doi:10.2527/jas2012-5372

INTRODUCTION

Because of the implementation of carcass merit pricing and evaluation systems, the use of ractopamine hydrochloride (RAC; Ractosuin; Ourofi no Animal

Health, Cravinhos, Brazil) has become one of the most successful nutritional strategies to improve pork carcass leanness. Ractopamine is a β-adrenergic agonist that directs nutrients toward lean muscle accretion. In this regard, RAC effectively improves G:F and fat-free lean yield (Weber et al., 2006; Apple et al., 2008). However, the magnitude of the reductions in backfat thickness remains controversial (Dunshea et al., 1993, 1998, 2005).

Ractopamine is commonly used in fi nishing pig diets for 28 d before slaughter, but some researchers have reported that pigs receiving RAC at a constant dosage showed maximum growth response during,

1The authors express their appreciation to the São Paulo Research Foundation (FAPESP) for its fi nancial support for this experiment. A scholarship for V. V. Almeida graduate study was generously provided by FAPESP. Additionally, we gratefully acknowledge the help of B. Berenchtein for carcass evaluation and J. L. F. Andrade for assistance with the halothane genotype analysis.

2Corresponding author: [email protected] April 11, 2012.Accepted December 17, 2012.

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Almeida et al. 812 Almeida et al. 812

Kelly et al., 2003; Schinckel et al., 2003a). Although

pigs are well documented, additional studies should be conducted to better characterize carcass traits for RAC feeding periods of 21 d or less (Kelly et al., 2003).

Armstrong et al. (2004) indicated that performance improvements are expected for shorter RAC feeding duration, but a longer feeding duration can be an

RAC-treated pigs fed a higher-CP diet (17.8%) showed a linear increase in carcass yield as the RAC feeding duration increased from 7 to 35 d before slaughter

the RAC treatment duration that maximizes N use are known in the literature. Therefore, the objective of the study was to determine the effect of RAC feeding duration on growth performance, plasma urea N (PUN)

MATERIALS AND METHODS

All experimental procedures were approved by the Luiz de Queiroz College of Agriculture Animal Care and Use Committee before initiation of this study.

Animals, Experimental Design, and Diets

BW of 69.4 ± 7.9 kg were transported from a commercial farm to the Luiz de Queiroz College of Agriculture Swine Research Unit (Piracicaba, Brazil) and used in a 28-d feeding trial. Using the PCR-RFLP technique, halothane (Hal) genotypes [(homozygous halothane-negative (HalNN), heterozygous halothane-negative (HalNn), and homozygous halothane-positive (Halnn)] were determined in this study. The pigs were housed in pens with a partially

2 (1.20 by 2.90 m). Each pen was equipped with a single spaced feeder and one nipple drinker, which provided ad libitum access to feed and water throughout the experimental period. The pigs were randomly assigned to pens according to initial BW, totaling 5 treatments with 8 replicate pens per treatment and 2 pigs per pen. The dietary treatments consisted of a corn–soybean meal basal diet with no added RAC (control treatment) or 10 mg of RAC/kg fed for 7, 14,

included in the control treatment and replaced with RAC in the other treatments. The amount of standardized ileal digestible (SID) Lys, Met, Thr, and Trp were increased by 30% over the requirements recommended by Rostagno et al. (2005) to contain 1.0% total dietary Lys as suggested by Schinckel et al. (2003a). All diets were formulated

to contain equal amounts of SID Lys, Met, Thr, and Trp (Table 1). The AA content in the diets was balanced by supplementation with crystalline AA, such as L

DL-Met, L-Thr, and L-Trp, to maintain constant ratios in relation to SID Lys. All other nutrients were formulated to meet or exceed the estimated requirements for 70- to 100-kg pigs (Rostagno et al., 2005).

