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The effect of dietary ractopamine concentration and duration of feeding on growth performance, carcass characteristics, and meat quality of finishing pigs

T. A. Armstrong^*,1, D. J. Ivers^*, J. R. Wagner^*, D. B. Anderson^*, W. C. Weldon^* and E. P. Berg

^* Elanco Animal Health, A Division of Eli Lilly and Co., Greenfield, IN 46140; and and Animal Sciences Department, University of Missouri, Columbia 65211-5300

	Abstract

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A total of 400 barrows from Dekalb EB and 83 terminal sires mated to 43 and 45 maternal lines were used to evaluate the effects of dietary ractopamine (RAC; Paylean, Elanco Animal Health, Greenfield, IN) concentrations (0, 5, 10, or 20 ppm; as-fed basis) and feeding durations (6 to 34 d) on growth, efficiency, carcass, and meat quality characteristics of finishing pigs. Barrows were weighed and sorted into five weight blocks, each block consisting of 16 pens (five pigs per pen). Weight blocks were allocated to feeding duration treatments and assigned consecutively by weight from lightest to heaviest to represent 34, 27, 20, 13, and 6 d on test, respectively. The lightest and heaviest blocks averaged 79.8 and 103.8 kg, respectively, at the start of the test. Within a weight block, pens (four per treatment) were randomly assigned to one of four dietary concentrations of RAC in a basal diet containing 18.5% CP and 1.13% lysine. The experiment-wide target slaughter weight was 109 kg, and pigs and feeders were weighed weekly. Weight blocks (80 barrows per block) were slaughtered at a commercial packing plant after 6, 13, 20, 27, or 34 d on test. Overall, RAC supplementation improved (P < 0.05) ADG; however, ADG was not different (P > 0.08) from controls for pigs fed 5, 10, and 20 ppm RAC for 27, 34, and 6 d, respectively. During each feeding period, RAC-fed pigs had improved (P < 0.05) G:F, and, after 20, 27, and 34 d on test, pigs fed 20 ppm RAC had greater (P < 0.05) G:F compared with those fed 0 or 5 ppm RAC. Hot carcass weight was increased (P < 0.05) by RAC feeding after 13 and 27 d of feeding, and by feeding 10 and 20 ppm RAC after 20 d of feeding. After 34 d, pigs fed 20 ppm RAC had heavier (P < 0.05) hot carcass weight than pigs fed 10 ppm RAC. Fat-free lean estimates and the 10th-rib LM area were increased (P < 0.05) by feeding 10 and 20 ppm RAC after 27 d, and by feeding 20 ppm RAC after 34 d compared with controls. Japanese and American color scores, as well as L*, a*, and b* values of the LM, were not affected (P > 0.11) by 5 and 10 ppm RAC compared with controls during each feeding period. Visual marbling score for the LM was decreased (P < 0.05) when RAC was fed at 10 and 20 ppm compared with 0 ppm RAC when fed for 34 d. Dietary RAC improved growth performance at all feeding durations, whereas carcass composition was improved at longer feeding durations. In addition, 5 and 10 ppm RAC did not affect objective and subjective measures of pork quality.

Key Words: Carcass • Growth • Meat Quality • Ractopamine • Swine

	Introduction

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Ractopamine hydrochloride (RAC; Paylean, Elanco Animal Health, Greenfield, IN) is a phenethanolamine-repartitioning agent that redirects nutrients away from adipose and toward lean tissue deposition (Ricks et al., 1984; Moody et al., 2000). Feeding diets containing RAC resulted in improvements in live performance, N retention, carcass leanness, and dressing percent of finishing swine (Anderson et al., 1987a; Veenhuizen et al., 1987; Watkins et al., 1990). These performance benefits are the result of increased protein synthesis (Bergen et al., 1989; Helfrich et al., 1990; Adeola et al., 1992).

Performance improvements associated with RAC-feeding in pigs are affected by several factors, including but not limited to nutrient concentrations of the diet, dietary RAC concentration, and duration of RAC feeding (Moody et al., 2000). Nutrient concentrations in the diet must be increased to maximize the RAC response (Anderson et al., 1987b; Jones et al., 1988; Weldon and Armstrong, 2001). Feeding 5 ppm RAC resulted in improved live animal performance, with small improvements in carcass composition, whereas increased RAC concentrations (10 to 20 ppm) and longer feeding durations (28 to 35 d) have resulted in further improvements in carcass characteristics (Watkins et al., 1990; Stites et al., 1991; Williams et al., 1994).

