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J. Anim. Sci. 2003. 81:1508-1516
© 2003 American Society of Animal Science

The effect of feeding ractopamine (Paylean) on muscle quality and sensory characteristics in three diverse genetic lines of swine1,2

G. M. Stoller*, H. N. Zerby*, S. J. Moeller*,3, T. J. Baas{dagger}, C. Johnson{dagger} and L. E. Watkins{ddagger}

* The Ohio State University, Columbus 43210; and {dagger} Iowa State University, Ames 50010; and and {ddagger} Elanco Animal Health, Greenfield, IN 46140

3 Correspondence:
122 Animal Science Bldg., 2029 Fyffe Road (phone: 614-688-3686; fax: 614-292-3513; E-mail:
moeller.29{at}osu.edu).


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
The present experiment utilized Berkshire (n = 76), Duroc (n = 81), and high-lean commercial crossbred (n = 75) barrows and gilts with an initial BW of approximately 85.1 kg. Pigs were fed a standard commercial diet (17.6% CP, 1.02% lysine) supplemented with ractopamine hydrochloride at a level of 0 or 10 ppm for 28 d. The experiment was conducted in a randomized complete block design, with animals blocked within genetic line according to litter, gender, and weight, for a total of four blocks per genetic line for each treatment. Pigs were harvested at a commercial abattoir and chilled for 24 h at 1 to 4°C. At 24 h postmortem, wetness and firmness scores and ultimate muscle pH were measured in the center of the longissimus muscle (LM) at the 10th to 11th rib interface. Visual and instrumental color and marbling score were measured at 48 h postmortem on a fresh cut LM surface. Percentage of chemically extracted intramuscular fat (IMF) was measured, and a trained sensory panel evaluated cooked LM chops for juiciness, tenderness, and chewiness. Cooking loss (%) and instrumental measurement of tenderness also were measured on cooked LM chops. Ractopamine treatment increased ADG (P < 0.01) and LM area (P < 0.05), but had no effect (P > 0.05) on LM quality, sensory attributes, or instrumental measures of palatability. Berkshire LM received higher tenderness and juiciness (P < 0.05) scores and had lower cooking losses (P < 0.05) and instrumental tenderness (P < 0.05) than LM from the Duroc and high-lean lines. Loins from barrows were firmer (P < 0.05), had lower drip loss percentages (P < 0.05), and received greater tenderness scores (P < 0.05) than the LM from gilts. Genetic line x treatment and gender x treatment interactions were detected for IMF. The LM of Berkshire pigs fed ractopamine had lower (P < 0.05) IMF than Berkshires fed the control diet, with no interaction in the other lines. Purebred barrows (Berkshire and Duroc) had greater (P < 0.001 and P < 0.05, respectively) IMF than their respective purebred gilts, with no gender difference in IMF in the high-lean line. Results from the present study indicate that feeding ractopamine does not affect most muscle quality and palatability characteristics. However, the genetic line x treatment interaction for loin IMF suggests that feeding ractopamine might reduce IMF within the loin muscle of genetic lines that have a propensity to produce greater levels of IMF.

Key Words: ß-Adrenergic Agonists • Palatability • Pigs • Pork


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
Under present marketing systems, hogs are valued almost exclusively on fat:lean ratios derived from backfat and/or loin muscle depth measurements. To maximize economic benefits, the primary focus of swine producers has been improving carcass leanness and production efficiency with limited economic incentive for improving muscle quality. The use of ractopamine (Paylean) in swine diets has been shown to improve growth rate (See et al., 2002), feed efficiency (Main et al., 2002), and carcass leanness (Anderson et al., 1987; Crome et al., 1996), resulting in enhanced opportunity to improve enterprise profitability. However, limited research is available that evaluates the effects of feeding ractopamine on muscle quality and sensory attributes in populations of pigs that are currently filling niche markets based on muscle quality attributes.