Growth Performance and Blood Sample Collection

Individual pig BW and pen feed disappearance were recorded at 7-d intervals during the experimental period to determine BW changes, ADG, ADFI, and G:F. On d 28, at the completion of the growth study,

Table 1. Composition of the basal diet (as-fed basis)Item AmountIngredient, %

Corn 82.49Soybean meal, 46% CP 14.14Dicalcium phosphate 1.29Limestone 0.59Salt 0.50Mineral premix1 0.10Vitamin premix2 0.10L 0.47DL-Met 0.07L-Thr 0.16L-Trp 0.04Kaolin3 0.05Total 100.00

Calculated compositionME, Mcal/kg 3.23CP, % 13.83Total Lys, % 1.00SID4 Lys, % 0.88SID Lys:ME, g/Mcal 2.72SID Met, % 0.27SID Thr, % 0.59SID Trp, % 0.17Ca, % 0.60Total P, % 0.51Available P, % 0.27SID Met:Lys 0.31SID Thr:Lys 0.67SID Trp:Lys 0.191Provided per kilogram of the complete diet: I as potassium iodine, 1.5 mg;

Co as cobalt sulfate, 1 mg; Cu as copper sulfate, 10 mg; Zn as zinc oxide, 100 mg; Fe as ferrous sulfate, 100 mg; Mn as manganese sulfate, 40 mg; and Se as sodium selenite, 0.3 mg.

2Provided per kilogram of the complete diet: vitamin A, 6,000 IU; vitamin D3, 1,500 IU; vitamin E, 15 IU; vitamin K3, 1.5 mg; thiamine, 1.35 mg;

12, 0.02 mg, nicotinic acid, 20 mg; folic acid, 0.6 mg; D-biotin, 0.08 mg; and D-pantothenic acid, 9.35 mg.

3

Cravinhos, Brazil) was added at the expense of kaolin to provide diets with 10 mg of ractopamine/kg of the complete diet.

4SID = standardized ileal digestible.

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Time-dependent effect of ractopamine on pigs 813

blood samples (8 mL) were collected from each pig via anterior vena cava puncture and transferred to tubes containing 10% EDTA. The pigs were fasted from 1200 to 0100 h of the next day, allowed free access to feed from 0100 to 0200 h, and fasted again from 0200 to 0700 h, when blood samples were collected. Plasma was obtained by centrifugation at 1,600 × g for 30 min at 4°C and stored at –20°C until it was analyzed for PUN concentrations. The concentrations of PUN were determined using a commercial enzymatic kit (Labtest Diagnostica, Belo Horizonte, Brazil).

Carcass Data Collection

After a 24-h fasting period, fi nal preshipment BW was taken and the pigs were then transported to the slaughter facility of the University of São Paulo (Pirassununga, Brazil). The animals were slaughtered via electrical stunning followed by exsanguination, scalding, dehairing, and evisceration. Immediately after evisceration, HCW was recorded to calculate the dressing percentage by dividing the HCW by the fi nal preshipment BW, with this quotient multiplied by 100. The carcasses were split longitudinally and chilled overnight at 4°C. At 24 h postmortem, the carcass length, fat depth at the fi rst rib, last rib, and last lumbar vertebrae, LM area, and fat area were measured on the left side of the carcass according to the Brazilian Method for Carcass Evaluation (ABCS, 1973). Both the LM area and fat area were recorded at the last rib, and the muscle-to-fat ratio was calculated by dividing the LM area by the fat area. The carcass LM depth and fat depth were measured 6 cm from the dorsal carcass midline at the last rib (P2). The carcass lean percentage was predicted by the equation (R. Irgang, Federal University of Santa Catarina, Florianópolis, Brazil, personal communication) carcass lean (%) = 60 – P2 fat depth (mm) × 0.58 + LM depth (mm) × 0.10.

Statistical Analysis

The data were analyzed as a randomized complete block design using the MIXED procedure (SAS Inst. Inc., Cary, NC), and the pen was considered the experimental unit. Halothane-positive pigs (Halnn) were not detected in the population used in this trial, but 6 pigs with the HalNn genotype were found. Therefore, these animals were removed from the data sets and only pigs with the HalNN genotype were considered for statistical analysis (n = 74). The model included the fi xed effect of dietary treatment and the random effect of block. For the carcass data, the fi nal BW was used as a covariate to adjust carcass variables to a common slaughter BW. The least squares means (LSMEANS) statement was used to calculate the adjusted means for the dietary treatments. Linear and quadratic contrasts were performed to determine the time-dependent effect of RAC feeding, and only the highest-order signifi cant contrast was discussed. Differences were considered statistically signifi cant at P < 0.05 and treatment trends were discussed at 0.05 < P < 0.10.

A data set with 160 weekly pen mean observations for ADG, ADFI, and G:F was created to determine the weekly RAC response. The data were analyzed using the MIXED procedure of SAS as a randomized complete block design with repeated measures. The model included the week on trial and week on RAC feeding as the fi xed effects and the block and pen (experimental unit) as the random effects. The repeated statement included the week on trial, and the pens were treated as the subject. The treatment means were computed using the LSMEANS statement, and differences were considered statistically signifi cant at P < 0.05.