Due to changes in genetics and management practices within the swine industry, this study was conducted to define the relationship between duration of feeding and dietary concentration of RAC in a modern lean genetic line. Thus, the objective of this study was to determine the effects of RAC fed at varying dietary concentrations and for various durations on live performance, carcass characteristics, and meat quality of finishing pigs.

	Materials and Methods

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Animal Care and Facilities
Approximately 700 barrows from Dekalb EB and 83 terminal sire lines and Dekalb 43 and 45 maternal lines (Dekalb Choice Genetics, St. Louis, MO) were acquired from a single commercial finishing facility and transported to the Swine Research Complex of Elanco Animal Health in Greenfield, IN. Barrows were of similar age (2 to 3 wk), and had a weight range of 63.6 to 95.5 kg upon arrival at the facilities. Throughout the trial, pigs were housed in one of four identically designed, environmentally controlled rooms (8.23 x 36.58 m), with the temperature maintained at approximately 18.3°C. Rooms were on a 12-h light:12-h dark cycle, switching at 0600 and 1800. Each pen (1.83 x 3.05 m) within a room had a concrete slatted floor located over a manure pit, and was equipped with a stainless steel feeder and nipple waterer. The study was divided into two phases, an acclimation phase (d 0 to 15) and an experimental phase (d 16 to 50). Throughout the acclimation and the experimental phases, all barrows had ad libitum access to feed and water. All experimental procedures, care, and handling of animals were approved by the Animal Care and Use Committee at Elanco Animal Health.

Acclimation Phase
From the approximately 700 barrows that were purchased, 576 barrows were selected to begin the acclimation phase. Selection was based on BW, structural soundness, and health status of the pigs. Barrows were sorted into six blocks (16 pens per block; six pigs per pen) based on BW, with Block 1 containing the heaviest pigs and Block 6 containing the lightest pigs. Five days before the initiation of the experimental phase (d 11), all barrows were weighed, and one pig from each pen was removed to leave five pigs of similar weight in each pen and to equalize the weight between the 16 pens within a weight block. During the acclimation phase, all barrows received a diet that contained 18.5% CP and 1.13% lysine in meal form (Table 1). The diet contained 44 ppm (as-fed basis) tylosin (Tylan; Elanco Animal Health, Greenfield, IN) during the acclimation phase, supplemented at the expense of ground corn.

Animal Slaughter
All groups arrived at the packing plant on Sunday evenings and were the first pigs slaughtered on Monday morning (0700). All pigs were killed according to the packing plant’s slaughter protocol in compliance with the Humane Slaughter Act federal standards for humane slaughter. Pigs were stunned with CO₂, shackled, exsanguinated, scalded, eviscerated, and each carcass was split into left and right sides. Individual hot carcass weight (HCW) was collected, and dressing percent was calculated as HCW divided by final preshipment live weight, with this quotient multiplied by 100. The time from stunning to the cooler was approximately 35 min.

Forty-eight carcasses per feeding period were preselected based on weight (three pigs per pen, with final preshipment live weights closest to the pen average) and identified in the packing plant. Individual pig identification number was written in edible carcass crayon on the ham, loin, belly, and picnic shoulder, and the 10th-/11th-rib juncture was marked. Carcasses were spray-chilled for the initial 6 h and conventionally chilled for the remaining 14 to 18 h at 1°C.

Carcasses proceeded to fabrication after a 20- to 24-h chill, and right sides were removed from the fabrication line in three sections: shoulder (Boston butt, picnic, jowl, and forefoot), loin/belly (intact midsection), and leg (ham, tail bones, and hind foot). The three sections were shipped under refrigeration to the University of Missouri Meat Laboratory in Columbia within 48 h of slaughter.