Increasing interest in fresh and processed pork quality by packing companies and niche consumer markets (100% Pure Berkshire, Five-Star Duroc Pork), and the generally antagonistic genetic correlations between lean percentage and muscle quality attributes (Sellier, 1998) support the need to more closely evaluate the impact of feeding ractopamine on muscle quality characteristics of swine. The ability to capture the economic improvement associated with improved efficiency of production and increased market value while maintaining acceptable levels of muscle quality to meet niche market targets is critical to long-term market access. Thus, the objectives of this experiment were to evaluate: 1) the impact of feeding ractopamine and potential interactions of ractopamine with genetic line on muscle quality and sensory attributes of pork from genetic lines currently used in commercial and niche markets, and 2) the effect of feeding ractopamine and potential interactions of ractopamine with gender on muscle quality and sensory attributes of pork.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results and Discussion
 Implications
 Literature Cited
 
Genetic Lines
Purebred Duroc, purebred Berkshire, and high-lean, terminal crossbred pigs were used in the present study. The Berkshire breed was chosen to represent a breed that previous research (NPPC, 1995) has shown consistently produces pork with greater ultimate pH, darker color, and improved tenderness with reduced cooking and drip loss when utilized as a purebred or in terminal crossbreeding systems. Whereas the Berkshire is known to produce high quality pork, pigs with Berkshire-sired genetics are often slower growing and less efficient with fatter, less muscular carcasses (NPPC, 1995). Durocs were used to represent a genetic line demonstrated to produce pork with darker color, greater levels of intramuscular fat, and improved loin tenderness while producing faster-growing, very efficient pigs with carcasses that are lean and heavy muscled (NPPC, 1995). Both Berkshire and Duroc breeds are currently used in niche markets (100% Pure Berkshire and Duroc Five-Star Pork, respectively) where premiums are paid for high quality pork. The high-lean, terminal commercial crossbred line represents a typical, commercially produced genetic line of pigs. The dam of the high-lean pigs was a cross of Landrace and Yorkshire, and the sire was a synthetic line specifically selected to improve rate of lean growth, carcass lean, and carcass muscle yield that did not contain Hampshire ancestry.

Pairs of littermate barrows and gilts within each genetic line were selected for use in the present study. The assignment of littermate pairs to treatments provides the opportunity to remove common litter effects in the analysis and thus more accurately estimate treatment effects. When possible, two barrows and two gilts within a litter were selected at weaning. Within the Duroc population, small litter size farrowed and disproportionate sex ratios required the selection of barrow or gilt pairs from within some litters. In these situations, at least two litters per sire were identified to provide adequate gender equality and to provide a paternal genetic tie. Progeny from 25 Berkshire (n = 76), 42 Duroc (n = 81), and 29 high-lean litters (n = 75) entered the testing facility as weaned pigs.

Management
Pigs used in the study were obtained through the cooperation of 10 U.S. swine producers. Early-weaned pigs (11 to 22 d of age) were collected from participating producers and transported to the Iowa Swine Testing Station in Ames. Pigs were penned by genetic line and litter within a mechanically ventilated, totally slotted-floored, double-stocked, wean-to-finish building providing approximately 0.25 m2 of floor space with 25 to 28 pigs per pen. Average entry age and weight were 18 d and 4.9 kg respectively, with a range in entry weight of 1.8 to 10 kg. At approximately 22.7 kg of BW, pigs were allocated within genetic line to ractopamine-fortified or control treatment pens based on litter, gender, and live weight resulting in four pairs of mixed-gender pens with equal gender ratios within each genetic line. Pen stocking rates ranged from 11 to 13 pigs/pen, corresponding to 0.74 to 0.92 m2 of pen space/pig.

Paired pens (littermate, treatment and control pairs) of pigs were started on test on a weekly basis when the average weight of the pens was approximately 85.1 kg. Corn-soybean meal-based diets containing synthetic crystalline lysine (Table 1Go) were fed during the treatment phase with the feeding program formulated to meet or exceed NRC (1998) nutrient requirements to optimize lean growth rate in the high-lean genetic line. Ractopamine was provided in the feed at a dosage rate of 10 ppm for a period of 28 d, and pigs were allowed to ad libitum access to their diets and water. The dosage rate and dosage duration chosen in the current experiment represent what is considered economically effective in most U.S. commercial swine production facilities. The start weight of 85.1 kg was chosen based on the projected average daily gain of the group with a target off test weight near the FDA approved maximum of 109.1 kg. Starting, ending, and carcass weight means for ractopamine and control treatments within genetic lines are presented in Table 2Go. The high-lean line started and finished test at slightly heavier weights than the Berkshire and Duroc pigs, with the difference in starting weight being primarily a function of weekly weighing and the need to maintain cross-classification of genetic lines within harvest date.