RESULTS

Growth Performance and Plasma Urea N ConcentrationsOver the 28-d feeding period, the fi nal BW and

ADFI were not affected by the RAC treatment (Table 2).

Table 2. Effect of ractopamine hydrochloride (RAC) feeding duration on growth performance and plasma urea N (PUN) concentrations of fi nishing pigs1

ItemRAC2 feeding duration, d Pooled

SEMModelP-value

P-value0 (control) 7 14 21 28 Linear Quadratic

Pens (pigs) 8 (14) 8 (15) 8 (16) 8 (15) 8 (14) – – – –Initial BW, kg 70.1 69.5 69.3 69.0 69.3 1.2 0.81 0.21 0.20Final BW, kg 102.0 101.7 102.4 103.2 103.0 1.4 0.89 0.35 0.94ADG, kg 1.184 1.193 1.225 1.267 1.248 0.019 0.50 0.09 0.74ADFI, kg 3.52 3.40 3.35 3.32 3.37 0.06 0.60 0.20 0.28G:F 0.338 0.354 0.369 0.383 0.372 0.006 0.02 0.003 0.12PUN, mg/dL 30.31 23.34 22.18 23.84 23.20 0.77 0.001 0.006 0.004

1Finishing pigs were fed a corn–soybean meal-based diet with no added RAC (control) or 10 mg of RAC/kg fed for 7, 14, 21, or 28 d before slaughter. The data represent the least squares means of 8 replicate pens containing 1 or 2 pigs each, totaling 14, 15, or 16 pigs per dietary treatment.

2Ractosuin, Ourofi no Animal Health (Cravinhos, Brazil).

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The longer RAC feeding period led to a tendency (P = 0.09) for a linear increase in ADG. Increasing the RAC feeding duration resulted in a linear increase (P = 0.003) in G:F and a quadratic decrease (P = 0.004) in PUN concentrations. The reduction in PUN concentrations was observed for all RAC treatments, even in those pigs fed RAC for 21 and 28 d before slaughter.

The pigs receiving diets containing RAC for 1 wk grew faster (P < 0.05) and were more effi cient (P < 0.05) than the control pigs (Table 3). However, there was no effect of RAC on weekly ADFI. Both ADG and G:F of RAC-fed pigs during wk 2 and 3 were similar compared with those fed RAC during wk 1. The pigs fed RAC for 4 wk showed decreased (P < 0.05) ADG and used the feed less effi ciently (P < 0.05) than those fed RAC for 1, 2, and 3 wk (Fig. 1).

Carcass Traits

At the completion of the 28-d experiment, the HCW (P = 0.01) and dressing percentage (P = 0.03) increased linearly as the RAC feeding duration increased (Table 4). The fi rst rib, last rib, and last lumbar fat measurements were unaffected by the RAC feeding periods. No effect of RAC treatment was observed on P2 fat depth, fat area, and carcass length. However, the LM depth (P = 0.001), LM area (P < 0.001), muscle-to-fat ratio (P = 0.004), and predicted carcass lean percentage (P = 0.02) increased in a linear fashion with increasing RAC feeding duration.

DISCUSSION

The present study was conducted to evaluate the time-dependent effect in continuous RAC-fed fi nishing pigs. Several researchers have demonstrated that RAC feeding for the last 28 d of the fi nishing period provided a 3.5% increase in the fi nal BW (Apple et al., 2008) and a 13 to 19% improvement in overall ADG (Webster et al., 2007; Poletto et al., 2009; Halsey et al., 2011). In the current study, no RAC treatment effect on fi nal BW was observed, but there was a trend toward a linear increase in overall ADG as the RAC feeding period increased. Furthermore, RAC treatment did not affect ADFI in fi nishing pigs, which is in agreement with previous studies (Carr et al., 2005; Weber et al., 2006; Kutzler et al., 2010).

Overall G:F was improved linearly as the RAC feeding duration increased. It has been shown that including RAC in fi nishing swine diets, regardless of dietary concentration and treatment duration, improves G:F (Rikard-Bell et al., 2009; Edmonds and Baker, 2010; Hinson et al., 2011). The improvement in G:F is related to the lesser amount of energy required

for depositing muscle tissue than for adipose tissue (Schinckel et al., 2003a).