Carcass Evaluation
Carcass evaluation was performed at the University of Missouri Meat Laboratory. Rough-cut loins were separated from the bellies along a line perpendicular to the outside skin surface, extending in a straight line from a point on the most anterior rib, which was immediately adjacent to the ventral edge of the psoas major. This initial separation left the rough-cut loin with the outer skin surface and overlying fat intact. The loin was split between the 10th and 11th ribs and measured for fat depth (measured three-quarters of the length of the LM from the chine bone with a ruler) and LM cross-sectional area (NPPC, 2000). Fat-free carcass lean was estimated using an equation that included fat depth, LM area, and HCW (NPPC, 2000). After an approximate 30-min bloom period, the LM was visually evaluated for marbling (NPPC, 1999) and color based on the American (NPPC, 1999) and Japanese (Nakai et al., 1975) color standards. In addition, the LM was evaluated for instrument color (L*, a*, and b*) at the 10th-/11th-rib juncture using a HunterLab Miniscan XE Plus 45/0 with a D65 illuminant (Hunter Association Laboratory Inc., Reston, VA). Color, texture, and exudation were determined on the ham using the categories of PSE, reddish-pink, firm and nonexudative, and DFD (NPPC, 2000). Each quality category was subjectively classified by a trained evaluator and was not determined by setting objective thresholds.

Statistical Analyses
Data were analyzed by ANOVA using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC). Average daily gain, ADFI, and G:F were calculated from d 0 of the experimental phase (d 16 of the study) to the day of shipment for each feeding period. The final model included dietary treatment and room, and was conducted for each period. Pen was considered the experimental unit, resulting in four experimental units per dietary treatment within a feeding period. Least squares means and average SEM were calculated, and statistically separated by the PDIFF option of SAS. Significance was declared at P 0.05. In addition, simple correlation coefficients were calculated to determine the level of association between L* values and the American and Japanese color scores.

	Results

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Feeding RAC at 5 and 10 ppm increased (P < 0.05) ADG compared with the control pigs after 6 d of feeding (Table 3). Daily gain was not affected (P > 0.33) by feeding 20 ppm RAC, which may be primarily a result of the removal of one pen of pigs assigned to receive 20 ppm RAC for 6 d due to health reasons. The on-site veterinarian removed the pen of pigs from the trial due to symptoms indicative of a viral challenge. Daily feed intake was decreased (P < 0.05) in pigs that received diets supplemented with 20 ppm for 6 d compared with the control pigs and pigs fed 10 ppm RAC (Table 3), and this may also be explained by the removal of one pen of pigs due to health reasons. Efficiency (G:F) was improved (P < 0.05) for pigs fed RAC after the 6-d feeding period when compared with pigs fed the control diet (Table 3); however, no differences (P > 0.47) were observed between dietary RAC treatment concentrations.

After 20 d on feed, ADG and G:F were improved (P < 0.05) by feeding 5, 10, and 20 ppm RAC when compared with control (0 ppm) pigs, and pigs consuming diets supplemented with 20 ppm RAC had further improvements (P < 0.05) in ADG and G:F than pigs consuming diets formulated with 5 ppm RAC (Table 3). Conversely, ADFI was similar (P > 0.12) among the dietary treatments.

Following the 27-d feeding period, ADG of pigs consuming diets supplemented with 10 and 20 ppm RAC was greater (P < 0.05) than that of the control pigs, and ADG of pigs consuming 5 ppm RAC was not different (P > 0.08) from pigs fed 0, 10, or 20 ppm RAC (Table 3). Feeding RAC did not affect (P > 0.09) ADFI after 27 d on test. All dietary RAC concentrations resulted in improved (P < 0.05) G:F compared with the controls (0 ppm), and pigs receiving 10 and 20 ppm RAC were more efficient (P < 0.05) than pigs receiving 5 ppm RAC.

After the 34-d feeding period, ADG was increased (P < 0.05) by feeding 5 and 20 ppm RAC when compared with 0 and 10 ppm RAC (Table 3). Daily feed intake was also increased (P < 0.05) by feeding 5 ppm RAC, and all pigs consuming RAC-formulated diets were more efficient (P < 0.05) than control pigs. Moreover, pigs consuming 20 ppm RAC were more efficient (P < 0.05) than pigs consuming either 5 or 10 ppm RAC after the 34-d feeding period.