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  		Table 1. Dietary specifications for corn-soybean meal-based diets, as-fed basis
 

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  		Table 2. Mean starting, ending, and carcass weights for pigs fed ractopamine-fortified and control diets by genetic lineab
 
DNA Genotyping
Isolated DNA, obtained from whole blood samples, was utilized to test for the presence of the Halothane sensitivity gene, HAL-1843 (Fuji et al., 1991), and the Rendement Napole (RN) gene (Milan et al., 2000). Due to the previously reported impact of HAL-1843 mutation (Christian and Rothschild, 1981; Leach et al., 1996; NPPC, 1997) and the RN- mutation (Le Roy et al., 1990; Lundstrom et al., 1996; Enfalt et al., 1997b) on pork quality, HAL-1843 genotype (GeneScreen, Inc., Dallas, TX) and RN genotype (GeneSeek, Inc., Lincoln, NE) were verified to avoid confounding pork quality results with genes known to influence quality.

Carcass and Muscle Quality Procedures
At the end of the feeding trial, paired pens of pigs were weighed off test and transported 192 km to a commercial abattoir (Hormel Foods, Austin, MN). Following a minimum of 8 h of rest prior to harvest, pigs were electrically stunned, exsanguinated, and processed according to industry-accepted procedures. Pigs were harvested on four dates with genetic lines cross classified across dates to ensure statistical estimability. Hot carcass weights were recorded, and the carcasses were chilled at 1 to 4°C for 24 h prior to collection of carcass measurements.

Carcass collection conformed to procedures outlined in Composition and Quality Assessment Procedures (NPPC, 2000). Carcasses were ribbed between the 10th and 11th ribs, and fat depth and longissimus muscle (LM) area were measured to the nearest 1.27 mm and 0.32 cm2, respectively. Percentage of fat-free carcass lean was estimated using the carcass measurement equation of NPPC (2000).

After a minimum 10-min bloom period, visual assessment of LM firmness (1 = soft, cut surface distorts easily, 2 = firm, cut surface tends to hold shape, 3 = very firm, cut surface very smooth and no distortion of shape; NPPC, 2000) and wetness (1 = exudative with excess fluid on the cut surface, 2 = moist surface with little or no free water, 3 = dry surface with no evidence of free water; NPPC, 2000) was recorded in whole numbers. Ultimate pH of the loin was measured by using a portable pH meter (pH-Star, SFK Inc, Denmark) fitted with a Mettler Toledo (Columbus, OH) glass penetration electrode and inserted into the center of the exposed loin surface being careful to avoid intramuscular fat deposits.

A four- to five-rib section of bone-in, skin-on LM, excised posterior to the 10th- to 11th-rib interface, was captured during fabrication, individually bagged, and transported to the Iowa State University meat laboratory. At 48 h postmortem, LM sections were deboned, defatted, and cut into four 2.5-cm-thick loin chops for further quality assessment. Following a minimum 10-min bloom, subjective color (1 = pale pinkish gray to white, 2 = grayish pink, 3 = reddish pink, 4 = dark reddish pink, 5 = purplish red, 6 = dark purplish red; NPPC, 2000) and marbling (1 = 1% intramuscular fat, 2 = 2% intramuscular fat, 3 = 3% intramuscular fat, 4 = 4% intramuscular fat, 5 = 5% intramuscular fat, 6 = 6% intramuscular fat; NPPC, 2000) scores were collected at the LM surface corresponding to the 11th to 12th rib by trained personnel and recorded in whole numbers. Lightness (L*) was objectively measured on the 11th- to 12th-rib LM surface by using the model CR-310 Minolta Chroma Meter (Minolta Corp, Ramsey, NJ) fitted with a 50-mm diameter orifice using a D65 illuminant, and standardized against a white tile. Percentage drip loss was measured using the filter paper procedure of NPPC (2000), where the amount of fluid (mg) absorbed on a standard-sized filter paper was measured and converted to drip loss percentage using the following formula:


The 11th- to 12th-rib LM section was subsequently homogenized for assessment of intramuscular fat percentage according the methods of Bligh and Dyer (1959). The two posterior loin chops were vacuum-packaged and frozen for use in sensory and instrumental tenderness assessment.