There is evidence that growth performance enhancements in modern high-lean-gain pigs are achieved with short periods of RAC feeding (Schinckel et al., 2002, 2003a). In this study, the most marked improvements in weekly ADG and G:F occurred during the fi rst 7 d of RAC feeding. These improvements were maintained for an additional 14 d and then declined after 21 d of RAC feeding. Our fi ndings are in agreement with a previously published report, in which the maximum response in ADG

Figure 1. Average daily gain and G:F as a function of week on ractopamine hydrochloride (RAC; Ractosuin; Ourofi no Animal Health, Cravinhos, Brazil) feeding. Finishing pigs were fed a corn–soybean meal-based diet with no added RAC (0; control) or 10 mg of RAC/kg fed for 1, 2, 3, or 4 wk. The values (♦) for each treatment are least squares means of 80, 32, 24, 16, and 8 replicate pens containing 1 or 2 pigs each, totaling 148, 60, 45, 29, and 14 pigs. (A) The quadratic regression equation is y = –0.06602x2 + 0.2145x + 1.1304 (R2 = 0.297, P < 0.001). (B) The quadratic regression equation is y = –0.01784x2 + 0.06384x + 0.3461 (R2 = 0.264, P < 0.001).

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Time-dependent effect of ractopamine on pigs 815

was observed in pigs fed 44.7 mg of RAC/d for 7 to 21 d followed by a reduced response when longer treatment durations were used (Williams et al., 1994). In a summary of 12 research trials, Kelly et al. (2003) reported that the benefi t in BW of pigs fed RAC at both 4.5 and 9 mg/kg declined linearly over a 28-d feeding period, and feed intake increased during the fi rst 14 d of RAC feeding but decreased thereafter. Prolonged exposure to RAC feeding may contribute to the desensitization of β-adrenergic receptors, resulting in attenuation of the response of the animal (Benovic et al., 1988; Lefkowitz, 2007). The initial step in desensitization appears to involve the uncoupling of the receptors from their G proteins, but chronic receptor stimulation by a β-adrenergic agonist may ultimately lead to downregulation responses (Mills, 2002).

The concentrations of PUN decreased quadratically as the RAC feeding period increased, reaching the

lowest level in pigs fed RAC in the 14 d before slaughter. The decrease in PUN concentrations in RAC-treated pigs is consistent with the results of Moreno et al. (2008). According to Dunshea and King (1994), PUN concentrations decreased after 24 h of exposure to RAC treatment, but on the fi fth day after RAC withdrawal, a marked increase of 21% in the circulating PUN was detected. In a study with clenbuterol-fed lambs, MacRae et al. (1988) suggested that the β-adrenergic agonist may initially reduce protein breakdown, and then the maintenance of anabolism provides protein accretion. This may explain why decreased PUN concentrations were observed in RAC-fed pigs in the present study. The increase in muscle protein synthesis promoted by RAC requires increased N use, resulting in decreased PUN concentrations (See et al., 2004). Our results indicated that PUN concentrations increased after 14 d

Table 3. Effect of ractopamine hydrochloride (RAC) feeding duration on weekly performance of fi nishing pigs1

ItemWeek on RAC2 feeding

Model P-value0 (control) 1 2 3 4Pens (pigs) 80 (148) 32 (60) 24 (45) 16 (29) 8 (14) –ADG, kg 1.114 ± 0.031bc 1.313 ± 0.039a 1.287 ± 0.044a 1.197 ± 0.054ab 0.955 ± 0.077c <0.001ADFI, kg 3.34 ± 0.11 3.22 ± 0.11 3.25 ± 0.12 3.15 ± 0.14 3.04 ± 0.17 0.35G:F 0.340 ± 0.011b 0.409 ± 0.013a 0.399 ± 0.014a 0.384 ± 0.017a 0.308 ± 0.023b <0.001

a−cWithin a row, means without a common superscript differ (P < 0.05).1Finishing pigs were fed a corn–soybean meal-based diet with no added RAC (control) or 10 mg of RAC/kg fed for 1, 2, 3, or 4 wk. The data represent the

least squares means ± SE of 80, 32, 24, 16, and 8 replicate pens containing 1 or 2 pigs each, totaling 148, 60, 45, 29, and 14 pigs per dietary treatment.2Ractosuin, Ourofi no Animal Health (Cravinhos, Brazil).

