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Effects of dietary lysine and energy density on performance and carcass characteristics of finishing pigs fed ractopamine¹

J. K. Apple^*,2, C. V. Maxwell^*, D. C. Brown^*, K. G. Friesen^*, R. E. Musser^,3, Z. B. Johnson^* and T. A. Armstrong

^* Department of Animal Science, University of Arkansas, Fayetteville 72701; and The Pork Group, a Division of Tyson Foods, Inc., Rogers, AR 72757; and and Elanco Animal Health, a Division of Eli Lilly and Co., Greenfield, IN 46140

	Abstract

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Two hundred sixteen crossbred barrows and gilts (84.3 kg BW) were used to test the effects of dietary energy density and lysine:energy ratio (Lys:ME) on the performance, carcass characteristics, and pork quality of finishing pigs fed 10 ppm ractopamine. Pigs were blocked by BW and gender, allotted to 36 pens (six pigs per pen), and pens were assigned randomly within blocks to dietary treatments (as-fed basis) arranged in a 2 x 3 factorial design, with two levels of energy (3.30 or 3.48 Mcal/kg) and three Lys:ME (1.7, 2.4, or 3.1 g lysine/Mcal) levels. Pigs were fed experimental diets for 28 d, and weights and feed disappearance were recorded weekly to calculate ADG, ADFI, and G:F. Upon completion of the feeding trial, pigs were slaughtered and carcass data were collected before fabrication. During carcass fabrication, hams were analyzed for lean composition using a ham electrical conductivity (TOBEC) unit, and loins were collected, vacuum-packaged, and boxed for pork quality data collection. Energy density had no (P > 0.22) effect on ADG or ADFI across the entire 28-d feeding trial; however, pigs fed 3.48 Mcal of ME were more (P < 0.02) efficient than pigs fed 3.30 Mcal of ME. In addition, ADG and G:F increased linearly (P < 0.01) as Lys:ME increased from 1.7 to 3.1 g/Mcal. Carcasses of pigs fed 3.48 Mcal of ME were fatter at the last lumbar vertebrae (P < 0.08) and 10th rib (P < 0.04), resulting in a lower (P < 0.03) predicted fat-free lean yield (FFLY). Conversely, 10th-rib fat thickness decreased linearly (P = 0.02), and LM depth (P < 0.01) and area (P < 0.01) increased linearly, with increasing Lys:ME. Moreover, FFLY (P < 0.01) and actual ham lean yield (P < 0.01) increased as Lys:ME increased in the diet. Dietary energy density had no (P > 0.19) effect on pork quality, and Lys:ME did not (P > 0.20) affect muscle pH, drip loss, color, and firmness scores. Marbling scores, as well as LM lipid content, decreased linearly (P < 0.01) as Lys:ME increased from 1.7 to 3.1 g/Mcal. There was a linear (P < 0.01) increase in shear force of cooked LM chops as Lys:ME increased in the finishing diet. Results indicate that 3.30 Mcal of ME/kg (as-fed basis) is sufficient for optimal performance and carcass leanness in pigs fed ractopamine. The Lys:ME for optimal performance and carcass composition seems higher than that currently used in the swine industry; however, feeding very high Lys:ME (> 3.0 g/Mcal, as-fed basis) to ractopamine-fed pigs may result in decreased marbling and cooked pork tenderness.

Key Words: Carcass Characteristics • Energy • Lysine • Pork Quality • Ractopamine • Swine

	Introduction

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Ractopamine hydrochloride (Paylean; Elanco Animal Health, Greenfield, IN) effectively repartitions nutrients from fat deposition (Watkins et al., 1990; Dunshea et al., 1993) toward increased protein synthesis (Bergen et al., 1989) and muscle protein accretion without impacting pork quality (McKeith et al., 1988; Stoller et al., 2003). The observed increase in lean growth in pigs fed diets containing ractopamine requires increased dietary protein content and/or quality to sustain protein synthesis and accretion in finishing swine (Adeola et al., 1992; Dunshea et al., 1993). Additionally, it seems that ractopamine is capable of producing maximal lean composition in pork carcasses at low energy intakes by repartitioning energy for maximal protein deposition (Williams et al., 1994).