Sensory and Tenderness Assessment
Sensory and instrument tenderness were assessed at the Iowa State University Food Science Department. A three-person trained taste panel (Huff-Lonergan et al., 2002) assessed loin cooked LM quality attributes. Both loin chops from each carcass were thawed under refrigeration at approximately 3°C, and then cooked simultaneously to an internal temperature of 71°C in an electric oven broiler (Amana model ARE 640, Amana, IA). Individual chop temperatures were monitored using Chromega/Alomega thermocouples (0.02 diameter, 1.8 m length) attached to an Omega digital thermometer (model DSS-650, Omega Engineering, Inc., Stamford, CN). Pre- and postcooked weights were recorded and used to calculate cooking loss percentage. Sensory traits were assessed on three 1.3-cm3 cubes removed from the center of the broiled loin chop. Each panelist was seated at an individual booth with red light overhead and presented with a single cube of pork loin, which was evaluated on a 10-point, end-anchored category scale for juiciness (1 = dry and 10 = juicy), tenderness (1 = tough and 10 = tender), and chewiness (1 = not chewy and 10 = very chewy). Between samples, room-temperature, deionized, distilled water and unsalted crackers were served to cleanse the palate.

An objective, instrumental measure of Instron tenderness was evaluated on one chop per pig by using a circular, five-pointed star probe attached to an Instron Universal Testing Machine (model 1122; Instron Corp., Canton, MA). The star probe was 9 mm in diameter with 6 mm between each point. The angle from the end of each point to the center is 48°. A 100-kg load cell was used with a cross-head speed of 200 mm/s. The amount of force (kg) required to puncture and compress the chop to 80% of sample height was recorded, and the mean of three measurements per chop was used for statistical analysis.

Statistical Analysis
Dependent variables were grouped into subsets defined as growth, carcass, muscle quality, and sensory attributes for statistical analysis with pig as the experimental unit. Dependent variables were analyzed using mixed model procedures (SAS, Inst. Inc., Cary, NC). Genetic line, gender, treatment, and the two-way interactions of genetic line x gender, gender x treatment, and genetic line x treatment were included as independent variables initially in all statistical models. Interactions were removed from subsequent analyses of dependent variables only if all dependant variables within a defined group of traits had no significant interaction detected. Muscle quality data were analyzed with harvest date included as a random variable to reduce a known source of environmental variation. Growth data were analyzed with a linear covariate for starting weight, and carcass data were analyzed using a linear covariate for carcass weight to adjust for known sources of variation. Least squares means and standard errors were calculated for independent variables and interactions and differences among means were compared by using the PDIFF option (SAS, Inst. Inc.) of the mixed model procedure for all preplanned comparisons among independent variables.


   Results and Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
Implications
 Literature Cited
 
DNA Genotyping
All pigs were genotyped nonmutant normal (NN) at the HAL-1843 locus. The mutant RN- allele was present in each genetic line and was distributed as follows: Berkshire (71 rn+/rn+ and 5 RN-/rn+), Duroc (75 rn+/rn+ and 6 RN-/rn+), and high-lean, terminal crossbred (70 rn+/rn+, 5 RN-/rn+ and 3 RN-/RN-). The detection of three RN-/RN- pigs within the high-lean line was not expected considering the both the sire and dam lines of the high-lean did not contain genes of Hampshire origin, and previous studies of the RN- mutation have shown the mutation is only present in breeds of Hampshire origin (Enfalt et al., 1997a). Thus, the three RN-/RN- pigs were removed from quality and sensory analyses. Due to confounding of the RN-/rn+ genotype within treatment for each genetic line, RN gene effects were not estimable and were not utilized in the final statistical models. The calculated frequency of the RN gene within the entire test population was 3.44%.

Growth and Carcass Traits
Results of growth rate and carcass composition analyses are presented in Table 3Go. Average daily gain for the 28-d test period, adjusted to a common starting weight, was greater (95 g/d; P < 0.01) for pigs fed the ractopamine-fortified diet compared with those fed the control diet. The results of the present study support previous research findings where the addition of ractopamine improved growth rate across environmental temperature (Spencer et al., 2002), nutritional levels (Weldon and Armstrong, 2001), and under various dosage regimens (Herr et al., 2001b). Growth rate differences were also evident among genetic lines. High-lean pigs grew faster (P < 0.05) than the purebred pigs, and purebred Berkshire pigs grew faster (P < 0.05) than purebred Duroc pigs. Additionally, growth rate for barrows was greater (P < 0.01) than for gilts. However, in the Duroc line, no difference in growth rate was detected between the genders, resulting in a genetic line x gender interaction (P < 0.05).


