J. Anim Sci. 2006. 84:3079-3088. doi:10.2527/jas.2006-137
© 2006 American Society of Animal Science
Treadmill exercise is not an effective methodology for producing the dark-cutting condition in young cattle1,2
J. K. Apple3,
E. B. Kegley,
D. L. Galloway,
T. J. Wistuba4,
L. K. Rakes and
J. W. S. Yancey
Department of Animal Sciences, University of Arkansas, Fayetteville 72701
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Abstract
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Holstein steer calves (n = 25) were used to evaluate the effects of treadmill exercise (TME) on blood metabolite status and formation of dark-cutting beef. Calves were blocked by BW (156 ± 33.2 kg) and assigned randomly within blocks to 1 of 5 TME treatments arranged in a 2 x 2 factorial design (4 or 8 km/h for a duration of 10 or 15 min) with a nonexercised control. Venous blood was collected via indwelling jugular catheters at 10, 2, and 0 min before TME and at 2-min intervals during exercise. Nonexercised steers were placed on the treadmill but stood still for 15 min. Serum cortisol levels, as well as plasma concentrations of glucose, lactate, and NEFA, were similar (P > 0.05) before TME. Serum cortisol concentrations were unaffected (P > 0.05) during the first 6 min of TME, but between 8 and 15 min of TME, cortisol concentrations were greater (P < 0.05) in steers exercised at 8 km/h than those exercised at 4 km/h or controls (speed x time, P < 0.001). Although TME did not affect (P > 0.05) plasma glucose levels, plasma lactate concentrations in steers exercised at 8 km/h increased (P < 0.05) sharply with the onset of the TME treatment and remained elevated compared with steers exercised at 4 km/h or unexercised controls (speed x time, P < 0.001). Exercised steers had the lowest (P < 0.05) plasma NEFA concentrations during the first 6 min of TME compared with unexercised steers; however, NEFA concentrations were similar after 10 and 12 min of TME, and by the end of TME, steers exercised at 8 km/h had greater (P < 0.05) NEFA levels than nonexercised controls or steers exercised at 4 km/h (speed x time, P < 0.001). Even though muscle glycogen levels and pH decreased (P < 0.001) and muscle lactate concentrations increased (P < 0.001) with increasing time postmortem, neither treadmill speed nor TME duration altered postmortem LM metabolism. Consequently, there were no (P > 0.05) differences in the color, water-holding capacity, shear force, or incidences of dark-cutting carcasses associated with preslaughter TME. It is apparent that preslaughter TME, at the speeds and durations employed in this study, failed to alter antemortem or postmortem muscle metabolism and would not be a suitable animal model for studying the formation of the dark-cutting condition in ruminants.
Key Words: cattle • dark-cutting condition • meat quality • postmortem metabolism • treadmill exercise
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INTRODUCTION
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Antemortem depletion of muscle glycogen reservoirs can curtail lactate accumulation in postmortem muscle, resulting in the formation of the dark-cutting condition (DCC). Dark-cutting meat is characterized by an ultimate muscle pH value in excess of 6.0; high water-holding capacity; a dry, firm, and sticky lean; and a dark-red to almost-black lean color. Moreover, this persistent quality defect costs the US beef industry between $132 and 170 million annually (Smith et al., 1992
, 1995).
The lack of a reliable animal model to study DCC greatly hinders the replication of experimental results and, more importantly, obstructs the ability of researchers to accurately test possible treatments or management practices, or both, to reduce or eliminate the formation of dark-cutting meat. In an attempt to develop an animal model to study DCC under controlled experimental conditions, sheep subjected to a physical stressor (treadmill exercise; TME) did not produce dark-cutting carcasses (Apple et al., 1994); however, 100% of the sheep subjected to a single 6-h bout of an emotional stressor (restraint and isolation stress) produced dark-cutting carcasses (Apple et al., 1995). More recently, Apple et al. (2005) reported that subjecting lightweight Holstein calves to 6 h of restraint and isolation stress effectively elicited DCC; however, because cattle are quite lethargic, Tarrant (1989) implied that physical, not emotional, stressors were the underlying cause of DCC in cattle.
Therefore, the aim of this study was to evaluate preslaughter TME as a potential animal model that could reliably produce DCC using lightweight Holstein steer calves.