Ractopamine may be included in swine diets containing 16% CP, and is approved by the U.S. FDA for finishing pigs from 41 to 109 kg BW. Integrated swine operations typically feed diets with approximately 3.4 Mcal of ME/kg, coupled with decreases in essential AA to optimize profitability during the final stage of the finishing period; however, these finishing diets are supplemented with synthetic AA to meet the ideal AA ratios of Chung and Baker (1992). These decreased AA concentrations may not be sufficient to meet the requirements of pigs fed ractopamine (Schinckel et al., 2000, 2003; Webster et al., 2002b). Furthermore, the lysine:energy (Lys:ME) ratio in ractopamine-fed pigs may have a more profound effect on performance and carcass composition than absolute energy intake values. Therefore, the objective of this study was to determine the interactive effect, if any, of energy density and Lys:ME ratios on performance, and carcass yield and quality traits of finishing swine fed ractopamine.

	Materials and Methods

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Animals and Diets
Two hundred sixteen crossbred barrows and gilts (U.S. Yorkshire x U.S. Landrace females mated to De-Kalb EB sires) with an average BW of 84.3 kg, were purchased from The Pork Group (a division of Tyson Foods, Inc., Rogers, AR), and moved to the University of Arkansas Swine Growing-Finishing Facility. Pigs were blocked by BW and sex, and allotted to 36 pens (six pigs per pen). After a 1-wk adjustment period when a common diet (devoid of ractopamine) was fed, pens were assigned randomly within blocks to one of six dietary treatments arranged in a 2 x 3 factorial design, with two levels of energy (3.30 or 3.48 Mcal/kg, as-fed basis) and three Lys:ME ratios (1.7, 2.4, or 3.1 g of lysine/Mcal, on an as-fed basis). Ractopamine was included in all diets at a level of 10 mg/kg (as-fed basis), and diets (Table 1) met or exceeded NRC (1998) requirements for 80- to 110-kg pigs.

Pigs were housed in a curtain-sided building on partially slatted concrete floors, and each pen was 1.49 x 3.96 m, affording at least 1.18 m²/pig. Additionally, each pen was equipped with a single-opening feeder and nipple-waterers, allowing pigs ad libitum access to feed and water. Pigs were fed the experimental diets including ractopamine for 28 d. Individual pig weights and feed disappearance were recorded at 7-d intervals during the experiment to calculate ADG, ADFI, and G:F. Furthermore, ultrasound measurements of 10th-rib fat depth and LM area were recorded on d 0, 14, and 28 of the experiment by a trained, certified ultrasound technician using an Aloka ultrasound diagnostic machine (model SSD-500V, Aloka Co., Ltd., Tokyo, Japan).

Carcass Data Collection
At the completion of the finishing period, pigs were transported approximately 10 h to a commercial pork slaughter/fabrication plant (Excel Corp.; Beardstown, IL). After a 6-h rest at the plant, all pigs were slaughtered according to industry-accepted procedures. Then, 10th-rib fat and LM depths were measured on-line with a Fat-O-Meater automated probe (SFK Technology A/S, Cedar Rapids, IA) and hot carcass weight was recorded. Following a 24-h rapid chilling period, midline backfat depth opposite the first rib, last rib, and last lumbar vertebrae was recorded, and loins were marked between the 10th and 11th ribs to measure LM area on arrival at the University of Arkansas Red Meat Abattoir. Carcasses were then fabricated into primal cuts, and bone-in hams from left sides were analyzed for lean composition using a ham electrical conductivity (TOBEC) unit (Cargill Meat Solutions, Wichita, KS). Prediction equations used to calculate ham fat-free lean composition could not be presented because they are the intellectual property of Cargill Red Meat Sector. Bone-in pork loins from left sides were collected during fabrication, and subsequently vacuum-packaged, boxed, and transported to the University of Arkansas for pork quality data collection.

