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Effect of a ractopamine feeding program on growth performance and carcass composition in finishing pigs

M. T. See^*,1, T. A. Armstrong and W. C. Weldon

^* Department of Animal Science, North Carolina State University, Raleigh 27695-7621 and and Elanco Animal Health, Greenfield, IN 46140

	Abstract

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Barrows and gilts (n = 100 per gender) were used to determine the effects of an increasing, decreasing, or constant ractopamine (RAC) dietary concentration on growth performance and carcass characteristics. Pigs, within a gender, were assigned randomly to pens (five pigs per pen and 10 pens per treatment). Pens were assigned randomly to one of four dietary treatments at a starting weight of 71.2 kg, to target an average ending weight of 109 kg. The four dietary treatments (as-fed basis) were 1) control = 0 ppm RAC, wk 0 to 6; 2) RAC step-up = 5.0 ppm, wk 1 to 2; 10.0 ppm, wk 3 to 4; and 20.0 ppm, wk 5 to 6; 3) RAC step-down = 20.0 ppm, wk 1 to 2; 10.0 ppm, wk 3 to 4; and 5.0 ppm, wk 5 to 6; and 4) RAC constant = 11.7 ppm, wk 0 to 6. Feed allocation was recorded daily, and pigs were weighed and feed was weighed back every 2 wk. Jugular blood samples were obtained from two randomly selected pigs per pen on d –3, 7, 21, 35, and 41 for determination of plasma urea nitrogen (PUN) concentrations. Two pigs were selected randomly per pen and sent to a commercial slaughter facility at the end of the 6-wk experimental period. Carcass data were evaluated on an equal time basis and on an equal weight basis by using hot carcass weight (HCW) as a covariate. Overall, ADG and G:F were improved (P < 0.05) for pigs fed RAC compared with control, with no differences among RAC feeding programs. In wk 3 and 4, improvements (P < 0.05) in ADG and G:F were realized with the implementation of a RAC step-up program compared with control pigs. The concentrations of PUN were decreased (P < 0.05) at d 7 and 21 with RAC feeding, and a RAC step-up program maintained the decrease (P < 0.05) in PUN through d 35 and 41. A RAC step-up and constant program increased (P < 0.05) HCW and percent yield. Loin muscle area and percentage of fat-free lean increased (P < 0.05) and backfat thickness decreased (P < 0.05) in pigs fed RAC. If pigs were considered to be on feed for an equal time period, advantages (P < 0.05) were observed for weight of boneless trimmed ham, shoulder and loin for the step-up and constant RAC treatments compared with the controls. Feeding a RAC step-up or constant feeding program resulted in favorable responses in growth performance and yielded more lean pork.

Key Words: Carcass • Growth • Pigs • Ractopamine

	Introduction

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Feeding ractopamine (RAC; Paylean, Elanco Animal Health, Greenfield, IN) to finishing pigs for the final 28 to 35 d prior to marketing results in improvements in live performance and carcass characteristics (Watkins et al., 1990; Stites et al., 1991). Recent data indicated that the improvement in ADG with RAC feeding was similar regardless of dietary RAC concentration (Armstrong et al., 2002). In addition, the RAC response is not constant over the course of the feeding period (Dunshea et al., 1993; Williams et al., 1994; Kelly et al., 2003). Specifically, the RAC response in the live animal increases rapidly, plateaus, and then seems to decrease during the course of the RAC feeding period (Dunshea et al., 1993; Williams et al., 1994; Kelly et al., 2003). The RAC response diminishes over time, due to either downregulation or desensitization of the ß₁-adrenergic receptors (Moody et al., 2000).