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MATERIALS AND METHODS
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Animals and Exercise Treatments
Animal treatment and experimental protocols were approved by the University of Arkansas Interdepartmental Animal Care and Use Committee. Twenty-five Holstein steer calves (156 kg of average BW) were purchased from a local backgrounder and blocked by weight into 5 blocks. Within blocks, steers were assigned randomly to 1 of 5 treatments arranged in a 2 x 2 factorial design with 2 treadmill speeds (4.0 or 8.0 km/h) and 2 exercise durations (10 or 15 min); the fifth treatment was an unexercised control. Steers were allowed ad libitum access to a high-concentrate diet (Table 1) and water for a minimum of 4 wk before each replicate.
Seven days before TME treatments, each block of steers was moved from the University of Arkansas Beef Cattle Research Unit to the University of Arkansas Calf Research Facility, and subjected to a single 10-min training/screening session, where all steers were exercised at 2 km/h. Afterwards, steers were stanchioned in 1.65 x 0.64-m metabolic crates with coated, woven-wire floors (raised 0.38 m off the ground) and individually fed twice daily the same high-concentrate diet at a rate of 2.5% of their BW. Additionally, steers had ad libitum access to water via automated bowl waterers attached to each crate.
Twenty-four hours before TME, steers were fitted with 1.27-mm i.d., 2.28-mm o.d. plastic catheters (Tygon formula S-54-HL, Norton Performance Plastics, Akron, OH) inserted into the right jugular vein by percutaneous venipuncture to facilitate repeated blood sampling. Feed was withheld for 16 h before stressor application, but water was freely available. A sample of blood was collected from each calf 10 min before moving from its home stanchion to the treadmill (designated as the –10-min sample). After a 4-min acclimation period on the treadmill (Cullwing, Sandusky, OH), a blood sample was collected (designated as the –2-min sample), and the treadmill speed was gradually increased from 0 to the appropriate speed (4 or 8 km/h) within a 2-min period. When the TME speed was achieved, a sample of blood was collected (designated as the 0-min sample), and the calf was exercised for 10 or 15 min (additional blood samples were collected at 2-min intervals until completion of exercise). Unexercised control steers were also moved from their home stanchions and placed on the treadmill but were allowed to stand still (not exercised) for 15 min and, with the exception of blood sampling, were exposed to minimal handling and stress. Upon completion of the specific TME treatment, steers were transported approximately 100 m to the University of Arkansas Red Meat Research Abattoir and humanely slaughtered within 10 min of the cessation of exercise according to industry-accepted procedures.
Blood Collection and Assays
Aliquots of the blood samples were placed into 3 tubes: 1 plain-glass tube for serum cortisol; 1 tube containing sodium fluoride and potassium oxalate (BD Vacutainer, Franklin Lakes, NJ) for plasma glucose and lactate; and 1 tube containing EDTA (BD Vacutainer) for plasma NEFA. Blood samples were kept on ice until transportation to the laboratory, where samples were centrifuged at 1,200 x g for 30 min, and the serum or plasma was harvested and stored at –20°C until analyzed.
Serum cortisol concentrations were measured using direct RIA, in which the appropriate antibody was coated to plastic tubes (Diagnostic Products Corp., Los Angeles, CA). The inter- and intra-assay CV for serum cortisol were 7.9 and 7.2%, respectively, with a minimum detection level of 0.2 mg/dL. Glucose and lactate were measured in plasma fractions using commercially available diagnostic kits (Sigma Chemical, St. Louis, MO), whereas plasma NEFA concentrations were quantified with a commercially available enzymatic kit (Wako Chemicals USA, Dallas, TX). Plasma glucose, lactate, and NEFA assays had detection limits of 0.3 mg/dL, 0.22 mmol, and 0.39 µEq/L, respectively, whereas inter- and intra-assay CV were 2.2 and 1.3% for glucose, 16.0 and 7.5% for lactate, and 0.8 and 0.3% for NEFA, respectively.
Animal Slaughter and Muscle Collection and Analysis
Steers were rendered unconscious and insensitive to pain via a nonpenetrating captive bolt and were subsequently exsanguinated. Immediately after exsanguination, two 1.27-cm-diam. cores were removed from the LM, perpendicular to the long axis of the LM, on the right side at the level of the 7th thoracic vertebrae. Subsequently, LM samples were removed at 0.75, 1.5, 3, 6, 12, 24, and 48 h after exsanguination at 5-cm distances caudal to the previous sample. Approximately 2 g of LM were removed from 1 core and used for pH determinations, whereas the remainder of the core, as well as the second core, were immediately frozen in liquid N2 and stored at –20°C for determination of LM glycogen and lactate concentrations.
