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J. Anim. Sci. 2005. 83:1633-1645
© 2005 American Society of Animal Science

ANIMAL PRODUCTS

Effects of dietary magnesium and short-duration transportation on stress response, postmortem muscle metabolism, and meat quality of finishing swine1

J. K. Apple2, E. B. Kegley, C. V. Maxwell, Jr., L. K. Rakes, D. Galloway and T. J. Wistuba3

Department of Animal Science, University of Arkansas, Fayetteville 72701


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
Crossbred pigs, heterozygous for the halothane gene, were used to determine the effects of long-term dietary supplementation of magnesium mica (MM) and short-duration transportation stress on performance, stress response, postmortem metabolism, and pork quality. Pigs were blocked by weight, penned in groups (six pigs per pen), and pens (three pens per diet) were assigned randomly either to a control corn-soybean meal diet or the control diet supplemented with 2.5% MM (as-fed basis; supplemented at the expense of corn). Diets were fed during the early-finisher (0.95% lysine, as-fed basis; 43.7 to 68 kg) and late-finisher (0.85% lysine, as-fed basis; 68 to 103 kg) periods. At the conclusion of the 71-d feeding trial, 12 pigs from each dietary treatment were selected randomly and subjected either to no stress (NS) or 3 h of transportation stress (TS). Dietary MM had no effect (P ≥0.40) on ADG or ADFI; however, G:F was improved (P < 0.05) during the early-finisher period when pigs were fed MM-supplemented diets. Plasma glucose concentrations were increased in TS pigs fed the control diet, but transportation did not affect plasma glucose in pigs fed 2.5% MM (diet x transportation stress; P = 0.02). Dietary MM did not affect blood lactate, cortisol, insulin, NEFA, Ca, or Mg concentrations in response to TS (diet x transportation stress; P ≥0.13); however, circulating lactate, cortisol, and glucose concentrations increased in TS pigs (transportation stress x time; P < 0.01). The LM from TS pigs fed MM had higher initial (0-min) and 45-min pH values than the LM from NS pigs fed the control diet (diet x transportation stress x time; P = 0.07). Lactic acid concentration and glycolytic potential were greater in the LM of TS pigs fed MM than TS pigs fed control diets (diet x transportation stress; P ≤0.01). Although some trends were identified, neither MM (P ≥0.15) nor TS (P ≥0.11) altered the color or water-holding capacity of the LM and semi-membranosus. The transportation model elicited the expected changes in endocrine and blood metabolites, but dietary MM did not alter the stress response in pigs. Conversely, although pork quality traits were not improved by dietary MM, delaying postmortem glycolysis and elevating 0- and 45-min muscle pH by feeding finishing diets fortified with MM may benefit the pork industry by decreasing the incidence of PSE pork in pigs subjected to short-duration, routine stressors.

Key Words: Magnesium • Pigs • Pork Quality • Postmortem Metabolism • Stress • Transportation


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results and Discussion
 Implications
 Literature Cited
 
Supplementing swine diets with Mg has been shown to have beneficial effects on pork color and to decrease the incidence of PSE pork (Otten et al., 1993Go; Schaefer et al., 1993Go; D’Souza et al., 1998Go). Although the mechanism is unclear, research has demonstrated that supplementing swine diets with Mg decreases blood cortisol and catecholamine concentrations (Kietzman and Jablonski, 1985Go; D’Souza et al., 1998Go, 1999Go) and produces visibly calmer pigs after long-distance transportation (Kuhn et al., 1981Go).

Magnesium mica (MM) is an inorganic, layered silicate product containing approximately 8% Mg, and has been reported to improve pork color and decrease the incidence of PSE pork (Apple et al., 2000Go). However, subsequent trials testing the effects of long-term MM supplementation failed to elicit any beneficial or detrimental effects on pork quality (Apple et al., 2000Go, 2002Go). It is evident that the response in pork quality to dietary Mg is related to the stress-susceptibility (or resistance) of the swine studied (Schaefer et al., 1993Go), as well as the level of stress imposed on Mg-supplemented pigs (D’Souza et al., 1998Go, 1999Go). Moreover, when our laboratory tested the effects of dietary MM on pork quality of halothane-carrier pigs, pigs were well rested before slaughter (Apple et al., 2002Go), and no measure of stress response or postmortem muscle metabolism was recorded. Therefore, the objective of the present study was to test the effects of supplementing swine finishing diets with 2.5% MM (as-fed basis; 0.2% total Mg) on performance, and the interactive effects, if any, of dietary MM and short-duration transportation stress on the stress response, postmortem metabolism, and pork quality of finishing swine.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results and Discussion
 Implications
 Literature Cited
 
Animals and Diets
Seven Yorkshire x Landrace females that were homozygous dominant (NN), or negative, for the halothane gene were bred with semen from a synthetic-breed boar line (PIC, Franklin, KY) tested and guaranteed homozygous recessive (nn), or reactor, for the halothane gene. The resulting offspring were heterozygous (Nn) carriers of the halothane gene.

Thirty-six halothane-carrier pigs (43.7 ± 4.0 kg; 24 barrows and 12 gilts) from the aforementioned matings, were blocked by BW (three blocks), and penned in groups of six (two pens per block), stratifying across gender and litter origin. Pens (three pens per diet) were assigned randomly to either a control corn-soybean meal diet or the control diet supplemented with 2.5% (as-fed basis) MM (Micro-Lite, Inc., Chanute, KS). Within the MM-treated diet, MM was added at the expense of corn (Table 1Go), and the transition from the early- to late-finisher diets occurred when the average block BW was 68 kg. All diets were formulated to meet or exceed NRC (1998)Go requirements for finishing swine, and the early- and late-finisher diets contained 0.95 and 0.85% lysine (as-fed basis), respectively. Additionally, the control early-and late-finisher diets supplied 3.45 and 3.47 Mcal of ME/kg, respectively, whereas the MM-supplemented early- and late-finisher diets contained 3.36 and 3.38 Mcal of ME/kg, respectively.