Previous research has focused on the feeding of a constant dietary concentration of RAC for a fixed period of time. Further benefits beyond a constant RAC dietary concentration have been reported when the dietary concentration of RAC is increased during the course of the RAC feeding period (Trapp et al., 2002). Consequently, it seems possible to maintain the RAC response in the live animal for a longer period of time by changing the dietary RAC concentration throughout the feeding period compared with a constant dietary RAC concentration. Therefore, the objective of this study was to determine whether the RAC response would be altered by an increasing or decreasing dietary RAC concentration compared with a constant dietary RAC concentration over a 6-wk period. In addition, the effects of RAC on carcass characteristics were evaluated under an equal length of time on treatment or common final weight-marketing scenarios.

	Materials and Methods

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One hundred barrows and 100 gilts from a commercial farm, using a Newsham Hybrids genetic program, were transported to the North Carolina Swine Evaluation Station (Clayton, NC) and randomly assigned to pens (five pigs per pen) at approximately 40.9 kg BW. Gilts and barrows were housed separately. Pigs were housed on solid concrete floors and provided with ad libitum access to feed and water. After 33 d of acclimation, dietary treatments were imposed at an average weight of 71.2 kg. This starting weight was calculated based on the expected performance of pigs in this facility to target an average weight at the end of the 6-wk experimental period of 109 kg. Diet changes occurred at 2-wk intervals over the 6-wk experimental period. Due to scheduling at the packing plant, the trial concluded at 41 d rather than 42 d, which resulted in 13 d during the third feeding period (wk 5 and 6). All experimental procedures, care, and handling of animals were approved by the North Carolina State University Animal Care and Use Committee.

Pens, within a gender, were assigned randomly to receive one of four dietary treatments. There were 10 pens per treatment, with five pens per treatment of barrows and five pens per treatment of gilts. The treatments were designed to evaluate the effects of an increasing and decreasing dietary concentration of RAC compared with a negative control and a constant dietary RAC concentration. Dietary treatments (as-fed basis) were 1) control = 0 ppm RAC, wk 0 to 6; 2) step-up RAC = 5.0 ppm RAC, wk 1 to 2; 10.0 ppm RAC, wk 3 to 4; and 20.0 ppm RAC, wk 5 to 6; 3) step-down RAC = 20.0 ppm RAC, wk 1 to 2; 10.0 ppm RAC, wk 3 to 4; and 5.0 ppm RAC, wk 5 to 6; and 4) constant RAC = 11.7 ppm RAC, wk 0 to 6. All diets were formulated (as-fed basis) to contain approximately 20% CP and 1.2% lysine (Table 1).

Eighty pigs (two randomly selected pigs per pen) were sent to a commercial slaughter facility at the completion of the 41-d feeding period and were slaughtered approximately 16 h after being weighed off treatment. Carcass data collected at the slaughter facility included hot carcass weight (HCW), leaf fat weight, 10th-rib LA, 10th-rib backfat depth, and carcass length. Percent carcass yield was calculated using off-test weight and HCW, and percentage of fat-free lean was estimated using equations recommended by the NPPC (2000). The right side of each carcass was separated into the four primal cuts: ham, loin, belly, and shoulder. Each primal cut was then boned and trimmed to 0.32 cm, and the weights of the boneless loin, tenderloin, boneless ham, belly, and shoulder were collected. The belly was boned but not trimmed to 0.32 cm.

Statistical analyses of the data were performed using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). Dietary treatment, gender, and their interaction were examined to determine their effects on growth and carcass characteristics. Differences between treatment means were compared using Duncan’s multiple range test (Steel et al., 1997). Least squares means for carcass data also were evaluated with HCW as a covariate to adjust carcass variables and primal characteristics to a common slaughter weight. Pen was the experimental unit for all dependent variables. Significance was declared at a P value less than or equal to 0.05.

	Results and Discussion

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There were no dietary treatment x gender interactions for any of the performance or carcass variables that depended on the physiological response of the RAC molecule evaluated in the current study. Dietary treatments did not differ in initial weight, with an average initial weight across treatments of 71.2 kg (Table 2). Dietary treatments did not affect live weight at the end of each 2-wk period; however, at the end of each 2-wk period, pigs receiving diets containing RAC had live weights that were numerically greater than those of the control pigs (Table 2).