Samples for pH were homogenized in 20 mL of 5 mM sodium iodoacetate in 150 mM KCl (Bendall, 1973). 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.).
For muscle lactate analysis, approximately 1 g of frozen LM was homogenized in 2.0 mL of 6% (vol/vol) perchloric acid for 30 s with a homogenizer (model PRO250, PRO Scientific Inc., Monroe, CT) on high. The sample was brought to a final volume of 19.3 mL/g of tissue with additional perchloric acid and was homogenized again for 1 min. The homogenate was filtered under vacuum, and a 4-mL aliquot of filtrate was neutralized with 0.2 mL of 5 M potassium carbonate and centrifuged at 1,500 x g for 10 min. The supernatant was aspirated into plastic tubes, capped, and frozen in liquid N2 until determination of lactate concentration using a commercially available kit (Sigma Chemical) according to the procedure of McGinnis et al. (1989).
To quantify muscle glycogen concentration, 30 to 60 mg of frozen LM was combined with 0.5 mL of 30% KOH and boiled in test tubes for approximately 25 min in a 100°C water bath. Samples were cooled on ice, and 0.7 mL of 95% ethanol was added to each tube, placed on ice for an additional 30 min, and then centrifuged at 2,000 x g for 20 min. The supernatant fluid was aspirated and discarded before 3 mL of distilled water were added to the precipitate and vortexed. Then, 1 mL of 5% phenol and 5 mL of concentrated H2SO4 were added to a 1-mL aliquot of each sample, vortexed, cooled, and the absorbance of the sample was read at 490 nm according to the procedure of Lo et al. (1970). Standard curves were run with each assay to determine LM glycogen concentrations, which were reported as micromoles per gram of wet tissue weight.
Carcass Data
After chilling at 1°C for 48 h, the left sides of each carcass were ribbed between the 12th and 13th ribs, and the wholesale rib was fabricated from the forequarter. Three bone-in, 2.54-cm thick LM chops were removed, beginning at the caudal end of the rib. Two chops were designated for visual and instrumental color measurements, whereas the third chop was used for moisture determinations. After a 30-min bloom period at 2°C, a 3-member panel evaluated subjective color using the Japanese color standards for pork (Nakai et al., 1975). The Japanese pork color scoring system consists of 6 plastic disks with meat-like texture and appearance developed from objective colorimetry (1 = pale gray to 6 = dark purple) and has been widely used as the standard for visual appraisal of veal color (E. D. Mills, Pennsylvania State University, University Park, personal communication). Instrumental color (L*, a*, and b* values) of the LM chops was determined from a mean of 8 random readings (4/LM chop) using a Hunter MiniScan XE (model 45/0-L, Hunter Associates Laboratory, Reston, VA) using illuminant C and a 10°standard observer. After color data collection, both chops were wrapped in freezer paper and frozen at –20°C for Warner-Bratzler shear force (WBSF) determinations.
LM Moisture and Water-Holding Capacity
Muscle moisture content was determined using the freeze-drying procedure of Apple et al. (2001). Additionally, LM water-holding capacity was measured with the compensating planar planimeter method of Urbin et al. (1962). Approximately 500 mg of minced LM and a piece of Whatman No. 1 filter paper (Maidstone, UK) were weighed. Then the sample was placed on the filter paper, between 2 Plexiglas sheets, and pressed for 1 min at 35.2 kg/cm2 of force in a Carver press (Fred S. Carver Inc., Summit, NJ). The Plexiglas sheets were separated, and the inner (meat film area) and outer (total surface area) moisture edges were carefully traced, and the areas of each circle were measured using a compensating planimeter. The percentage of free moisture/water was calculated as follows: [(total surface area – meat film area) x 61.1]/total moisture content of the LM samples, with the results multiplied by 100. The percentage of bound moisture/water was calculated as 100 – percentage of free moisture/water.