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  		Table 1. Composition of experimental diets, as-fed basis
 
Pigs were housed in a curtain-sided building on partially slatted concrete floors, and each pen was 1.49 x 3.96 m. Additionally, each pen was equipped with a single-opening, wet-dry feeder (Crystal Spring Hog Equipment, Ltd., St. Pierre Johys, Manitoba, Canada), which allowed ad libitum access to both feed and water. Individual pig weights were measured at the beginning and end of the 71-d feeding trial, as well as the transition from early- to late-finisher diets, and feed disappearance was recorded at 7-d intervals during each phase to calculate ADG, ADFI, and G:F.

Stressor Treatment
On d 74, and again on d 81, six pigs from each dietary treatment were selected randomly and assigned within dietary treatments to either 3 h of transportation stress (n = 12) or nontransported controls (n = 12), resulting in a 2 x 2 factorial arrangement of treatments. Forty-eight hours before each experimental day, pigs were surgically fitted with indwelling jugular catheters, and, after a 3- or 4-h recovery period, were moved to individual pens in an environmentally controlled building. Pigs were fitted with tethers and restrained so that the pigs could lie and stand, but had a limited range of movement. Pigs had ad libitum access to their assigned dietary treatment (rubber pan feeders) and water (nipple waterers); however, 15 h before stressor treatments, feed was removed.

On d 76 and 83 (at approximately 0730), a –90-min blood sample was collected from each pig, and subsequent pretransportation blood samples were collected at –60, –30, and 0 min for quantification of plasma glucose, lactate, insulin, and NEFA concentrations, as well as circulating serum cortisol, Ca, and Mg concentrations. Immediately following the 0-min blood sample, pigs assigned to the transportation stressor (TS) were moved from their individual pens, loaded onto a trailer (allowing 1.08 m2/pig), and tethered within the trailer to facilitate blood sampling. On each day, the six pigs that were transported had visual contact with each other, but tactile contact between pigs was prohibited. Pigs were subjected to a single 3-h bout of transportation stress, stopping every 30 min to allow for blood sampling. With the exception of blood sampling, unstressed (NS) pigs remained in their home pens, and were subjected to minimal handling and stress

Analysis of Blood Samples
Blood was drawn from the cannula with a syringe and immediately aliquoted into three glass tubes. Blood for analysis of plasma insulin, glucose, and lactate was transferred into a storage tube containing potassium oxalate and sodium fluoride. Blood for analysis of serum cortisol, Mg, and Ca was transferred into a plain glass storage tube, whereas blood for plasma NEFA was transferred into a tube containing EDTA, and all blood tubes were kept on ice until centrifugation. Tubes were centrifuged for 20 min at 1,200 x g, and serum and plasma were stored at –18°C before analysis.

Serum cortisol and plasma insulin concentrations were determined by RIA using antibody-coated tubes (Diagnostic Products Corp., Los Angeles, CA). The inter-and intraassay CV for cortisol were 3.6 and 6.3%, respectively, whereas the inter- and intraassay CV for insulin were 10.6 and 9.1%, respectively. Plasma lactate concentrations were determined spectrophotometrically using the procedure of Brandt et al. (1980)Go, modified to use microtiter plates. Plasma glucose concentrations were determined using a spectrophotometric procedure in a commercially available kit (Sigma Chemical Co., St. Louis, MO), whereas plasma was analyzed for NEFA concentrations by a commercial enzymatic procedure (NEFA-C kit, ACS-ACOD Method, Wako Chemicals USA, Inc., Richmond, VA) as modified by Johnson and Peters (1993)Go. Serum Mg and Ca concentrations were determined by atomic absorption spectroscopy (model 5000, Perkin Elmer, Norwalk, CT).

Carcass Data Collection
After transportation stress, pigs were transported approximately 26 km (15 min) to the University of Arkansas Red Meat Abattoir, where transported pigs were slaughtered first at 20-min intervals. The NS pigs were transported to the abattoir approximately 2 h after transported pigs in an attempt to minimize the stress associated with commingling pigs, transportation, and preslaughter handling. To avoid accelerated postmortem metabolism associated with electrical stunning, all pigs were rendered unconscious and insensitive to pain by a nonpenetrating, captive-bolt stunning method. Immediately after stunning, two 1.27-cm-diameter cores were removed from the LM perpendicular to the length of the LM on the right side of each carcass at the level of the 7th thoracic vertebra. Subsequent LM samples were removed at 45, 90, 180, 360, 720, and 1,440 min after stunning at 3-cm distances caudally from the previous sample. Additionally, LM temperature was recorded at each sampling time with a digital thermometer (model KM28, Comark Instruments Inc., Beaverton, OR). One core was homogenized in sodium iodoacetate for pH determinations, and the second core was frozen immediately after removal in liquid nitrogen for determination of glycolytic potential at a later date. Carcasses were chilled conventionally at 1°C for 48 h until fabrication into primal cuts.