Pork Loin Fabrication and Quality Data Collection
Upon arrival, pork loins were removed from the packaging material, cut between the 10th and 11th ribs, and the area of the LM was traced onto acetate paper (Bee Paper Co. Inc., Wayne, NJ). The LM area was measured using a compensating planimeter at a later date. Then, LM chops were removed from the posterior portion of the loin in the following order: 1) two 2.5-cm-thick chops used for subjective and objective pork quality measurements; 2) two 3.8-cm-thick chops used for drip loss determination; and 3) one 2.5-cm-thick LM chop trimmed free of all bone, external fat, and connective tissue, vacuum-packaged, and frozen for LM moisture and proximate analysis.

After a 30-min bloom period at 4°C, the 2.5-cm-thick LM chops were visually evaluated for marbling (1 = devoid [1% i.m. lipid] to 10 = abundant [10% i.m. lipid]; NPPC, 1999), firmness (1 = very soft and watery to 5 = very firm and dry; NPPC, 1991), and color based on both the American (1 = pale, pinkish gray to 6 = dark purplish red; NPPC, 1999) and Japanese color standards (Nakai et al., 1975). Commission International de l’Eclairage (CIE, 1976) L*, a*, and b* values were determined from a mean of four random readings (two readings for each 2.5-cm-thick LM chop) made with a Hunter MiniScan XE (model 45/0-L; Hunter Associates Laboratory, Reston, VA) using illuminant C and a 10°standard observer. The saturation index, or chroma (C*), was calculated as C* = (a*² + b*²)^1/2 and is a measure of the total color, or vividness of the color, of the LM. After quality data collection, both LM chops were wrapped in white, polycoated, heavyweight freezer paper (Paper Con, Dallas, TX), and frozen at –20°C before cooking and Warner-Bratzler shear force (WBSF) determinations.

Drip loss percent was determined following a modified suspension procedure of Honikel et al. (1986). Briefly, a 3.8-cm-diameter core was manually removed from each of the 3.8-cm-thick LM chops, weighed, and suspended on a fishhook (barb removed) attached to the lid of a plastic container (46 x 66 x 38 cm deep Dur-X Food Box; Rubbermaid Commercial Products LLC, Winchester, VA). Containers were sealed and stored at 2°C for 48 h, after which cores were removed from hooks, blotted dry on paper towels, and reweighed. Differences between core weights were used to calculate drip loss percent. Additionally, 2 g of LM from each chop after core removal was homogenized in 20 mL of distilled, deionized water. The pH of the homogenate was measured with a temperature compensating, combination electrode (model 300731.1; Denver Instrument Co., Arvada, CO) attached to a pH/Ion/FET-meter (model AP25; Denver Instrument Co.).

Moisture content was determined according to the freeze-drying method of Apple et al. (2001), in which duplicate 25-g samples of LM were weighed, placed in 30-mL beakers, and reweighed. Beakers were then placed into vacuum flasks attached to the manifold of a Labconco freeze dryer (model 4.5; Labconco Corp., Kansas City, MO) with a temperature setting of –50°C and a vacuum of less than 10 µm of Hg. Samples were freeze-dried for 60 h, and beakers were reweighed. The difference between initial and dried beaker weights was used to calculate percentage of moisture. Freeze-dried samples were subsequently pulverized in a Waring blender (model 38BL54; Waring Commercial, New Hartford, CT), and analyzed for protein, ether extractable lipid, and ash according to AOAC (1990) procedures.

Longissimus muscle chops were thawed for 16 h at 2°C, weighed, and then cooked to an internal temperature of 71°C in a commercial convection oven (Zephaire E model; Blodgett Oven Co., Burlington, VT) preheated to 165°C. Internal temperature was monitored with Teflon-coated thermocouple wires (Type T; Omega Engineering, Inc., Stamford, CT) placed into the geometric center of each LM chop and attached to a multichannel data logger (model 245A; VAS Engineering Inc., San Diego, CA). Chops were turned once during the cooking process, when the internal temperature reached 35°C. Immediately after removal from the oven, chops were blotted dry on paper towels and weighed, and the difference between precooked and cooked weights was used to calculate cooking loss percent. Chops were allowed to cool to room temperature, and five 1.27-cm-diameter cores were removed parallel to the muscle fiber orientation. Then, each core was sheared once through the center with a WBSF device attached to an Instron Universal Testing Machine (model 4466; Instron Corp., Canton, MA) with a 55-kg tension/compression load cell and a crosshead speed of 250 mm/min.