Average daily feed intake was not affected during the first 28 d, but control pigs consumed more feed (P < 0.05) than the RAC step-up (20.0 ppm RAC) and constant (11.7 ppm RAC) treatments during the last 13 d (Table 2). Control pigs tended (P = 0.07) to consume more feed relative to RAC-fed pigs, regardless of the dietary RAC feeding regimen, over the entire experimental period. This finding agrees with previous reports, which indicated that ADFI was decreased when RAC was included in the diet at 10.0 to 20.0 ppm for durations of approximately 42 d (Watkins et al., 1990; Bark et al., 1992; Crome et al., 1996).

Our data agree with previous research indicating improvements in ADG and feed efficiency with RAC feeding (Uttaro et al., 1993; Crome et al., 1996; Dunshea et al., 1998). Previous research has investigated the effects of a constant dietary concentration of RAC during a fixed time period, and data indicate that the RAC response on live performance was not constant over time (Dunshea et al., 1993; Williams et al., 1994; Kelly et al., 2003). This diminishing type of response may be the result of either a downregulation or a desensitization of the ß₁-adrenergic receptors for the RAC molecule (Moody et al., 2000). From these current data, the implementation of a RAC step-down feeding regimen results in live performance responses that are inferior to the RAC step-up or constant feeding programs, beginning at d 28 and continuing through d 41, which agrees with the conclusion of Herr et al. (2001). Therefore, it may be possible that the RAC step-down program enhances the downregulation or desensitization of these ß₁-adrenergic receptors through the introduction of the highest dietary RAC concentration early in the feeding period. In addition, the improvements in ADG and G:F that were realized with a RAC step-up feeding program compared with a RAC step-down feeding program may allow for the targeted feeding of specific groups of animals to optimize profitability as the population of pigs changes within a barn as commercial swine operations ship pigs to market. However, further research is necessary to determine the logistics and economics of this type of targeted RAC feeding program.

Plasma urea nitrogen (PUN) concentrations were not affected by dietary treatment at d –3 (Table 3). At d 7 and 21, pigs receiving diets containing RAC had PUN concentrations that were decreased (P < 0.05) compared with the controls. In addition, at d 21, pigs assigned to the RAC step-down dietary regimen had PUN concentrations that were increased (P < 0.05) compared with the RAC step-up and constant dietary treatments. At d 35, pigs assigned to receive RAC step-up or constant feeding regimens had PUN concentrations that were less (P < 0.05) than those of the controls, and pigs that received the RAC step-up treatment had PUN concentrations that were decreased (P < 0.05) compared with the RAC step-down and constant treatments. Plasma urea nitrogen concentrations were decreased (P < 0.05) in pigs receiving the RAC step-up treatment compared with the controls and pigs that received the RAC step-down feeding regimen at d 41. Our findings agree with those of Dunshea et al. (1993), who reported a decrease in PUN concentrations with RAC feeding; however, they do not agree with the results of Yen et al. (1990), in which no difference in PUN concentrations was detected between control and RAC-fed pigs.

Carcass characteristics were evaluated on an equal-time basis (Table 4) and on an equal-weight basis (Table 5) by including HCW as a covariate in the statistical model. Carcass length was not affected by dietary treatment when evaluated under either marketing scenario. Hot carcass weight and percent yield were improved (P < 0.05) with the implementation of a RAC step-up or constant feeding program when data were evaluated on an equal-time basis; however, there was no difference between the control and RAC step-down feeding programs. Percent yield was increased (P < 0.05) with the implementation of a RAC step-up or constant feeding program when evaluated by an equal-weight scenario, with no difference between the RAC step-down and control feeding regimens. Previous research has demonstrated the increases in HCW and percent yield with a constant RAC supplementation (Watkins et al.; 1990; Stites et al., 1991; Crome et al., 1996), and these data agree with the report of Herr et al. (2001) with respect to the effects of a RAC step-up feeding program on HCW and percent yield. It is evident from our data that a RAC step-down feeding program did not result in the advantages in HCW and percent yield that were obtained with a RAC step-up or constant feeding program.