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  		Table 3. Least squares means and standard errors for growth and carcass traits by genetic line, treatment, and gender for pigs fed a control or ractopamine-fortified diet
 
A genetic line x treatment interaction (P < 0.05) was observed where 10th-rib fat depth was reduced (18.6 vs. 15.6 mm; P < 0.01) in only the high-lean genetic line by feeding ractopamine (Figure 1Go). Bark et al. (1992), in a study conducted to evaluate ractopamine response in high- and low-lean tissue growth genotypes, concluded that a genotype x ractopamine interaction existed for accretion rate and amount of total carcass muscle and dissectible fat with the high-lean genotype having greater total carcass muscle than the low-lean genotype; however, both high- and low-lean genotypes responded with a reduction in 10th-rib fat depth of similar magnitude when fed ractopamine. This finding was not consistent with the results of the present study. Carcasses from gilts had less (P < 0.01) fat at the 10th rib than barrows (Figure 1Go), yet the largest difference between genders for 10th-rib fat depth was within Berkshire pigs (8.8 mm) compared with Duroc (5.1 mm) and high-lean (4.4 mm) genetic lines (genetic line x gender; P < 0.05). The presence of a genetic line x treatment interaction for 10th-rib fat depth suggests the response to feeding ractopamine is genetic line dependent, and the economic implications of reduced fat may not be realized across all genetic lines of swine.



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  		Figure 1. Interactive effects on 10th-rib fat depth. Within each panel, least squares means that do not have a common superscript letter differ (P < 0.001). The genetic line x dietary treatment interaction (P < 0.05) is presented in Panel A. The mean difference between control- and ractopamine-fed pigs within Berkshire, Duroc, and High-lean breed-types was 1.3, 1.3, and 3.0 mm, respectively. The genetic line x gender interaction (P < 0.05) is presented in Panel B. The mean difference between barrows and gilts within Berkshire, Duroc, and High-lean breed-types was 8.8, 5.1, and 4.4 mm, respectively.

 
The addition of ractopamine in the diet increased (P < 0.05) LM area by 1.4 cm2 (Table 3Go) compared with the control diet, and no (P > 0.05) interactions with genetic line or gender were observed. The observed increase in LM area when feeding ractopamine is consistent with findings of Bark et al. (1992), Crome et al. (1996), and Weber et al. (2002). Estimated percentage of lean, a function of 10th-rib fat depth, LM area, and hot carcass weight, was not affected by feeding ractopamine in the present study as might be expected given no differences among treatments for 10th-rib fat depth.

Muscle Quality Characteristics
There were no (P > 0.05) differences in any visual (color, firmness, and marbling) or instrumental quality trait measured between loins from pigs fed the control or ractopamine-fortified diet (Table 4Go). These results are similar to the findings of Hancock et al. (1987), Watkins et al. (1990), and Crome et al. (1996), and suggested that feeding ractopamine at the 10-ppm level will not affect a consumer’s subjective assessment of fresh loin quality and/or palatability. The inclusion of supplemental ractopamine had no (P > 0.05) effect on CIE L*, which is consistent with the findings for subjective visual color score. Loins from Berkshire and Duroc pigs had greater (P < 0.05) marbling and wetness scores, and loins from Duroc pigs had greater (P < 0.01) color scores than the high-lean line. The results of the present study support the findings of NPPC (1995), where Duroc and Berkshire genetic lines had greater loin marbling scores when compared to standard, terminal crossbred sire lines.


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  		Table 4. Least squares means and standard errors for genetic line, treatment, and gender effects for fresh loin muscle quality attributes of pigs
 
Loins from barrows were firmer (P < 0.05), with greater (P < 0.01) marbling scores than loins from gilts (Table 4Go). Loins from Duroc barrows had a greater (P < 0.01) amount of marbling than loins from Duroc gilts, and loins from Berkshire and Duroc pigs received higher marbling scores than those from high-lean pigs, regardless of gender (genetic line x gender interaction; P < 0.01; Figure 2Go).