Statistical Analyses
All data were analyzed as a randomized complete block design with treatments arranged in a 2 x 3 factorial design, and pen as the experimental unit. Analysis of variance was generated using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC), with energy density, Lys:ME ratio, and energy x Lys:ME as the main effects in the model. Least squares means were calculated for all data, and pair-wise t-tests (PDIFF option) were used to statistically separate treatment least squares means when a significant F-test (P < 0.05) was observed. Additionally, linear and quadratic contrasts were used to detect the response of Lys:ME across energy levels.

	Results and Discussion

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Performance
Energy density had no effect (P > 0.72) on ADG during the first 14 d of the feeding trial; however, ADG increased linearly during the first half of the trial (0 to 7 d, P < 0.01; 7 to 14 d, P = 0.02) as Lys:ME increased in the diet (Table 2). Growth rate during the third week (14 to 21 d) was not (P > 0.20) affected by either energy density or Lys:ME, but pigs fed 3.48 Mcal/kg tended to have higher (P < 0.08) ADG than pigs fed 3.30 Mcal/kg during the last week (21 to 28 d) of the trial. Moreover, across the entire 28-d feeding trial, ADG increased linearly (P < 0.01) as Lys:ME increased from 1.7 to 3.1 g/Mcal, but was not (P > 0.23) affected by diet energy density.

Results of the present study support the notion that increasing the energy density of late-finishing swine diets has no appreciable effect on ADG (Le Dividich et al., 1987; Matthews et al., 1998, 2003). Furthermore, neither Williams et al. (1994) nor Dunshea et al. (1998) found an interactive effect of ME density and ractopamine on pig growth rates. Conversely, ADG has been shown to increase in response to increasing dietary CP (Cromwell et al., 1978, 1993; Chen et al., 1995), lysine (Goodband et al., 1990; Gatel and Grosjean, 1992; Friesen et al., 1994), and, consistent with the present study, lysine:energy ratio (Castell et al., 1994; Cameron et al., 1999; Szabó et al., 2001). In addition, other studies have shown that ractopamine increased growth rate in pigs fed 17.0 or 18.0% CP, but not in pigs fed 13.0% CP diets (Xiao et al., 1999; Adeola et al., 1990). Webster et al. (2002b) reported that ADG also increased in ractopamine pigs as dietary lysine increased, which is consistent with results of the present study.

Neither energy density nor Lys:ME affected (P > 0.22) ADFI; however, G:F increased linearly (P < 0.01) as Lys:ME increased in the diet (Table 2). Additionally, G:F was greater (P < 0.02) in ractopamine-fed pigs consuming 3.48 vs. 3.30 Mcal/kg. Castell et al. (1994) also reported no effect of lysine:energy ratio on ADFI, whereas Campbell et al. (1984), Gatel and Grosjean (1992), and Witte et al. (2000) found no effect of dietary lysine content on ADFI. However, in accordance with the present results, Castell et al. (1994) and Batterham et al. (1990) observed that feed efficiency improved linearly as the lysine:energy ratio increased in swine diets, and G:F has been shown to increase in response to increasing lysine levels (Campbell et al., 1984; Goodband et al., 1990; Friesen et al., 1994) or CP content (Cromwell et al., 1978, 1993; Chen et al., 1995) in swine diets. Moreover, Adeola et al. (1990), Mitchell et al. (1991), and Xiao et al. (1999) reported that ractopamine-fed pigs were more efficient than their untreated counterparts when fed diets containing 17.0 to 18.0% CP, but feed efficiency was similar between control and ractopamine pigs when diets contained 12.0 to 13.0% CP.