Warner-Bratzler Shear Force and Cooking Loss
Frozen chops were thawed at 2°C for 16 h, deboned, weighed, and cooked to an internal temperature of 71°C in a commercial convection oven (Zephaire E, Blodgett Oven Co., Burlington, VT) preheated to 165°C. Chop internal temperature was monitored with Teflon-coated thermocouple wires (Type T, Omega Engineering Inc., San Diego, CA) placed in the geometric center of each chop and attached to a multichannel data logger (model 245A, VAS Engineering Inc., San Diego, CA). Chops were turned once during cooking, when the internal temperature reached 35°C. Immediately after removal from the oven, chops were blotted dry on paper towels, weighed, and the difference between pre- and postcooked weights was used to calculate cooking loss percentage. Chops were chilled for 24 h at 4°C before six 1.27-cm cores were removed parallel to the muscle fiber orientation and sheared through the center with the WBSF attachment on an Instron Universal Testing Machine (model 4466, Instron Corp., Canton, MA), equipped with a 55-kg tension/compression load cell and a crosshead speed of 250 mm/min. Peak shear force values of the 6 cores from each chop were averaged for statistical analyses.
Statistical Analyses
All data were analyzed as a randomized complete block design with calf as the experimental unit. Blood data (pooled across TME speeds), as well as postmortem muscle data, were analyzed as repeated measures using PROC MIXED (SAS Inst. Inc., Cary, NC), with sampling time as the repeated variable, calf as the subject, and TME treatment and the TME speed x sampling time interaction included in the model as fixed effects; random effects included block and the block x TME treatment interaction. Additionally, the ANOVA for all beef quality data was generated with PROC MIXED, with TME speed and duration as the fixed effects and block as the lone random effect included in the model. Least squares means were computed for all main and interaction effects and separated statistically using pairwise t-tests (PDIFF option of SAS) when the F-test was significant (P 
0.05). In the analysis of the beef quality data, orthogonal contrasts were included in the model to more accurately compare unexercised to exercised, treadmill speed (4.0 vs. 8.0 km/h), and exercise duration (10 vs. 15 min).
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RESULTS
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Physiological Responses
Serum cortisol concentrations were similar (P > 0.05) among treatments before exercise (–10 and –2 min) and during the first 6 min of exercise (Figure 1). Yet, during the last 7 min of TME (between 8 and 15 min of TME), serum cortisol concentrations were greater (P 0.05) in steers exercised at 8 km/h than in unstressed controls and steers exercised at 4 km/h (TME speed x time interaction, P < 0.001). Interestingly, cortisol levels of steers exercised at 4 km/h did not (P > 0.05) differ from unstressed controls at any time measured during TME.
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Figure 1. Effect of treadmill speed on serum cortisol concentrations (speed x time, P < 0.001). Each data point from –10 to 10 min for unexercised controls (0.0 km/h) or those exercised at 4.0 and 8.0 km/h represents the least squares mean (± SE) of 5, 10, and 10 steers, respectively, whereas data points at 12 and 15 min represent the least squares means (±SE) of 5 steers/speed. *Data points are different (P < 0.05) within a time.
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Even though there was no TME speed x time interaction (P = 0.204) on plasma glucose concentrations (Figure 2), plasma glucose concentrations were greater (P = 0.03) in steers exercised at 8 than 4 km/h (results not shown). Even though plasma lactate concentrations were similar (P > 0.05) between unexercised steers and steers exercised at 4 km/h at all sampling times, lactate concentrations increased sharply during the first 2 min of TME in steers exercised at 8 km/h (TME speed x time, P < 0.001; Figure 3). Plasma lactate levels in steers exercised at 8 km/h steadily declined between 2 to 15 min after initiation of TME, but plasma lactate concentrations were still greater (P 0.05) than those measured in steers exercised at 4 km/h or unexercised controls.
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Figure 2. Effect of treadmill speed on plasma glucose concentrations (speed x time, P = 0.204). Each data point from –10 to 10 min for unexercised controls (0.0 km/h) or those exercised at 4.0 and 8.0 km/h represents the least squares mean (±SE) of 5, 10, and 10 steers, respectively, whereas data points at 12 and 15 min represent the least squares means (±SE) of 5 steers/speed.
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Figure 3. Effect of treadmill speed on plasma lactate concentrations (speed x time, P < 0.001). Each data point from –10 to 10 min for unexercised controls (0.0 km/h) or those exercised at 4.0 and 8.0 km/h represents the least squares mean (±SE) of 5, 10, and 10 steers, respectively, whereas data points at 12 and 15 min represent the least squares means (±SE) of 5 steers/speed. *Data points are different (P < 0.05) within a time.