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  		Figure 2. Genetic line x gender interaction (P < 0.05) effects on marbling score. Least squares means that do not have a common superscript letter differ (P < 0.05). The mean difference between barrows and gilts within Berkshire, Duroc, and High-lean breed-types was 0.32, 0.89, and 0.05 units, respectively.

 
Neither loin ultimate pH nor drip loss percentage was affected (P > 0.05) by the addition of ractopamine in the diet (Table 4Go). Ivers et al. (2000), in a study conducted using genetically lean pigs, also found no difference in ultimate pH when comparing the loins of pigs fed ractopamine-fortified or control diets. Uttaro et al. (1993) and Aalhus et al. (1990) reported no difference in drip loss when comparing loins from ractopamine-treated and untreated pigs. Loins from the high-lean genetic line had a greater (P < 0.01) drip loss percentage compared with loins from either the Duroc or Berkshire pigs (0.67 and 0.51% greater, respectively). The higher percentage drip loss in the loins from the high-lean genetic line compared with the purebred pigs coincides with, and may be partially related to, the reduction in loin ultimate pH observed within the high-lean line. The absence of a genetic line x treatment interaction for percentage drip loss and ultimate pH indicates that the effect of feeding ractopamine is independent of genetic background.

Sensory Attributes
Taste panel observations of loin tenderness, juiciness, and chewiness were not (P > 0.05) different between ractopamine-treated and untreated pigs (Table 5Go). These results agree with those reported by McKeith et al. (1988), where no differences in taste panel juiciness, tenderness, flavor intensity or off-flavor intensity were observed among treatments when ractopamine was fed at levels of 0, 5, or 10 ppm. Loin chops from Berkshire pigs were the most palatable among the genetic lines, receiving greater (P < 0.01) tenderness and juiciness scores, as well as lower (P < 0.01) chewiness scores, when compared to chops from Duroc and high-lean pigs (Table 5Go). Chops from barrows received greater (P < 0.05) tenderness scores than chops from gilts, which concurs with results by Uttaro et al. (1993), who reported loins from barrows required less force to shear compared with loins from gilts.


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  		Table 5. Least squares means and standard errors for genetic line, treatment, and gender effects for fresh pork loin sensory attributes, objective tenderness measurement, and percentage of intramuscular fat
 
Even though taste panel tenderness was not affected by feeding ractopamine, there was a trend for LM chops from pigs fed the control diet to have lower (P = 0.07) instrumental tenderness values when compared to LM chops from ractopamine-fed pigs (Table 5Go). Uttaro et al. (1993) reported ractopamine-fed pigs produced loins that were 0.49 kg tougher than loins from pigs fed a control diet, and Herr et al. (2001a) reported an increase in Warner-Bratzler shear force of 0.19, 0.23, and 0.39 kg for loins from pigs fed ractopamine at 5, 10, and 20 ppm, respectively, when compared with loins from untreated pigs. In contrast to the findings of Uttaro et al. (1993) and Herr et al. (2001a), McKeith et al. (1988) and Merkel et al. (1990) reported no difference in shear force values for loins from control pigs and pigs fed up to 20 ppm ractopamine. Although mechanical measures of tenderness were not of the magnitude to be detected by a trained sensory panel, it appears that mechanical measurements provide conflicting results with regard to the impact of feeding ractopamine fortified diets on loin tenderness.

As expected, loins from Berkshire pigs were more tender, receiving greater (P < 0.01) tenderness scores and lower (P < 0.01) instrumental tenderness values than loins from the Duroc and high-lean pigs (Table 5Go). Consistent with sensory panel observations, LM chops from barrows tended to require less (P = 0.07) mechanical force to puncture than LM chops from gilts. Uttaro et al. (1993) recorded a 0.72-kg difference in Warner-Bratzler shear force (P < 0.01) between loins from barrows and gilts with the loins from barrows being more tender.

No difference (P > 0.05) in cooking loss was observed between LM chops from ractopamine-fed and control pigs (Table 5Go). In contrast, Uttaro et al. (1993) reported that loin chops from pigs treated with ractopamine had less cooking losses than chops from pigs fed a control diet. The lack of a dietary effect on percentage cooking loss in the present study may have been expected given that no differences were observed for loin wetness score, ultimate pH, or percent drip loss when comparing LM chops from ractopamine-fed and control diets.