Some researchers have shown that increasing the energy density in swine diets decreases ADFI (Campbell and Taverner, 1986; Southern et al., 1989), whereas others have failed to detect differences in feed intake by increasing dietary energy (Matthews et al., 1998, 2003). However, it is clear that increasing dietary energy improves feed efficiency in pigs not fed ractopamine (Seerley et al., 1978; Campbell and Taverner, 1986; Southern et al., 1989). In accordance with results of the present trial, both Williams et al. (1994) and Dunshea et al. (1998) reported that increasing energy density of swine finishing diets containing ractopamine resulted in improved G:F; however, there was no interactive effect of energy density and ractopamine on feed efficiency.

Ultrasound-measured fat thickness was greater on d 14 (P < 0.05) and 28 (P < 0.02) in pigs fed the high-energy diets compared with pigs consuming the low-energy diets; however, neither scanned LM area nor live weight was affected (P > 0.42) by dietary energy level (Table 2). Conversely, ultrasound-measured LM area increased linearly on d 14 (P = 0.01) and d 28 (P < 0.01), whereas there was a linear decrease (P = 0.04) in scanned fat thickness on d 28, as Lys:ME ratio increased from 1.7 to 3.1 g/Mcal. In pigs not fed ractopamine, Castell et al. (1994) reported that ultrasonically measured fat thickness decreased linearly as the lysine:energy ratio increased from 1.6 to 2.6 g/Mcal DE. Moreover, Cameron et al. (1999) observed that scanned backfat was decreased, and LM depth was increased, in pigs fed 3.32 and 4.89 g of lysine/Mcal of ME compared to those fed 1.76 g of lysine/Mcal of ME.

Carcass Characteristics
Energy density of finishing diets had no effect (P > 0.86) on hot carcass weights and dressing percents of pigs fed ractopamine (Table 3). Carcasses from pigs fed the high-energy (3.48 Mcal/kg) diets were fatter than those of pigs fed the low-energy (3.30 Mcal/kg) diets, especially opposite the last lumbar vertebra (P < 0.08) and at the 10th rib (P < 0.04), resulting in lower (P < 0.03) predicted fat-free lean yields compared with carcasses from pigs fed 3.30 Mcal/kg. Even though calculated fat-free lean yields differed between diet energy densities, neither whole ham weight nor ham lean weight and percent was different (P < 0.35) between pigs fed 3.30 and 3.48 Mcal/kg.

Dressing percent was not (P > 0.14) affected by Lys:ME, although carcasses of pigs fed Lys:ME levels of 2.4 and 3.1 g/Mcal were heavier (P < 0.01) than carcasses of pigs fed 1.7 g/Mcal (Table 2). Both last lumbar vertebrae backfat and 10th-rib fat depths decreased linearly (P = 0.02) as Lys:ME increased from 1.7 to 3.1 g/Mcal. Furthermore, LM depth (P < 0.01) and area (P < 0.01), as well as carcass fat-free lean yield (P < 0.01), increased as Lys:ME ratio increased from 1.7 to 3.1 g/Mcal. Improvements in carcass muscling and leanness with increasing Lys:ME ratio were further manifested in the observation that ham weight, ham lean weight, and yield also increased linearly (P < 0.01) as Lys:ME ratio was increased from 1.7 to 3.1 g/Mcal in ractopamine-supplemented diets.

In agreement with the current results, Batterham et al. (1990) and Castell et al. (1994) reported that average backfat decreased linearly with increasing Lys:ME. Furthermore, LM area has been repeatedly shown to increase in response to increased dietary Lys:ME (Castell et al., 1994; Grandhi and Cliplef, 1997; Cameron et al., 1999), lysine (Goodband et al., 1990; Witte et al., 2000), and CP (Cromwell et al., 1978, 1993; Chen et al., 1995) content. Calculated, or dissected, lean yield percent has also been increased by increasing the lysine:energy ratio (Grandhi and Cliplef, 1997; Szabó et al., 2001), lysine level (Dourmad et al., 1996; Witte et al., 2000), and CP content (Davey, 1976; Cromwell et al., 1978, 1993) in swine finishing diets. More importantly, Castell et al. (1994) observed that ham weight increased linearly with increasing lysine:energy ratio, and Grandhi and Cliplef (1997) found that the percentage of lean in the ham increased as Lys:ME increased in the diet. Including ractopamine in swine finishing diets increases carcass leanness and muscling (Crome et al., 1996; Jones et al., 2000; Webster et al., 2002b), but when all pigs were fed ractopamine-supplemented diets, carcass cutability improved linearly with increasing Lys:ME, lending support to the theory that lysine requirements for optimal carcass composition are higher for ractopamine-fed pigs than for pigs fed diets devoid of ractopamine.