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Plasma NEFA concentrations were similar (P > 0.05) 10 min before exercise (Figure 4), but NEFA levels were decreased (P 0.05) in TME steers compared with unexercised controls just 2 min prior to exercise (TME speed x time, P < 0.001). Moreover, circulating NEFA concentrations in unexercised steers were higher (P 0.05) than exercised steers, regardless of TME speed. Although plasma NEFA levels were similar (P >0.05) between exercised and unexercised steers after 10 and 12 min of exercise, NEFA concentrations appeared to increase in exercised steers, and by the end of TME (15 min), steers exercised at 8 km/h had greater (P 0.05) NEFA levels than unexercised steers and steers exercised at 4 km/h.
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Figure 4. Effect of treadmill speed on plasma NEFA concentrations (speed x time, P < 0.001). Each data point from –10 to 10 min for unexercised controls (0.0 km/h) or those exercised at 4.0 and 8.0 km/h represents the least squares mean (±SE) of 5, 10, and 10 steers, respectively, whereas data points at 12 and 15 min represent the least squares means (±SE) of 5 steers/speed. a,bWithin a time, data points lacking common letters differ (P < 0.05).
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Postmortem Metabolism and LM Quality Attributes
Postmortem muscle glycogen, lactate, and pH decline results are presented in Figures 5, 6, and 7, respectively. As expected, LM glycogen concentrations decreased (P < 0.001) and lactate levels increased (P < 0.001) during the first 24 h postmortem; however, there were no TME treatment x time postmortem interactions for LM glycogen (P = 0.245; Figure 5) or lactate (P = 0.968; Figure 6). Moreover, LM pH declined with time postmortem (P < 0.001), but postmortem LM pH decline was similar among TME treatments (TME treatment x time postmortem, P = 0.926), with mean ultimate (48-h) pH values ranging from 5.67 to 5.73 (Figure 7).
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Figure 5. Effects of treadmill speed (4.0 vs. 8.0 km/h) and exercise duration (10 vs. 15 min) on postmortem glycogen decline in the LM (treatment x time, P = 0.245; time, P < 0.001). Each data point for the treatment combination represents the least squares mean (±SE) of 5 steers. a–dTimes postexsanguination lacking common letters differ (P < 0.05).
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Figure 6. Effects of treadmill speed (4.0 vs. 8.0 km/h) and exercise duration (10 vs. 15 min) on postmortem lactate accumulation in the LM (treatment x time, P = 0.968; time, P <0.001). Each data point for the treatment combination represents the least squares mean (±SE) of 5 steers. a–dTimes postexsanguination lacking common letters differ (P < 0.05).
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Figure 7. Effects of treadmill speed (4.0 vs. 8.0 km/h) and exercise duration (10 vs. 15 min) on postmortem pH decline in the LM (treatment x time, P = 0.926; time, P < 0.001). Each data point for treatment combination represents the least squares mean (±SE) of 5 steers. a–eTimes postexsanguination lacking common letters differ (P < 0.05).
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Neither TME speed (P 
0.33) nor duration (P 0.15) affected LM moisture content or water-holding capacity (Table 2). Japanese color scores (P = 0.08) and L* values (P = 0.04) indicated that the LM of steers exercised for 15 min was lighter than the LM of steers exercised for 10 min; otherwise, LM color was not affected by TME (P 0.31), TME duration (P 0.29), or TME speed (P 0.19). Even though LM chops from steers exercised at 8 km/h had lower (P = 0.03) cooking losses than chops from steers exercised at 4 km/h, WBSF values were similar (P = 0.85) between unexercised and TME steers.
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Table 2. Effects of treadmill speed and exercise duration on muscle metabolites and quality characteristics of the LM
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DISCUSSION
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It is generally understood that the DCC is caused by the depletion of glycogen stores antemortem, which limits lactate accumulation in the postmortem muscle and curtails postmortem muscle pH decline. Excessive preslaughter glycogenolysis may occur in response to any number of physical or psychological stressors, or both, occurring several minutes to several days prior to slaughter (Tarrant, 1989; Warriss, 1990). Yet, it has been difficult to repeat the results of many of the published experiments because of the difficulty in reproducing and controlling the environmental conditions and treatments imposed upon the animals. Thus, many attempts have been made to develop reliable animal models to study the DCC under controlled, experimental conditions.