Table 4. Effect of ractopamine hydrochloride (RAC) feeding duration on carcass traits of fi nishing pigs1

ItemRAC2 feeding duration, d Pooled

SEMModelP-value

P-value0 (control) 7 14 21 28 Linear Quadratic

Pens (pigs) 8 (14) 8 (15) 8 (16) 8 (15) 8 (14) – – – –Final preshipment BW,3 kg 97.5 97.8 99.0 99.1 99.7 1.4 0.63 0.11 0.91HCW,4 kg 78.38 79.03 79.62 79.29 80.71 1.23 0.05 0.01 0.80Dressing,4,5 % 79.99 80.19 80.27 80.57 81.30 0.24 0.30 0.03 0.43Carcass length,4 cm 91.12 90.79 89.60 89.87 90.24 0.36 0.17 0.18 0.12Fat fi rst rib,4 mm 35.42 36.63 35.20 34.07 35.43 0.07 0.44 0.56 0.84Fat last rib,4 mm 24.03 25.16 25.08 23.72 26.26 0.07 0.72 0.39 0.75Fat last lumbar,4 mm 20.27 18.26 19.64 19.99 19.59 0.06 0.74 0.77 0.57P2 fat depth,4,6 mm 15.65 14.28 14.51 13.29 14.02 0.48 0.38 0.19 0.38Fat area,4 cm2 18.31 18.22 17.79 16.93 18.04 0.55 0.75 0.75 0.51LM depth,4 mm 61.87 64.31 66.09 65.26 69.66 0.99 0.02 0.001 0.81LM area,4 cm2 39.45 43.21 43.39 44.29 48.32 0.96 0.002 <0.001 0.80Muscle-to-fat ratio4,7 2.17 2.44 2.54 2.68 2.71 0.07 0.04 0.004 0.36Predicted carcass lean,4,8 % 57.11 58.15 58.19 58.82 58.84 0.26 0.13 0.02 0.45

1Finishing pigs were fed a corn–soybean meal-based diet with no added RAC (control) or 10 mg of RAC/kg fed for 7, 14, 21, or 28 d before slaughter. The data represent the least squares means of 8 replicate pens containing 1 or 2 pigs each, totaling 14, 15, or 16 pigs per dietary treatment.

2Ractosuin, Ourofi no Animal Health (Cravinhos, Brazil).3The fi nal preshipment BW was taken after a 24-h fasting period.4The fi nal BW was included in the statistical model as a covariate for carcass data.5Calculated as the HCW divided by the fi nal preshipment BW, with the quotient multiplied by 100.6P2 fat depth = fat depth measured 6 cm from the dorsal carcass midline at the last rib.7Calculated as the LM area divided by the fat area.8Calculated using the Irgang equation: predicted carcass lean (%) = 60 – P2 fat depth (mm) × 0.58 + LM depth (mm) × 0.10.

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Almeida et al. 816

of exposure to RAC treatment, but the levels remained lower than those of the control pigs. Interestingly, pigs have shown decreased growth responses and estimated protein accretion rates after 21 d of RAC administration (Williams et al., 1994; Kelly et al., 2003; Schinckel et al., 2003a). Therefore, pigs fed RAC for 21 and 28 d were expected to have greater PUN concentrations compared with those fed RAC for 7 and 14 d.

Diets for fi nishing pigs weighing 70 to 100 kg should contain 0.772% total Lys (as-fed basis; Rostagno et al., 2005). However, pigs fed RAC require increased amounts of limiting AA, especially Lys, which should be included in the diet at a proportion of at least 1.0% total Lys (Schinckel et al., 2003a). To offer the same condition for all animals, the experimental diets (control and RAC diets) were formulated to contain 1.0% total dietary Lys (as-fed basis). The control animals ingested and probably absorbed an excess of AA in our study. The amount of AA absorbed beyond the requirements for maintenance and growth need to be catabolized because the body does not contain AA storage (Berg et al., 2002). Therefore, the control animals likely spent more energy on urea synthesis to excrete the excess N, reducing the energy available for growth.