Longissimus Muscle Quality
Dietary energy density had no effect (P > 0.19) on any quality trait measured (Table 4). Moreover, muscle pH, drip loss, Japanese and American color scores, firmness score, and redness (a*) value of the LM were not (P > 0.17) affected by Lys:ME. However, marbling scores decreased as Lys:ME increased in the diet (linear effect, P < 0.01). There was a quadratic decrease in L* values (P < 0.03) with increasing Lys:ME ratio, with the LM from pigs fed 2.4 g/Mcal being darker than the LM from pigs fed 1.7 g/Mcal. The yellowness (b* value) and total color (C* value) of the LM decreased linearly (P = 0.02) as Lys:ME increased from 1.7 to 3.1 g/Mcal; however, differences among Lys:ME levels were quite small and seemingly irrelevant.

The consensus is that formulating diets based on CP, lysine, or lysine:energy ratio has no influence on ultimate muscle pH (Goerl et al., 1995; Witte et al., 2000; Szabó et al., 2001), drip loss percent (Castell et al., 1994; Witte et al., 2000; Szabó et al., 2001), or other measures of water-holding capacity (Gatel and Grosjean, 1992; Goerl et al., 1995), subjective color scores (Goodband et al., 1990; Friesen et al., 1994; Witte et al., 2000), firmness scores (Goodband et al., 1990; Friesen et al., 1994; Grandhi and Cliplef, 1997), and L* values (Goerl et al., 1995; Cameron et al., 1999; Witte et al., 2000). However, results from the present study are consistent with those of Cameron et al. (1999), who reported that pork from pigs fed a diet containing 1.76 g of lysine/Mcal of ME was redder and more yellow (higher a* and b* values, respectively), resulting in a more vivid (higher C* value) color, than pork from pigs fed either 3.32 or 4.89 g of lysine/Mcal of ME. Moreover, Goerl et al. (1995) demonstrated that a* and b* values decreased linearly as dietary CP increased from 10 to 25% in swine finishing diets.

Several researchers have demonstrated that including ractopamine in swine finishing diets has no effect on pork quality traits (Crome et al., 1996; Spencer et al., 2002; Stoller et al., 2003). Even though L* values were not affected by ractopamine inclusion in swine diets (Webster et al., 2002a; Stoller et al., 2003), Aalhus et al. (1990) and Uttaro et al. (1993) reported that pork from pigs fed diets devoid of ractopamine had higher a* and b* values than pork from ractopamine-fed pigs. When reporting the results of several experiments, Watkins et al. (1990) failed to detect an effect of ractopamine dosage on pork color or firmness; however, in the second experiment, they found that pork color and firmness actually improved over controls when 10 to 20 ppm ractopamine was included in the diet.

In agreement with the effect of Lys:ME on marbling scores, i.m. lipid content also decreased linearly (P < 0.01) with increasing Lys:ME ratio (Table 4). Additionally, similar to marbling scores, energy density of the diet did not (P > 0.19) affect LM i.m. fat content.

The effect of ractopamine on marbling and/or i.m. lipid content is unclear. Both Watkins et al. (1990) and Aalhus et al. (1990) reported that the LM from ractopamine-fed pigs received higher marbling scores than pork from control pigs, whereas several other studies observed similar marbling scores between ractopamine-treated and control pigs (Adeola et al., 1990; Crome et al., 1996; Stoller et al., 2003). Conversely, Uttaro et al. (1993) reported that i.m. fat content was reduced by the addition of ractopamine in the diet, and Engeseth et al. (1992) observed that LM i.m. fat content was only reduced in pigs fed ractopamine-diets for 4 and 6 wk. The variation in the reported response in LM marbling may also be influenced by the genetic base of the swine population used in the experiment, as well as the nutritional feeding program employed before and/or during the ractopamine feeding period.