The left side of each carcass was fabricated into primal cuts according to National Association of Meat Purveyors (NAMP) specifications (NAMP, 1992Go). The bone-in loin section posterior to the 11th rib was removed and subsequently fabricated into two 3.8-cm-thick LM chops and two 2.5-cm-thick LM chops. Additionally, the semimembranosus (SM) was removed from the ham (NAMP #401) and cut into two 3.8-cm-thick and two 2.5-cm-thick slices perpendicular to the muscle fiber orientation. The 3.8-cm-thick chops/slices from both the LM and SM were used to measure muscle drip loss, whereas the 2.5-cm-thick chops/slices were used for subjective and objective pork quality measures. Firmness (1 = very soft to 5 = very firm; NPPC, 1991Go) and color based on the American (1 = pale pinkish gray to 6 = dark purplish red; NPPC, 1999Go) and Japanese color standards (Nakai et al., 1975Go) were subjectively evaluated on both 2.5-cm chops/slices by a three-person panel after a 30-min bloom period at 4°C. The Japanese color standards system consists of six plastic disks with meat-like texture and appearance developed from objective colorimetry, and scores range from 1 (pale gray) to 6 (dark purple).

Objective color (L*, a*, and b* values) was determined from a mean of three random readings on each LM chop and SM slice with a Hunter MiniScan XE (model 45/0-L, Hunter Associates Laboratory, Inc., Reston, VA) using illuminant C and a 10° standard observer. The spectro-colorimeter had a 22-mm aperture and was calibrated against a standard white tile (No. M04207, Hunter Associates Laboratory, Inc.).

Drip loss percent was determined on the 3.8-cm chops/slices following the modified suspension procedure of Honikel et al. (1986)Go, as described in detail by Apple et al. (2000)Go. After quality measures were collected, one 2.5-cm-thick LM chop, or SM slice, was vacuum-packaged in an oxygen-impermeable package and stored at –20°C until moisture and protein solubility determinations could be conducted. The second 2.5-cm-thick chop/slice was paper-wrapped and stored at –20°C for Warner-Bratzler shear force (WBSF) analysis.

Muscle pH and Glycolytic Potential Determinations
Approximately 1 g of excised muscle at each sampling time was homogenized with 10 mL of 5 mM sodium iodoacetate in 150 mM of potassium chloride (Bendall, 1973Go) with a PRO Scientific homogenizer (model PRO250, PRO Scientific, Inc., Monroe, CT). 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.).

Assay procedures used to estimate glycolytic potential (GP) were identical to those described by Miller et al. (2000)Go. Glycogen, glucose, and glucose-6-phosphate concentrations were determined simultaneously by the change in absorbance at 340 nm (Dalrymple and Hamm, 1973Go; Keppler and Decker, 1974Go), whereas muscle lactate concentration was measured using a commercially available lactate kit (826-A; Sigma-Aldrich Co.) by the change in spectrophotometric absorbance of NADH at 340 nm (Bergmeyer, 1974Go). Values for GP were calculated using the formula of Monin and Sellier (1985)Go: GP = 2 x ([glycogen] + [glucose] + [glucose-6-phosphate]) + [lactate].

Moisture Content and Protein Solubility
Moisture content was determined with duplicate 5-g samples of LM according to the freeze-drying method of Apple et al. (2001)Go. Additionally, the extractability of sarcoplasmic and myofibrillar proteins was determined using the procedures of Chaudry et al. (1969), as modified by Boles et al. (1992)Go, on quadruplicate 4-g samples of LM or SM. Protein content in sarcoplasmic and myofibrillar extracts was determined using the biuret protein assay of Robson et al. (1968)Go. Bovine serum albumin was used to construct a standard curve, and protein concentration (mg/mL) in each extract was determined by multiplying the absorbance by the reciprocal of the slope of the standard curve.

Warner-Bratzler Shear Force
Longissimus muscle and SM chops/slices 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, 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 or SM chop/slice and attached to a multichannel data logger (model 245A, VAS Engineering Inc., San Diego, CA). Chops/slices were turned once during the cooking process when the internal temperature reached 35°C. Immediately after removal from the oven, LM and SM chops/slices were blotted dry on paper towels and weighed, and the difference between precooked and cooked weights was used to calculate cooking loss percent. Cooked chops/slices were allowed to cool to room temperature, and six 1.27-cm-diameter cores were removed parallel to the muscle fiber orientation. Each core was then 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
Growth performance data were analyzed as a randomized complete block design using the GLM procedure of SAS (SAS Inst., Inc., Cary, NC), with pen as the experimental unit, and block and dietary treatment as the main effects included in the model. Blood data, as well as postmortem metabolism data, were analyzed as a 2 x 2 factorial design, with pig as the experimental unit. The cannula in one MM-fed, NS pig did not function; therefore, no blood was collected from that pig. After the study was completed and upon review of the GP distribution, it was evident from the bimodal distribution of GP data that one MM-fed, TS pig was a Rendement Napole gene carrier (this pig also had an abnormally low 1,440-min LM pH of 5.38), and all data from that pig were removed from the analysis.

Analysis of variance of blood and postmortem metabolism data was performed using the Mixed procedure of SAS, with a spatial structure, SP(POW), as the covariance structure. The model included dietary treatment (0.0 or 2.5% MM), stressor treatment (NS or TS), time (either blood sampling time or time postmortem), and all two- and three-way interactions. The subject of the repeated statement was pig. When a significant two- or three-way interaction with time was detected (F-test, P < 0.05), data from each time were compared using pairwise t-tests (PDIFF option of SAS). Additionally, pork quality data also were analyzed using the Mixed procedure of SAS with diet, transportation stress, and the diet x transportation stress interaction as fixed effects and block as the lone random effect in the model. Least squares means were computed for all main and interactive effects, and separated statistically using pairwise t-tests when a significant (P < 0.05) F-test was observed.


   Results and Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
Implications
 Literature Cited
 
Growth Performance
Neither ADG nor ADFI were affected by dietary treatments during the early-finisher (P ≥0.69) or late-finisher (P ≥0.40) periods, as well as over the entire 71-d feeding period (P ≥0.70; Table 2Go). During the early-finisher phase, pigs fed diets formulated with 2.5% MM had greater (P < 0.05) G:F than pigs fed the control diets; however, G:F did not (P ≥0.75) differ between dietary treatments during the late-finishing period or across the 71-d feeding trial.