Increasing the RAC feeding duration led to a linear increase in HCW so that pigs fed RAC for 28 d before slaughter had carcasses that were 2.33 kg heavier than untreated animals. In support of our fi nding, other studies have reported that the HCW of pigs receiving 7.4 mg of RAC/kg for 21 and 28 d were 3.74 and 5.47 kg, respectively, heavier compared with the control animals (Hinson et al., 2011; Rickard et al., 2012). Linear improvement in dressing percentage resulting from increased HCW was observed when the RAC feeding period was increased. This fi nding agrees with a meta-analysis of studies examining the use of RAC in the swine diets, in which dressing percentage was increased by only 0.6% points in pigs fed 10 mg of RAC/kg compared with the control pigs (Apple et al., 2007). In addition, similar to the results obtained by Carr et al. (2009), no differences were observed in the carcass length of pigs in any of RAC feeding periods tested in this trial.

Dietary RAC did not consistently affect any backfat measurements, differing from previous studies that reported reductions in carcass backfat depth by 37 and 12% in pigs fed RAC (Mimbs et al., 2005; Rickard et al., 2012). Conversely, the current results are corroborated by those of other works (Fernández-Dueñas et al., 2008; Leick et al., 2010). The commercial RAC is available as a mixture of 4 stereoisomers with RR, RS, SR, and SS confi gurations, but the RR isomer is likely the biologically active ligand (Ricke et al., 1999; Mills et al., 2003a). Because the RR stereoisomer of RAC triggers a better signal through the β2–adrenergic receptor, perhaps

greater reductions in fat accretion may not be achieved in adipose tissue where the β1–adrenergic receptor is the predominant subtype (Mills et al., 2003b). Additionally, RAC may lead to the downregulation of receptors, resulting in progressively decreased β-adrenergic receptors in porcine subcutaneous adipose tissue by 28% over 1 d and 53% over 8 d (Spurlock et al., 1994).

Repartitioning agents are generally used to increase lean mass, which is the major determinant of carcass value. The linear increase in the LM depth, LM area, and muscle-to-fat ratio with increasing RAC feeding duration observed in the present study may be partly accounted for by increased blood fl ow to muscle cells, which provides the substrates and energy needed for protein synthesis (Mersmann, 1998). There is considerable evidence that the protein synthesis rate increases in pigs fed RAC (Adeola et al., 1992). An alternative approach for the enhancement of muscle accretion may be the activation of satellite cells induced by RAC. As reviewed by Zammit (2008), quiescent muscle stem cells, called satellite cells, produce myonuclei for muscle hypertrophy in the postnatal period. According to cell culture assays, RAC stimulated the proliferation of chick (Grant et al., 1990) and mouse muscle satellite cells (Shappell et al., 2000). However, the recruitment of additional satellite cell nuclei may not be the most important contributor to the accretion of muscle mass because DNA concentrations were unaffected by the RAC treatment (Grant et al., 1993).

Because carcass value is strongly associated with carcass lean quantity, prediction equations for pork carcass lean mass and lean percentage are widely used to estimate carcass value (Schinckel et al., 2007). The predicted carcass lean percentage increased in a linear fashion as the RAC feeding duration increased. Other researchers have demonstrated an increase in the predicted lean percentage of 1 and 3.8% when pigs were fed RAC at concentrations of 7.4 and 10 mg/kg for the last 27 or 28 d of fi nishing, respectively (Groesbeck et al., 2007; Boler et al., 2011). Lean prediction equations that include standard carcass measurements of carcass weight, backfat depth, and LM depth, such as those used in our study, underestimate lean mass in RAC-fed pigs (Schinckel et al., 2003b). Prediction equations from direct carcass measurements represented approximately 50 to 60% of the RAC response of increased carcass leanness (Gu et al., 1992; Schinckel et al., 2003b). Therefore, in addition to chemical analyses, carcass dissection such as dissected loin lean or dissected ham lean is recommended to obtain more accurate predictions for carcass composition when pigs are fed RAC (Schinckel et al., 2003b).

In conclusion, this study demonstrated that increasing the RAC feeding duration tended to increase ADG and potentially improved G:F of fi nishing pigs. The maximal growth response occurred during the fi rst 21 d of RAC

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Time-dependent effect of ractopamine on pigs 817

feeding but declined thereafter. Furthermore, feeding diets containing 10 mg of RAC/kg had no effect on any backfat measurements or carcass length over a 28-d period. Conversely, all other results regarding carcass traits improved over 28 d of RAC feeding, supporting the notion that the magnitude of the carcass response seems to be directly dependent on the RAC feeding duration.

LITERATURE CITEDAdeola, O., R. O. Ball, and L. G. Young. 1992. Porcine skeletal muscle

myofi brillar protein synthesis is stimulated by ractopamine. J. Nutr. 122:488–495.

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