In agreement with results from the present study, several researchers have found that dietary energy level had no effect on marbling scores and i.m. lipid content (Seerley et al., 1978; Coffey et al., 1982; Myer et al., 1992). Nonetheless, others have observed that marbling and/or intramuscular lipid content increased with increasing dietary energy density (Cromwell et al., 1978; Le Dividich et al., 1987) and energy intake (Ellis et al., 1996; Wood et al., 1996; Lebret et al., 2001). Conversely, LM marbling and i.m. lipid have been shown to be reduced by dietary CP (Cromwell et al., 1978; Goerl et al., 1995; Kerr et al., 1995), lysine (Goodband et al., 1990; Friesen et al., 1994), and lysine:energy ratio (Castell et al., 1994; Grandhi and Cliplef, 1997; Cameron et al., 1999). It is not surprising, therefore, that Xiao et al. (1999) found that marbling and i.m. lipid content in the LM was decreased in ractopamine-treated pigs fed an 18% CP diet. Moreover, Webster et al. (2002a) reported that marbling scores decreased as the lysine content of the diet increased in ractopamine-fed pigs, and that i.m. fat content also decreased linearly as ractopamine and lysine levels increased in swine finishing diets. Because all pigs in the present study were fed ractopamine, the results concur with previously published information that as the dietary lysine content is increased, i.m. fat content and/or marbling scores will decrease, regardless of whether ractopamine is included in the finishing diet.

Moisture and ash content of the LM did not differ (P > 0.39) among Lys:ME levels; however, LM protein content increased linearly (P < 0.01) with increasing Lys:ME ratio (Table 4). It has been repeatedly shown that LM protein content increases with increasing dietary Lys:ME (Castell et al., 1994; Grandhi and Cliplef, 1997), lysine (Goodband et al., 1990), or CP (Cromwell et al., 1978; Goerl et al., 1995; Kerr et al., 1995). Moreover, neither Castell et al. (1994) nor Cameron et al. (1999) observed a change in LM moisture content as the lysine:energy content increased in swine diets; however, Goodband et al. (1990) and Goerl et al. (1995) reported that the moisture content in the LM actually increased in response to increasing lysine or CP content in the diet. Lastly, increasing dietary lysine or CP has no effect on the proportion of ash in the LM (Goodband et al., 1990; Goerl et al., 1995).

Cooking loss percents and WBSF values of the LM were not (P > 0.54) different between pigs fed 3.30 or 3.48 Mcal of energy/kg (Table 4). Additionally, cooking losses were similar (P > 0.50) among Lys:ME levels; however, there was a linear (P < 0.01) increase in WBSF of cooked LM chops as Lys:ME increased from 1.7 to 3.1 g/Mcal.

Pork from ractopamine-fed pigs has been shown to have similar (Jeremiah et al., 1994; Stoller et al., 2003), or lower (Uttaro et al., 1993), cooking loss percents than pork from untreated controls. Additionally, cooking losses were not affected by dietary energy density (Matthews et al., 2003) or energy intake (Lebret et al., 2001). Although Castell et al. (1994) reported that cooking loss percent decreased with increasing lysine:DE ratio, most research indicates that neither dietary CP (Goerl et al., 1995) nor lysine (Goodband et al., 1990; Witte et al., 2000) content altered cooking losses, which is consistent with results of the present study.

There have been conflicting reports on the effect of ractopamine on pork palatability, particularly cooked pork tenderness. Both Aalhus et al. (1990) and Uttaro et al. (1993) reported that ractopamine increased cooked pork shear force values; however, neither McKeith et al. (1988), Merkel et al. (1990), nor Jeremiah et al. (1994) detected an effect of ractopamine on shear force values. Even though Stoller et al. (2003) recently noted a trend for slightly higher WBSF values in chops from ractopamine-fed pigs, sensory panel tenderness ratings were not affected by ractopamine.