Feeding RAC to finishing swine has resulted in consistent increases in the yield of boneless cuts. Specifically, RAC increased the boneless cut weights of the ham, loin, tenderloin, Boston butt, and picnic when administered at a constant dietary concentration for a fixed period of time (Stites et al., 1991; Crome et al., 1996). In addition, the lean weight of the picnic, Boston butt, loin, and ham were increased as the dietary RAC concentration increased from 0 to 20.0 ppm (Aalhus et al., 1990). In the current study, the weight of the boneless, trimmed ham was increased (P < 0.05) in pigs fed RAC compared with the controls when evaluated under an equal-time and equal-weight scenario. Aalhus et al. (1990) reported that the greatest increases in muscle content following RAC supplementation were noted in the ham.

When these current data were evaluated on an equal-time basis, weights of the boneless, trimmed shoulder and loin were increased (P < 0.05) when a RAC step-up or constant feeding program were implemented. The RAC step-down feeding regimen did not affect the weights of the boneless, trimmed shoulder and loin when evaluated on an equal-time basis. These data support the work of Herr et al. (2001), in that a RAC step-up feeding regimen increased the weight of the shoulder, and both a RAC step-up and constant feeding program increased the weight of the loin. In addition, weight of the tenderloin tended (P = 0.10) to be increased in pigs fed RAC compared with the controls; however, there was no effect of dietary treatment on weight of the boneless belly when data were evaluated on an equal-time basis. When data were presented as feeding pigs to a common market weight, weights of the boneless, trimmed shoulder, loin, or belly and weight of the tenderloin were not affected by dietary treatment.

There were no dietary treatment x gender interactions for any of the performance or carcass variables evaluated in the current study. Dunshea et al. (1993) reported a dietary RAC x gender interaction for ADG, in that RAC supplementation at 20.0 ppm increased ADG in gilts and barrows but not in boars. Therefore, these data indicate that the physiological response to RAC is not gender specific, which agrees with previous data (Uttaro et al., 1993; Dunshea et al., 1998). However, gender effects existed for growth performance and carcass characteristics, and these gender differences agree with expected production differences. Specifically, gilts have an increased daily lean gain, larger LMA, and higher percent lean of the carcass at market weight than barrows, and barrows typically have an increased feed intake (Cline and Richert, 2001). During the course of the study, barrows were heavier and grew faster (P < 0.05) than gilts (Table 2). In addition, barrows had a greater (P < 0.05) ADFI than gilts; however, gilts were more efficient (P < 0.05) than barrows (Table 2). Overall, gilts were leaner (P < 0.05) and had less (P < 0.05) 10th rib backfat than barrows (Tables 4 and 5). In addition, concentrations of PUN were higher (P < 0.01) in barrows than gilts at d -3, 21, 35, and 41 (Table 3).

	Implications

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Feeding ractopamine to finishing swine resulted in improvements in growth performance and carcass characteristics and yielded more lean pork. In addition, the implementation of a ractopamine step-up or constant feeding regimen produced more consistent and desirable improvements than the implementation of a ractopamine step-down feeding regimen. However, further research is necessary to determine the optimal ractopamine feeding program. The ractopamine response decreases with time and was consistent across gender. Ultimately, the desired carcass composition should be considered in the decision to feed ractopamine and the feeding program and marketing scenario used with it.

¹ Correspondence: Box 7621 (phone: 919-515-8797; fax: 919-515-6316; e-mail: todd_see{at}ncsu.edu).

Received for publication October 30, 2003. Accepted for publication April 26, 2004.

	Literature Cited

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