Final preshipment live weights and HCW were not affected (P > 0.07) by RAC after 6 d of feeding; however, numerical advantages were present above the controls of 1.0, 2.4, and 0.6 kg of HCW for pigs fed 5, 10, and 20 ppm RAC, respectively (Table 4). After 13 d of feeding, preshipment live weights were increased (P < 0.05) in pigs fed 5 and 10 ppm RAC, and feeding 20 ppm RAC tended (P = 0.07) to increase preshipment live weights when compared with the controls. All dietary concentrations of RAC increased (P < 0.05) HCW after 13 d of feeding when compared with the controls; however, there were no differences (P > 0.24) among the dietary RAC treatments. After 20 d of feeding, pigs fed 10 and 20 ppm RAC had heavier (P < 0.05) live weights and HCW than controls. Preshipment live weight and HCW were heavier (P < 0.05) in RAC-fed pigs after 27 and 34 d of feeding compared with control-fed pigs. In addition, pigs that received diets supplemented with 20 ppm RAC had heavier (P < 0.05) preshipment live weights and HCW after 34 d of feeding than pigs fed diets formulated with 10 ppm RAC.

Ractopamine did not affect (P > 0.16) 10th-rib fat depth, regardless of feeding duration (Table 5). Even though dietary RAC did not affect (P > 0.25) LM area after 6, 13, or 20 d on feed, 10th-rib LM area was increased (P < 0.05) by feeding 10 and 20 ppm RAC for 27 d and by feeding 5, 10, and 20 ppm RAC for 34 d (Table 5). Fat-free carcass lean estimates were increased (P < 0.05) by feeding 10 and 20 ppm RAC compared with feeding the control diet after 27 d on feed, and feeding 20 ppm RAC resulted in greater (P < 0.05) fat-free carcass lean estimates than feeding 5 ppm RAC (Table 5). After 34 d of feeding, carcasses of pigs fed 20 ppm RAC had greater (P < 0.05) estimates of fat-free carcass lean than carcasses of pigs fed the control diet.

	Discussion

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In pigs, feeding RAC has been shown to result in dramatic improvements in growth, efficiency, and carcass leanness (Watkins et al., 1990; Stites et al., 1991; Dunshea et al., 1993), with no negative effects on meat quality (Stites et al., 1991; McKeith and Ellis, 2001). However, since these data were collected, changes in genetics and management practices have occurred within the swine industry. Pigs representative of the modern swine industry are leaner and are produced more efficiently (Hollis and Curtis, 2001), and it has been demonstrated that the response to RAC is greater in pigs with a high genetic potential for lean growth (Gu et al., 1991a,b; Bark et al., 1992). However, the response to RAC in modern lean genetic lines was unclear.

Growth rate was improved with RAC feeding in the current study, which is in agreement with previously published reports (Uttaro et al., 1993; Crome et al., 1996; Dunshea et al., 1998). The response to ADG was not related to the dietary concentration of RAC, in that 5 ppm RAC resulted in improvements similar to those seen with 20 ppm RAC. These results conflict with those of Watkins et al. (1990) and Stites et al. (1991), in which ADG was increased as dietary RAC concentration increased over a 45- and 48-d feeding period, respectively, even though the magnitudes of those increases were small. Watkins et al. (1990) reported that the maximal response to ADG occurred between 14 and 16 ppm RAC. In addition, current results demonstrated that ADG was increased by RAC after as little as 6 d, and improvements in ADG were also realized through the 34-d feeding period. An increased ADG was also previously reported after a 7-d feeding period (Williams et al., 1994), and these improvements were maintained through a 49-d feeding period.

Gain:feed was improved by dietary RAC, which agrees with previous work (Uttaro et al., 1993; Crome et al., 1996; Dunshea et al., 1998), and pigs in the current study consuming diets with 20 ppm RAC were more efficient than pigs consuming diets with 5 ppm RAC beginning at, and continuing after, 20 d of RAC feeding. This is hypothesized to be a response to changes in growth composition of pigs receiving higher dietary concentrations of RAC. Specifically, pigs consuming diets supplemented with 20 ppm RAC had larger LM areas beginning at, and continuing after, 27 d of RAC feeding, which is in agreement with previous studies (Aalhus et al., 1990; Stites et al., 1991; Crome et al., 1996). An increase in lean tissue would be more efficient because the deposition of lean tissue is more energetically efficient than deposition of adipose tissue (de Lange et al., 2001). These results agree with the work of Watkins et al. (1990), who demonstrated that the maximal improvement in G:F with RAC feeding occurred between 18 and 20 ppm RAC; however, Stites et al. (1991) reported an improvement in G:F with RAC feeding, without a dose dependent response.