Early animal models attempted to chemically induce glycogen depletion before slaughter. Lister and Spencer (1983) infused sheep with pyrazole carboxylic acid to inhibit lipolysis, and found that pyrazole carboxylic acid effectively depleted muscle glycogen and elicited DCC; however, plasma ACTH and cortisol levels were not altered as typically observed in stressed animals. Other researchers have used epinephrine injections in cattle to produce DCC, but when compared with mixed/commingled cattle, the patterns of glycogen depletion and muscle pH elevation were considerably different (Tarrant and Sherington, 1980; Lacourt and Tarrant, 1985). These exogenously induced conditions may result in depletion of muscle glycogen levels and elicit DCC; yet they fail to provoke the central nervous system in the cognition, integration, and homeostatic responses typically associated with an acute or chronic stressor (Minton, 1994).
Forced exercise has also been shown to produce dark-cutting carcasses (Forrest et al., 1964; Petersen, 1983; Bond et al., 2004). Forrest et al. (1964) and Bond et al. (2004) used dogs, whereas Chrystall et al. (1982) used humans, to chase sheep to exhaustion, and Petersen (1983) forced sheep to swim for several minutes prior to slaughter. Although the aforementioned studies successfully elicited normal physiological stress responses and produced dark-cutting meat, the stressors were relatively unnatural to sheep and cattle, and it is very difficult to distinguish the physical from the psychological component of the stressor on the sheep.
Because cattle are known to expend very little energy in a day, a predominantly physical stressor like TME would be hypothesized to produce a pronounced stress response and affect postmortem metabolism and the formation of DCC. Interestingly, plasma cortisol responses in Holstein steers to TME were consistent with the findings of Kuhlmann et al. (1985), who reported that subjecting Hereford calves to 10 min of TME resulted in elevated serum cortisol. Additionally, Apple et al. (1994) demonstrated that exercising sheep on a treadmill at 5.6, 7.2, or 8.8 km/h for 10 min, followed by a 10-min walk at 4.0 km/h, resulted in greater blood ACTH and cortisol concentrations than unexercised controls. Lugar et al. (1987) indicated that plasma cortisol concentrations increased in proportion to exercise intensity in trained and untrained humans; however, increases in cortisol or ACTH, or both, were only observed in humans (Farrell et al., 1983; Lugar et al., 1987) and rats (Shephard and Gollnick, 1976) exercised at or greater than 70% maximal oxygen consumption (VO2MAX). Thus, the activation of the hypothalamus-pituitary-adrenal axis in calves exercised at 8 km/h would indicate that this TME intensity was likely greater than 70% VO2MAX.
It is generally accepted that metabolism shifts from aerobic to anaerobic at, or near, 70% VO2MAX, which is often referred to as the lactic acid threshold. It is at 70% or greater VO2MAX that muscle relies solely on glycogenolysis, resulting in the production of lactic acid; thus, blood lactate concentrations would be consistently elevated. However, in the current study, TME did not alter plasma glucose levels (Figure 2
) or LM glycogen concentrations (Figure 5), indicators of muscle glycogenolysis, hepatic lipogenesis, or both (Brooks, 1986; Wasserman et al., 1987). Plasma lactate levels of calves exercised at 8 km/h increased robustly during the first 2 min of TME, but although lactate concentrations were greater in plasma from calves exercised at 8 km/h than controls or calves exercised at 4 km/h, lactate values declined during the remaining 10 to 13 min (Figure 3). Similar increases in lactate at the onset of exercise, with a steady decline in lactate levels, have been observed in humans when subjected to low-intensity exercise (Galbo et al., 1976; Stanley et al., 1985). Additionally, the sudden, dramatic decrease in plasma NEFA in response to exercise (Figure 4) would indicate a greater metabolic reliance upon ß-oxidation of lipids as the primary energy source. Moreover, the increase in plasma NEFA concentrations during the last 7 min of TME in calves exercised at 8 km/h would be indicative of enhanced lipolysis and energy mobilization; thus, the greatest exercise intensity employed in this study (8 km/h) failed to cause a shift in muscle metabolism from aerobic, ß-oxidation.