Intramuscular fat percentage (IMF) in the LM was not (P > 0.05) different between ractopamine-fed and control pigs (Table 5Go); however, loins from Berkshire and Duroc pigs had higher (P < 0.01) IMF percentages than chops from high-lean pigs. Moreover, LM chops from barrows had more (P < 0.01) IMF than LM chops from gilts.

Loin IMF levels for Berkshire pigs fed ractopamine were lower than loin IMF levels of Berkshires fed the control diet, and loin IMF levels were substantially greater than those of the high-lean pigs, regardless of diet (genetic line x treatment interaction; P < 0.05; Figure 3Go). Furthermore, dietary inclusion of ractopamine had no impact on IMF content in Duroc pigs, and these IMF percentages were similar to those of Berkshire pigs. The present results suggest that the impact of ractopamine on loin IMF is genetic line dependent and possibly related to the magnitude of IMF within the genetic lines studied. Interestingly, the reduction of loin IMF within the Berkshire pigs that received a ractopamine-fortified diet occurred without a reduction in 10th-rib fat depth (Figure 1Go). The authors hypothesize that the differences between genetic-lines for loin IMF may be attributed to differences in genetic potential for IMF deposition across genetic-lines. The significant reduction in loin IMF percentage for ractopamine-fed Berkshire pigs, without a reduction in 10th-rib fat depth, may be attributed to a difference in the growth curve and/or adipocyte maturation. Berkshires are an early-maturing breed that tends to produce carcasses with greater fat depths (NPPC, 1995; Jones, 1998) than other lines of swine at a constant weight and age. Given that the subcutaneous fat depth was greater for Berkshire pigs, the authors hypothesize that the subcutaneous adipocyte cells in the Berkshires were more mature at the start of ractopamine administration. Therefore, the effects of ractopamine may have led to either a reduction in IMF deposition or an increased catabolism of IMF depots within the LM without affecting subcutaneous fat depth due to adipocyte maturity. Another hypothesis is that the metabolic processes that partition nutrients toward the development of subcutaneous fat compared with IMF may be affected differently within a given breed following ractopamine administration, thus resulting in the differences observed in the current study.



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  		Figure 3. Interactive effects on percentage of intramuscular fat. Within each panel, least squares means that do not have a common superscript letter differ (P < 0.05). The genetic line x dietary treatment interaction (P < 0.05) is presented in Panel A. The mean difference between control- and ractopamine-fed pigs within Berkshire, Duroc, and High-lean breed-types was -0.40, -0.14, and +0.26%, respectively. The genetic line x gender interaction (P < 0.05) is presented in Panel B. The mean difference between barrows and gilts within Berkshire, Duroc, and High-lean breed-types was 0.82, 0.41, and 0.05%, respectively.

 
Loins from barrows had greater IMF percentages than loins from gilts for both Berkshire and Duroc pigs (Figure 3Go); however, there was no difference in IMF content between barrows and gilts in the high-lean pigs (genetic line x gender interaction; P < 0.05). Gender differences were greater in the purebred (Berkshire and Duroc) genetic lines that had the greater levels of IMF in the LM. The genetic line x gender interaction for IMF corresponded closely with marbling score, indicating that both the subjective and objective measurement techniques detected differences in the quantity of IMF deposited within the LM.


   Implications
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
Literature Cited
 
The results of this study indicate that adding ractopamine to the swine diet at 10 ppm, when fed for the last 28 d to a target weight of 109 kg, improves rate of body weight growth and carcass muscularity without affecting quality traits associated with visual, instrumental, and sensory attributes of pork. However, genetic line differences were found for many quality traits, indicating genetic background must be considered in the production of high-quality pork. Feeding ractopamine may lower percentage of loin intramuscular fat in pigs that have the genetic potential to produce fresh pork loins with higher amounts of intramuscular fat. Future research must further address the interaction of ractopamine with genetic line for percentage of intramuscular fat.


   Footnotes
 
1 Research support provided by Elanco Animal Health, a Division of Eli Lilly and Co. Back

2 Salaries and research support provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Center, The Ohio State University. Manuscript No. 36-01AS. Back

Received for publication February 11, 2001. Accepted for publication March 4, 2003.


   Literature Cited
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 


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