Lean prediction equations may contain prediction biases and, therefore, underestimate the magnitude of the effects of feeding RAC (Schinckel et al., 2003). Specifically, prediction equations that included standard carcass measurements (e.g., carcass weight, 10th-rib fat depth, and 10th-rib LM area), as was used in the current study, only accounted for approximately 50% of the increased carcass lean with RAC (Gu et al., 1992; Schinckel et al., 2003). Therefore, the estimate of calculated carcass lean in the current study may be under-predicted with respect to the actual value. To accurately predict the compositional growth of RAC-fed pigs, more precise measurements, such as carcass dissection (dissected ham or loin lean) or chemical analysis, should be implemented when RAC is fed (Schinckel et al., 2003).

Ractopamine feeding did not alter 10th-rib fat depth, which contradicts previous research reporting reductions in fat depth with RAC feeding (Williams et al., 1994; Crome et al., 1996). This discrepancy may be explained, in part, by improvements in genetics in the modern swine industry (Hollis and Curtis, 2001). For example, control (0 ppm RAC) pigs in the current study had an average 10th-rib fat depth measurement of 19.2 mm, whereas the control pigs used in the study of Uttaro et al. (1993) had an average 10th-rib fat depth of 22.5 mm. In addition, Schinckel et al. (2003) reported that control barrows had a lower total carcass fat mass compared with barrows evaluated 10 yr prior (Wagner et al., 1999; Schinckel et al., 2001).

The increases in final preshipment live weight with RAC feeding were realized as increases in the HCW, except when 5 ppm RAC was fed for 20 d. These results concur with previous work that demonstrated improvements in live weight and HCW in RAC-fed pigs (Stites et al., 1991; Crome et al., 1996). Improvements in dressing percent, resulting from the increased HCW, were realized when 10 ppm RAC was fed for 20, 27, and 34 d, and when 20 ppm was fed for 6, 13, 20, 27, or 34 d. Effects on dressing percent were more consistent within the current study when higher dietary concentrations of RAC were fed, which agrees with the work of Watkins et al. (1990).

Ractopamine did not consistently affect measures of meat quality, which support the work of Stites et al. (1991) and McKeith and Ellis (2001). These current results are desirable, in that feeding RAC does not impact the high meat quality standards within the swine industry. Specifically, 5 and 10 ppm RAC did not alter the subjective (American and Japanese color scores) or objective (L*, a*, and b* values) measures of LM color. These results agree with the work of Aalhus et al. (1990), Stites et al. (1991, 1994), and Crome et al. (1996) with respect to subjective color scores and L* (lightness) values. However, feeding 20 ppm RAC for 20 to 34 d did result in alterations in pork color, which is in agreement with the work of Aalhus et al. (1990) and Uttaro et al. (1993), who reported changes in a* and b* values of fresh loins and hams from pigs fed 20 ppm RAC. Similar differences were observed across treatments for the light reflectance variables in the aforementioned trials and in the current study. The consistent results within each trial are important, as it is difficult to make a direct comparison of the results in the current study to previously published trials. The different equipment used in each trial, standardized to different tiles with different apertures and angles of reflectance are difficult to compare.

Marbling scores for the LM were decreased by 10 and 20 ppm RAC after 34 d of feeding. Published results on subjective marbling score are not consistent. Some results indicate no effect of RAC on the visual assessment of marbling in the LM between the 10th and 11th ribs (Watkins et al., 1990; Stites et al., 1991; Crome et al., 1996), or on the extractable lipid content of the LM (Stites et al., 1994). Conversely, other studies indicate that RAC may increase the marbling scores (Aalhus et al., 1990; Watkins et al., 1990). In addition, the incidence of PSE or DFD hams was not affected by feeding RAC, which concurs with the work of Aalhus et al. (1990).

	Implications

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Feeding ractopamine to finishing pigs results in production advantages. Ractopamine resulted in improvements in average daily gain and gain:feed, and these improvements were realized after as little as 6 d of feeding. There were no increases in average daily gain with increasing dietary concentration of ractopamine; however, higher dietary concentrations of ractopamine fed for longer durations resulted in improvements in gain:feed, carcass weight, longissimus muscle area, and predicted fat-free carcass lean. Additionally, advantages to feeding ractopamine at 5 and 10 ppm were obtained without compromising meat quality.

¹ Correspondence: 2001 W. Main St. (phone: 317-655-0957; fax: 317-277-1414; e-mail: tarmstrong{at}lilly.com).

Received for publication March 1, 2004. Accepted for publication June 21, 2004.

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