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  		Table 2. Effect of magnesium mica (MM; as-fed basis) supplementation on growth performance of finishing pigs
 
Caine et al. (2000)Go reported that pigs fed diets formulated with Mg aspartate had greater ADG than pigs fed the control diets, and Apple et al. (2002)Go noted that pigs fed 1.25% MM were more efficient during the grower phase (34.0 to 68.2 kg) than pigs fed diets supplemented with 2.5% MM. However, neither Apple et al. (2000)Go nor O’Quinn et al. (2000)Go observed an effect of dietary Mg on ADG, ADFI, or G:F during any feeding phase or across the entire feeding trials. Because Mg is involved in the activation of several metabolic enzymes, Heaton (1973)Go concluded that increasing cellular Mg concentrations would result in greater energy utilization and efficiency. In the present study, as well as others (Apple et al., 2000Go, 2002Go), MM was included in diets at the expense of corn, resulting in decreased energy density of the experimental diets. Thus, the lack of a negative effect of dietary MM on ADG and ADFI, coupled with minor improvements in G:F, might be indicative of improved energy utilization/efficiency.

Stress Response Hormones and Blood Metabolites
Inclusion of MM in the diets of finishing pigs had no (P = 0.34) effect on serum cortisol concentrations, nor was there a diet x transportation interaction (P = 0.13) on circulating cortisol concentrations (Table 3Go). However, cortisol concentrations increased (P < 0.05) dramatically during the first 30 min of transportation, and remained elevated (P < 0.05) above that of NS pigs throughout the duration of transportation stress (transportation stress x time interaction; P < 0.001; Figure 1Go).


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  		Table 3. Main effects of magnesium mica (MM; as-fed basis) supplementation and short duration transportation (TS) on endocrine and blood metabolite concentrations of finishing pigs
 


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  		Figure 1. Effect of short-duration (3 h) transportation on serum cortisol concentrations (transportation stress x time; P < 0.001). Each point represents the least squares mean (±SE) of 11 nontransported, control pigs and 11 transported pigs. Points with an asterisk (*) differ within a specific time, P < 0.05.

 
The observation that dietary MM failed to alter serum cortisol concentrations is in contrast with several studies demonstrating that short-term (Kietzmann and Jablonski, 1985) and long-term (Otten et al., 1993Go, 1995Go) Mg supplementation effectively decreased cortisol concentrations in pigs. Moreover, in exercising humans (Golf et al., 1984Go) and Wistar rats subjected to 45 min of forced immobilization (Kaemmerer et al., 1984Go), supplemental Mg was shown to effectively decrease circulating cortisol concentrations. Nonetheless, Lim et al. (2004)Go recently reported that supplemental Mg, regardless of dietary source, had no appreciable effect on blood cortisol concentrations of pigs at slaughter, which is consistent with the results of the present study.

Reports of transportation effects on serum cortisol concentrations seem to depend on the duration pigs are in transit. Brown et al. (1999)Go reported that pigs transported for 8 and 24 h had plasma cortisol concentrations similar to nontransported pigs at slaughter; however, cortisol concentrations were dramatically increased in pigs transported 2 h or less compared with unstressed controls (Brown et al., 1998Go; Geverink et al., 1998Go; Parrott et al., 1998Go). Comparing the effects of a 15-min journey to 3 h of transportation, Pérez et al. (2002)Go noted that pigs transported 15 min had higher plasma cortisol concentrations at slaughter than pigs transported 3 h. The reason for the discrepancy was demonstrated by Bradshaw et al. (1996)Go, who showed that plasma cortisol concentrations increased robustly 30 min after loading pigs into trailers, but pigs became habituated to transportation, as evidenced by the slow, progressive decrease in cortisol to values similar to those of stationary pigs after 4.5 h of the 8-h journey.

Plasma glucose concentrations were greater (P < 0.05) in TS pigs than in NS controls at each sampling time during transportation (transportation stress x time interaction; P = 0.006; Figure 2Go). Moreover, circulating glucose concentrations of transported pigs fed 0.0% MM were higher (P < 0.05) than their unstressed contemporaries; however, glucose concentrations did not differ between TS and NS pigs fed 2.5% MM (diet x transportation stress interaction; P = 0.02; Table 3Go), possibly indicating a glycogen-sparing effect.



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  		Figure 2. Effect of short-duration (3 h) transportation on plasma glucose concentrations (transportation stress x time; P = 0.006). Each point represents the least squares mean (± SE) of 11 nontransported, control pigs and 11 transported pigs. Points with an asterisk (*) differ within a specific time, P < 0.05.

 
There are conflicting results concerning the effect of transportation on blood glucose concentrations in the pig. In agreement with the present results, Hicks et al. (1998)Go reported that 4 h of transportation increased circulating glucose concentrations above those of non-transported pigs, whereas Becker et al. (1989)Go noted that plasma glucose concentrations increased dramatically in pigs transported 700 km. Conversely, neither Pérez et al. (2002)Go nor Brown et al. (1998)Go detected a change in plasma glucose in pigs after journeys of 4 to 24 h. Interestingly, Otten et al. (1993Go, 1995)Go reported that feeding pigs Mg fumarate from 30 to 100 kg BW significantly lowered plasma glucose concentrations. However, Porta et al. (1991)Go demonstrated that feeding rats high levels (9,000 mg/kg Mg) of supplemental Mg aspartate did not alter blood glucose concentrations, and Haenni et al. (1998)Go reported no relationship between elevated blood Mg levels and glucose disappearance during an i.v. glucose tolerance test.