Muscle pH decreases curvilinearly after death from an initial value of approximately 7.2 to an ultimate value of 5.4 to 5.7 in response to lactic acid accumulation (Pearson and Young, 1989). Results of the current study indicate that LM lactate concentration increased, and LM pH declined, at a normal, curvilinear rate (Figures 6 and 7). More importantly, because LM pH declined at a normal rate, quality attributes such as muscle color, water-holding capacity, and shear force were not affected by TME treatment, nor did TME result in the formation of DCC in this study. Apple et al. (1994) failed to observe an effect of TME on muscle metabolism, lamb quality, or formation of dark-cutting meat. In pigs, TME immediately before slaughter reduced creatine phosphate and ATP levels, but did not necessarily reduce LM glycogen levels (Henckel et al., 2002) or alter LM pH decline (Henckel et al., 2000), and L*, a*, and b* values were similar between exercised and control pigs (Juncher et al., 2001).
Gardner et al. (2001) noted that glycogen concentrations in the semimembranosus (SM) and semitendinosus (ST) were similar between Angus steers exercised at 9 km/h for two or four 15-min intervals (with 15-min rest periods between each interval); however, glycogen levels were reduced in the SM and ST by 57.2 and 48.4%, respectively, when calves were exercised for five 15-min intervals. The discrepancy between the results of Gardner et al. (2001) and this study can be attributed to several factors. First, the duration of exercise was considerably different between studies even though exercise speed was relatively similar. In the current study, all calves received a single training period; however, serum cortisol concentrations were elevated by placing all calves, even the controls, onto the treadmill, which would indicate an emotional stress response. Thus, similar to forced exercise (Chrystall et al., 1982; Petersen, 1983; Bond et al., 2004), it would be difficult to ignore the psychological component of the model of Gardner et al. (2001). Secondly, Holstein calves used in the current study were approximately 4 to 6 mo of age, whereas the Gardner et al. (2001) study used 19-mo-old Angus steers. Older animals may be more susceptible to stress-induced ante-mortem glycogenolysis than younger animals because the percentage of fast-twitch glycolytic muscle fibers increases with increasing animal age (Vann et al., 2001). Thirdly, heavily muscled lamb (Martin et al., 2004) and cattle (Wegner et al., 2000) breeds have a greater proportion of fast-twitch glycolytic fibers than light muscle breeds. Lastly, Gardner et al. (2001) reported glycogen metabolism in the SM and ST, whereas, in the current study, the LM was used to measure postmortem muscle metabolism. Although all 3 are considered to be fast-twitch glycolytic muscles, the LM has a greater percentage of slow-twitch, oxidative fibers than the SM or ST (Hunt and Hedrick, 1977; Kirchofer et al., 2002).
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IMPLICATIONS
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Although serum cortisol and plasma lactate concentrations, indicative of a stress response, were elevated in response to treadmill exercise at a speed of 8 kilometers per hour, exercise did not affect antemortem muscle glycogen stores or postmortem muscle metabolism. Moreover, beef color and water-holding capacity were not altered by preslaughter exercise, regardless of speed or duration. Although higher speeds or longer durations of treadmill exercise, or both, may reduce antemortem muscle glycogen reserves, results of the current study clearly indicate that, under the conditions employed, treadmill exercise failed to produce any dark-cutting carcasses and would not be a suitable animal model for studying the dark-cutting condition.
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Footnotes
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1 This project was funded by the Arkansas Beef Council and Arkansas beef cattle producers through their $1.00 per animal check-off program. 
2 The authors thank P. Hornsby, G. Carte, and J. Sligar for animal care and management; J. Stephenson, J. Leach, J. Jimenez-Villarreal, R. Miller, and N. Simon for assistance with steer slaughter; W. J. Roberts and C. B. Boger for assistance with blood and muscle data collection; K. S. Anschutz for assistance in the laboratory; and Z. B. Johnson for assistance with statistical analyses.
4 Current address: Morehead State Univ., Morehead, KY 40351
3 Corresponding author: japple{at}uark.edu
Received for publication March 8, 2006.
Accepted for publication June 2, 2006.
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LITERATURE CITED
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Apple, J. K., J. R. Davis, L. K. Rakes, C. V. Maxwell, M. R. Stivarius, and F. W. Pohlman. 2001. Effects of dietary magnesium and duration of refrigerated storage on the quality of vacuum-packaged, boneless pork loins. Meat Sci. 57:43–53.[CrossRef]
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Abstract
INTRODUCTION