With the exception of Matthews et al. (2003), who reported a tendency for WBSF values to decline as the ME level in the diet increased from 3.2 to 3.4 Mcal/kg, neither energy density nor intake have been shown to affect objective or subjective measures of tenderness (Cromwell et al., 1978; Wood et al., 1996; Lebret et al., 2001). On the other hand, the effects of increasing dietary CP and/or lysine levels on pork tenderness are more consistent. Davey (1976) observed that pork tenderness was deceased in pigs fed diets formulated with 16% CP than in pigs fed diets formulated with 11% CP. Goerl et al. (1995) reported that WBSF increased from 2.98 to 3.65 kg as dietary CP level increased from 10 to 22%, Goodband et al. (1990) observed a linear increase in WBSF as dietary lysine increased in the diet, and Cameron et al. (1999) found that pigs fed diets with 3.32 and 4.89 g of lysine/Mcal of DE produced pork with greater WBSF values than pigs fed only 1.76 g of lysine/Mcal of DE. Moreover, sensory panel tenderness ratings indicated that pork became tougher as dietary lysine increased from 0.6 to 1.4% in the diet (Goodband et al., 1990) and as the lysine:energy ratio increased from 1.6 to 3.3 g/Mcal of DE (Castell et al., 1994). Because all pigs received 10 ppm ractopamine, the observed increase in WBSF in the present study was a result of increasing Lys:ME and not a response to ractopamine.

It is evident from the present results, as well as those of other studies (Williams et al., 1994; Dunshea et al., 1998), that energy density has minimal effects on the performance, carcass composition, and pork quality of ractopamine-fed pigs, and that 3.30 Mcal/kg is sufficient energy for optimal ADG, G:F, and lean tissue deposition in pigs fed ractopamine. Conversely, the level of lysine in finishing diets obviously affects the response of pigs to ractopamine. The lysine and ME requirements for high-lean gain pigs during the finisher phase are 0.60 to 0.69% and 3.26 Mcal, respectively (NRC, 1998), which equates to a Lys:ME of 1.84 to 2.11 g/Mcal. However, Schinkel et al. (2000) indicated that ractopamine-fed pigs required as much as 41.7% more lysine during the first 21 d (or first 20 kg live weight), and as much as 21.5% more lysine after 21 d on feed to optimize fat-free lean growth rate. These suggested increases would equate to dietary lysine levels of 0.85 to 0.98% and 0.73 to 0.84%, respectively, which were comparable to lysine levels in the 2.4 g/Mcal Lys:ME diets in the present study. However, performance and carcass composition increased with increasing Lys:ME to 3.1 g/Mcal (1.024 to 1.079% lysine), supporting the conclusions of Webster et al. (2002b), who indicated that ractopamine-fed pigs required at least 1.0% dietary lysine to optimize growth rate and carcass lean yield. Thus, results of the present study suggest that the Lys:ME ratio for optimal gain and efficiency, as well as economically important carcass traits, in pigs fed ractopamine are higher than that reported in the literature and higher than levels currently being used in the industry.

	Implications

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

 
Results of the present study indicate that 3.30 Mcal/kg of metabolizable energy (as-fed basis) is sufficient energy for optimal live pig performance and lean tissue deposition in pigs fed 10 ppm of ractopamine. Moreover, results of this study suggest that the lysine:energy ratio for optimal performance and carcass composition seems to be higher than that reported in the literature, and higher than levels currently being used in the industry. Yet, the major drawbacks to feeding very high (greater than 3.0 g of lysine/Mcal of metabolizable energy) lysine:energy ratios to late-finishing pigs may include decreased intramuscular fat and cooked pork tenderness.

	Footnotes

 
¹ The authors express their appreciation to Elanco Animal Health, a division of Eli Lilly and Co., for financial support of this experiment. Additionally, the authors gratefully acknowledge the assistance of A. Hays and R. Hinson for animal care, and M. E. Davis, C. B. Boger, W. J. Roberts, R. L. Miller, J. Stephenson, and L. K. Rakes for assistance in data collection.

³ Present address: Hubbard Feeds, Inc., Mankato, MN 56001.

² Correspondence: B-103C AFLS Admin. Bldg. (phone: 479-575-4840; fax: 479-575-7294; e-mail:japple{at}uark.edu).

Received for publication March 18, 2004. Accepted for publication July 14, 2004.

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