There was a transportation stress x time interaction (P < 0.001) for plasma lactate (Figure 3Go). Circulating lactate concentrations increased (P < 0.05) robustly during the first 30 min of transportation, but steadily decreased to pretransportation levels after 120 min of transportation. There was neither a main effect of dietary MM (P = 0.19) nor a diet x transportation stress interactive effect (P = 0.53) on plasma lactate concentrations (Table 3Go), which contrasts with the work of Otten et al. (1993Go, 1995)Go, who reported that pigs consuming diets with Mg fumarate had lower blood lactate concentrations at slaughter than pigs fed the unsupplemented diets.



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  		Figure 3. Effect of short-duration (3 h) transportation on plasma lactate concentrations (transportation stress x time; P < 0.001). Each point represents the least squares mean (±SE) of 11 nontransported, control pigs and 11 transported pigs. Points with an asterisk (*) differ within a specific time, P < 0.05.

 
To the authors’ knowledge, this is the first report of a plasma lactate response curve, and may explain some of the discrepancy between blood lactate concentrations of short- and long-distance/duration transportation in pigs. As results of the present study indicate, plasma lactate increased robustly during the first 30 min of transportation (Figure 3Go), probably due to increased physical stress associated with loading and movement (Warriss and Brown, 1985Go), which is consistent with reports noting higher blood lactate in pigs transported 10 km (Warriss et al., 1998Go) or 15 min (Pérez et al., 2002Go). Although plasma lactate concentrations were still higher in transported pigs after 60 and 90 min of transportation, lactate concentrations were rapidly decreasing and did not differ from NS pigs thereafter (Figure 3Go). This result would explain why circulating lactate concentrations were not affected in pigs subjected to 2 to 24 h of transportation (Warriss et al., 1990Go; Brown et al., 1999Go; Hambrecht et al., 2005Go).

Although there were no main effects of dietary MM (P = 0.44) or transportation stress (P = 0.41) on plasma NEFA concentrations (Table 3Go), there was an interesting transportation stress x time interaction (P < 0.001) on circulating NEFA concentrations (Figure 4Go). Nonesterified fatty acid concentrations were decreased (P < 0.05) dramatically after 30 min of transportation, indicating a greater metabolic reliance on NEFA as an energy source; however, NEFA increased thereafter, and actually were greater (P < 0.05) than their unstressed counterparts at 120, 150, and 180 min of transportation, indicating enhanced lipolysis and energy mobilization. These results may explain why pigs transported for 24 h (Brown et al., 1999Go), and calves transported for 19 h (Knowles et al., 1999Go), had substantially higher blood NEFA concentrations than nontransported animals.



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  		Figure 4. Effect of short duration (3 h) transportation on plasma NEFA concentrations (transportation stress x time; P < 0.001). Each point represents the least squares mean (± SE) of 11 nontransported control pigs and 11 transported pigs. Points with an asterisk (*) differ within a specific time, P < 0.05.

 
Even though unstressed pigs had higher (P < 0.01) plasma insulin concentrations than transported pigs, circulating insulin levels were not (P = 0.89) affected by dietary MM, nor were there diet x transportation stress (P = 0.40; Table 3Go) or transportation stress x time (P = 0.28; results not shown) interactive effects on plasma insulin concentrations. Nor are there reports on the effects of Mg-supplementation on insulin response in pigs. Apparently, results of this study are the first reporting the effects of transportation on circulating insulin concentrations in pigs. However, Zofková et al. (1988)Go reported that acute hypermagnesemia lowered insulin secretion during an i.v. glucose tolerance test, and Haenni et al. (1998)Go noted that insulin increment and area beneath the insulin response curve were inversely related to circulating Mg during an i.v. glucose tolerance test in elderly men. Furthermore, Paolisso et al. (1989)Go found an acute insulin response and improved glucose disappearance rate in elderly patients with type 2 diabetes who received chronic Mg-supplementation, whereas Porta et al. (1991)Go indicated that feeding rats high levels (9,000 mg/kg) of Mg from Mg aspartate decreased blood insulin concentrations, but feeding very low Mg levels (250 mg/kg) increased circulating insulin levels.

Serum Ca was greater (P = 0.02) in blood from transported pigs than in blood from unstressed pigs; however, Ca concentrations were not (P = 0.56) affected by the addition of 2.5% MM (Table 3Go). Moreover, neither dietary MM (P = 0.75) nor 3 h of transportation (P = 0.80) altered serum Mg concentrations.

The observation that 3 h of transportation increased serum Ca concentrations conflicts with the findings of Parker et al. (2003)Go, who failed to detect an effect of transportation on blood Ca concentrations in Bos indicus cattle. Additionally, the consensus of published literature is that short-term Mg supplementation of swine diets with Mg aspartate (D’Souza et al., 1998Go, 1999Go) or Mg sulfate (O’Quinn et al., 2000Go; D’Souza et al., 1999Go) increased circulating Mg concentrations. This discrepancy may be partly because long-duration Mg supplementation to humans (Golf et al., 1990Go) and cattle (Chester-Jones et al., 1990Go) increases red blood cell, liver, kidney, and bone Mg concentrations, but not plasma or muscle Mg concentrations, and because MM is a layered silicate product and absorbed primarily in the intestines, it may have a lower apparent absorption and retention rate than other Mg sources, which are absorbed primarily in the preintestinal region (Hurley et al., 1990Go; Ratchford-Milliken et al., 2001).

Postmortem Metabolism
The interactive effect of MM and transportation stress on postmortem pH decline in the LM is presented in Figure 5Go. The LM from pigs fed MM and subjected to 3 h of transportation had higher initial (0-min) and 45-min pH values than the LM from transported pigs fed the control diets and nontransported pigs, regardless of dietary treatment (diet x transportation stress x time; P = 0.067).



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  		Figure 5. Postmortem pH decline in the longissimus muscle as affected by dietary magnesium mica (MM; as-fed basis) and transportation stress (diet x transportation stress x time; P = 0.067). Each point represents the least squares mean (± SE) of six, six, six, and five nontransported (NS) pigs fed 0.0% MM, transported (TS) pigs fed 0.0% MM, NS pigs fed 2.5% MM, and TS pigs fed 2.5% MM, respectively. Within a specific time postmortem, an asterisk (*) indicates that the LM of TS pigs fed 2.5% MM had higher pH values than all other treatment combinations, P < 0.05.

 
The effect of preslaughter transportation on porcine muscle pH seems to be affected by the duration/distance of transportation. For example, subjecting pigs to short-duration (3 h or less) transportation has been shown to have no effect on LM pH (Brown et al., 1998Go; Pérez, 2002Go). Moreover, LM pH at 45 min postmortem was similar between pigs transported 1 and 4 h (Warriss et al., 1990Go) and 1 and 6 h (Warriss et al., 1983Go); however, long-duration (8 h or longer) transportation results in increased initial and ultimate LM pH values (McPhee and Trout, 1995Go; Brown et al., 1999Go; Leheska et al., 2003Go).

Results of this study are consistent with reports demonstrating that initial and/or early postmortem (20 to 45 min) LM pH were elevated in pigs fed Mg fumarate between 30 and 100 kg BW (Otten et al., 1992Go), fed Mg aspartate for 2 (Schmitten et al., 1984Go) or 7 d (D’Souza et al., 1998Go; Caine et al., 2000Go), and injected immediately before slaughter with a Mg sulfate solution (Lister and Ratcliff, 1971Go). Furthermore, ultimate (24 or 48 h) LM pH values have not been affected by long-term supplementation of swine diets with MM (Apple et al., 2000Go, 2002Go) and Mg fumarate (Otten et al., 1992Go), or short-term treatment of diets with Mg aspartate (Schaefer et al., 1993Go; D’Souza et al., 1999Go, 2000Go), Mg sulfate (van Laack, 2000Go; Hamilton et al., 2002Go, 2003Go), Mg chloride (D’Souza et al., 1999Go; Lim et al., 2004Go), Mg carbonate (Kuhn et al., 1981Go), Mg gluconate (Lim et al., 2004Go), Mg proteinate, and Mg proprionate (Hamilton et al., 2003Go), as well as swine drinking water treated with Mg sulfate (Frederick et al., 2004Go). Conversely, only Caine et al. (2000)Go observed that 48-h LM pH was actually increased by supplementing swine finishing diets with Mg aspartate for 7 d before slaughter, which contradicts previously published results as well as results of the current study.

Neither transportation (P = 0.33) nor dietary MM (P = 0.17) had an effect on LM temperature decline (Table 4Go); however, Brown et al. (1998)Go reported that transportation elevated LM temperature at 45 min postmortem, but Warriss et al. (1983)Go failed to detect an effect of transportation on SM temperature. Additionally, LM temperature measured within 45 min after slaughter was increased by supplementing swine diets with Mg aspartate for 5 (Schaefer et al., 1993Go) and 7 d (Caine et al., 2000Go); however, treating swine drinking water with Mg sulfate did not affect LM temperature measured at 45 min postmortem (Frederick et al., 2004Go).


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  		Table 4. Effects of magnesium mica (MM; as-fed basis) supplementation and short duration transportation (TS) on pH, temperature, and glycolytic potential of the LM from finishing pigs
 
There were no transportation stress x time interactions (P = 0.29) for LM carbohydrate (glycogen, glucose, and glucose-6-phosphate) or lactate concentrations or glycolytic potential (results not shown). Muscle carbohydrate reserves were not affected by transportation (P = 0.72), but the LM from pigs fed 2.5% MM tended (P = 0.07) to have higher carbohydrate reserves than those fed 0.0% MM (Table 4Go).

Consistent with results of the present study, transportation does not seem to affect muscle glycogen (Warriss et al., 1983Go; Brown et al., 1999Go) or carbohydrate concentrations (Leheska et al., 2003Go; Hambrecht et al., 2005Go). Neither D’Souza et al. (2000)Go nor O’Quinn et al. (2000)Go detected an effect of supplemental Mg on LM glycogen reserves; however, supplementing swine diets with Mg aspartate or Mg sulfate within 1 wk of slaughter resulted in greater glycogen reserves in the LM (D’Souza et al., 1999Go; Lim et al., 2004Go), and supplemental Mg aspartate had a glycogen-sparing effect when pigs were subjected to a preslaughter handling stressor (D’Souza et al., 1998Go).

For pigs fed 0.0% MM, transportation stress decreased LM lactate concentrations and GP, whereas the 3-h journey did not alter lactate concentrations or GP (diet x transportation stress; P < 0.01; Table 4Go). Leheska et al. (2003)Go found that transporting pigs 8 h to slaughter dramatically curtailed lactic acid accumulation in the LM, resulting in decreased GP compared with pigs transported 0.5 or 2.5 h, whereas Hambrecht et al. (2005)Go observed that pigs subjected to a long and rough journey had higher LM lactate concentrations and increased GP than those subjected to a short and smooth journey to slaughter. Muscle lactate concentrations were decreased dramatically by a preslaughter infusion of Mg sulfate (Lister and Ratcliff, 1971Go), as well as supplementing swine diets with Mg aspartate (D’Souza et al., 1998Go, 1999Go) or Mg sulfate (D’Souza et al., 1999Go). However, similar to the current results, Caine et al. (2000)Go and D’Souza et al. (2000)Go reported that supplemental dietary Mg actually increased muscle lactate concentrations early postmortem, whereas O’Quinn et al. (2000)Go failed to detect an effect of supplementary Mg on LM lactate.

The increase in plasma lactate concentrations of transported pigs may be an indicator of catecholamine-initiated glycogenolysis and subsequent clearance from muscle; however, there was a discrepancy between LM lactate concentrations and initial LM pH values of transported, MM-fed pigs. It has been proposed that incorporation of free H+ ions into mitochondrial respiration (Vaghy, 1979Go), as well as ATP hydrolysis increasing the production and accumulation of H+ in the cytosol (Busa and Nuccitelli, 1984Go), is responsible for the decrease in intracellular pH rather than lactic acid accumulation. Moreover, Stewart (1981)Go indicated that the strong ion difference and the partial pressure of CO2 had a greater effect on intracellular H+ ion concentrations than weak acids like lactic acid. Lastly, Robergs (2001)Go proposed that muscle lactate production actually retards rather than contributes to acidosis. Thus, the high initial LM pH may actually be related to the high LM lactate concentrations "buffering" the effects of H+ accumulation in response to ATP hydrolysis, and/or acid-base factors other than lactic acid.

Pork Quality Characteristics
There were no (P ≥0.15) diet x transportation stress interactions for any pork quality measurements on the LM or SM; therefore, only main effects will be reported. Neither dietary MM (P ≥ 0.51) nor 3 h of transportation (P ≥ 0.20) affected subjective color scores or L*, a*, or b* values of the LM (Table 5Go).


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  		Table 5. The main effects of dietary magnesium mica (MM; as-fed basis) and transportation stress on pork quality characteristics of the LMa
 
Leheska et al. (2003)Go reported that the LM of pigs subjected to an 8-h journey received higher color scores than pigs transported 30 min or 2.5 h, whereas Becker et al. (1989)Go reported that the LM of pigs transported 700 km received lower color scores than stationary pigs. Although Brown et al. (1998)Go observed that L*, a*, and b* values were not affected by 2 h of transportation, Warriss et al. (1990)Go reported that a 4-h journey elevated L* values and decreased b* values, but did not affect a* values, of the LM. Additionally, McPhee and Trout (1995)Go demonstrated that 10 h of transportation produced a darker (lower L* values), less red (lower a* values), and less yellow (lower b* values) LM compared with pigs transported less than 1 h to slaughter.

Apple et al. (2000)Go reported that American color scores were increased by including MM in the diets of growing-finishing pigs, but in other studies, supplementary MM (Apple et al., 2000Go, 2002Go), Mg sulfate (O’Quinn et al., 2000Go; van Laack, 2000Go), and Mg aspartate (Schaefer et al., 1993Go) did not alter American or Japanese color scores. Neither long-term supplementation with MM (Apple et al., 2000Go, 2002Go) nor short-term supplementation with Mg sulfate (D’Souza et al., 1999Go; O’Quinn et al., 2000Go), Mg aspartate (Schaefer et al., 1993Go; D’Souza et al., 1999Go), or Mg chloride (D’Souza et al., 1999Go) affected L* values of pork LM. Conversely, other researchers have shown that feeding pigs diets containing Mg resulted in darker (lower L* values) LM (D’Souza et al., 1998Go, 2000Go; Hamilton et al., 2002Go, 2003Go). Redness (a* values) of the LM from homozygous halothane-negative and heterozygous halothane-carrier pigs was not affected by 2.5% dietary MM (Apple et al., 2002Go). Moreover, short-term Mg supplementation failed to change a* values of the LM (O’Quinn et al., 2000Go; Hamilton et al., 2002Go, 2003Go). Although Hamilton et al. (2003)Go reported that the LM of pigs supplemented with Mg sulfate for 1 and 5 d had lower b* values, most of the previous research has not observed a change in LM b* values (Schaefer et al., 1993Go; Apple et al., 2002Go; Hamilton et al., 2002Go).

There were no MM (P ≥0.72) or transportation (P ≥ 0.39) effects on LM firmness scores (Table 5Go). Transportation does not seem to affect LM firmness scores, regardless of duration and/or distance (Becker et al., 1989Go; McPhee and Trout, 1995Go; Leheska et al., 2003Go). With the exception of Hamilton et al. (2002)Go, who reported a firmer LM in pigs consuming Mg-fortified diets for 2 and 5 d preslaughter, most studies have failed to observe an effect of dietary MM (Apple et al., 2000Go, 2002Go) or other Mg sources (Schaefer et al., 1993Go; O’Quinn et al., 2000Go; Hamilton et al., 2003Go) on LM firmness scores.

Drip loss and moisture content of the LM were similar (P ≥ 0.15) between pigs fed 2.5% MM or control diets (Table 5Go). However, the LM from transported pigs tended to have more (P = 0.09) moisture than nontransported pigs, even though drip loss percents were not (P = 0.66) affected by transportation.

Drip loss percents of the LM were not affected when pigs were subjected to 4 h or less of transportation (Warriss et al., 1990Go; Brown et al., 1998Go; Pérez et al., 2002Go); however, journeys of 8 h or more typically decrease LM drip losses (McPhee and Trout, 1995Go; Leheska et al., 2003Go). Apple et al. (2000Go, 2002)Go did not detect a difference in LM drip loss percents or moisture content with long-term dietary MM inclusion, and Caine et al. (2000)Go, O’Quinn et al. (2000)Go, and van Laack (2000)Go did not observe an effect of short-term Mg supplementation on LM drip loss percents; however, feeding pigs Mg aspartate (Schaefer et al., 1993Go; D’Souza et al., 1998Go, 1999Go) or Mg sulfate (Hamilton et al., 2002Go, 2003Go; D’Souza et al., 1999Go) for 5 to 7 d before slaughter effectively decreased drip losses of the LM.

Similar to the results on LM quality characteristics, neither feeding pigs 2.5% MM nor transportation affected American or Japanese color scores (P ≥ 0.18), L* (P ≥ 0.43), or a* (P ≥ 0.67) values of the SM (Table 6Go). Nonetheless, the SM from pigs fed 2.5% MM and transported pigs tended to be less yellow (lower b* values, P = 0.07 and 0.09, respectively) than the SM from pigs fed the control diets or nontransported pigs. Neither dietary MM nor transportation altered SM drip loss (P ≥0.96) or moisture content (P ≥ 0.37).


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  		Table 6. The main effects of dietary magnesium mica (MM; as-fed basis) and transportation stress on pork quality characteristics of the semimembranosusa
 
There is very little information concerning the effects of preslaughter transportation or Mg supplementation on pork quality measures of the SM. Although they did not report the effects of transportation on SM color, Brown et al. (1999)Go noted that transporting pigs 24 h to slaughter decreased SM moisture content compared with pigs transported 8 or 16 h. Moreover, Frederick et al. (2004)Go reported that neither SM L* nor a* values were affected by supplemental Mg, but treating the drinking water of pigs with Mg sulfate for 2 d before slaughter produced a less yellow (lower b* values) SM. Frederick et al. (2004)Go also demonstrated that water-treatment with Mg sulfate did not alter the drip loss percent of the SM; however, Kuhn et al. (1981)Go reported that treating pigs for 100 d with Mg carbonate decreased moisture losses from the SM compared with untreated pigs.

Protein Solubility and Warner-Bratzler Shear Force
There were no interactive (P ≥ 0.30) or main effects of dietary MM (P ≥ 0.52) or transportation (P ≥ 0.73) on the sarcoplasmic and myofibrillar protein solubility of the LM and SM (Table 7Go). The LM and SM from pigs fed 2.5% MM had similar cooking losses (P ≥0.26) and WBSF values (P ≥0.57) to pigs fed 0.0% MM. In additiion, transportation did not affect cooking loss percents (P ≥0.13) or WBSF values (P ≥0.40) of LM chops or SM slices.


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  		Table 7. The main effects of dietary magnesium mica (MM; as-fed basis) and transportation stress on protein solubility, cooking loss, and Warner-Bratzler shear force value of the LM and semimembranosusa
 
Muscle protein solubility is a measure of the degree of protein denaturation, and, in PSE pork, myofibrillar and sarcoplasmic protein solubility is decreased because denaturation of these proteins is considerable (Lopez-Bote et al., 1989Go). Moreover, Joo et al. (1999)Go observed a close, negative relationship between sarcoplasmic protein solubility/denaturation and muscle pH, L* values, and drip loss percents, accounting for 42, 71, and 52% of the variation in the respective pork quality measurements. However, neither the results of this experiment nor those of Warriss et al. (1990)Go indicated an effect of 3 to 4 h of transportation on muscle protein solubilities, and, to our knowledge, this is the first study to report the effect of Mg supplementation on the solubility of sarcoplasmic and myofibrillar proteins.

Similar to results of the present study, Brown et al. (1998)Go reported that WBSF values were similar between pigs transported for 2 h compared with NS pigs, and Hambrecht et al. (2005)Go noted no effect of transportation on shear force values of cooked LM. Leheska et al. (2003)Go reported that cooking losses were higher in pigs transported only 30 min compared with those of pigs transported 2.5 or 8 h. Moreover, subjecting pigs to long-distance (700 km; Becker et al., 1989Go) and/or long-duration (8 h; Leheska et al., 2003Go) transportation has been shown to produce more tender LM chops. Nonetheless, supplementing swine diets with Mg, regardless of source or length of Mg feeding, had no effect on cooking losses (Caine et al., 2000Go) or WBSF values (Caine et al., 2000Go; D’Souza et al., 2000Go; van Laack, 2000Go), which agrees with the present results.


   Implications
Top
 Abstract
 Introduction
 Materials and Methods
 Results and Discussion
 Implications
Literature Cited
 
The transportation model used in this experiment elicited the expected endocrine and blood metabolite responses to stress in pigs; however, supplemental magnesium mica did not have dramatic effects on these measurements. Although the addition of magnesium mica to finishing diets of pigs may have improved energy utilization/efficiency and markedly increased initial and 45-min pH values of the longissimus muscle from transported pigs, dietary magnesium had no appreciable effects on pork color, water-holding capacity, or cooked pork palatability.


   Footnotes
 
1 The authors are grateful to Micro-Lite, Inc., Chanute, KS, for donation of Mg mica and financial support of this project; Pig Improvement Co., Franklin, KY, for halothane-genotyping our females and donating boar semen; and F. K. McKeith at the Univ. of Illinois for use of his laboratory and training/assistance with the glycolytic potential assays. Additionally, the authors express their appreciation to A. Hays, L. Kirkpatrick, and J. Leibrandt for animal care and performance data collection; D. H. Hellwig for assistance in jugular cannulation of pigs; C. Bailey, D. C. Brown, G. Carte, M. E. Davis, P. Hornsby, S. Krumpelman, L. McBeth, J. Morris, R. Panivivat, N. Post, J. Sligar, J. Turner, and S. Williamson for assistance during blood sampling; J. Stephenson, K. Anschutz, R. Miller, and N. Simon for assistance in pig slaughter, carcass fabrication, and data collection; and Z. B. Johnson for statistical consultation. Back

3 Current address: Morehead State University, Morehead, KY 40351. Back

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

Received for publication November 16, 2004. Accepted for publication April 12, 2005.


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


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