J. Anim Sci. 2007. 85:1046-1053. doi:10.2527/jas.2006-262
© 2007 American Society of Animal Science
Effects of supplemental manganese on performance of growing-finishing pigs and pork quality during retail display1
J. T. Sawyer*,
A. W. Tittor
,2,
J. K. Apple*,3,
J. B. Morgan,
C. V. Maxwell*,
L. K. Rakes* and
T. M. Fakler
* Department of Animal Sciences, University of Arkansas, Fayetteville 72701;
and
Department of Animal Science, Oklahoma State University, Stillwater 74074; and
and
Zinpro Corporation, Eden Prairie, MN 55344
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Abstract
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Crossbred barrows and gilts (n = 168) were used to test the effects of supplemental Mn during the growing-finishing period on performance, pork carcass characteristics, and pork quality during 7 d of retail display. Pigs were blocked by BW and allotted within blocks to pens (5 pigs/pen in blocks 1, 2, 5, and 6, and 4 pigs/pen in blocks 3 and 4). A total of 36 pens was randomly assigned to 1 of 6 dietary treatments, where the basal diets were formulated with (PC) or without (NC) Mn in the mineral premix, and supplemented with 0 or 350 ppm (as-fed basis) of Mn from MnSO4 or a Mn-AA complex (AvMn). Pigs were slaughtered at a commercial pork packing plant when the lightest block of pigs averaged 113.6 kg. During fabrication, boneless pork loins were collected and transported to Oklahoma State University, where 2.5-cm-thick LM chops were packaged in a modified atmosphere (80% O2 and 20% CO2) and subsequently placed in display cases (2 to 4° C) under continuous fluorescent lighting (1,600 lx) for 7 d. Pig performance was not (P 
0.44) affected by supplemental Mn; however, during the grower-II phase, pigs fed the basal diets including Mn consumed less (P < 0.02) feed and tended to be more efficient (P < 0.09) than pigs fed the basal diets devoid of Mn. Throughout the entire feeding trial, neither dietary nor supplemental Mn altered (P 0.22) ADG, ADFI, or G:F. Chops from pigs fed the diets supplemented with MnSO4 received greater (P 
0.05) lean color scores and had a redder (greater a* and hue angle values), more vivid color than chops from pigs fed the diets supplemented with AvMn. Additionally, LM chops from pigs fed the PC diets supplemented with MnSO4 were darker (lower L* values; P < 0.05) than chops from pigs fed the NC diets or PC diets supplemented with 0 or 350 ppm of AvMn. Even though discoloration scores were similar during the first 4 d of display, chops from pigs fed the PC diets supplemented with MnSO4 were less (P < 0.05) discolored on d 6 and 7 of retail display than chops from pigs fed the PC or NC diets and diets supplemented with AvMn (dietary treatment x display time, P = 0.04). Results of this study indicate that feeding an additional 350 ppm of Mn from MnSO4 above the maintenance requirements of growing-finishing pigs does not beneficially affect live pig performance but may improve pork color and delay discoloration of pork during retail display.
Key Words: color • manganese • performance • pork • quality • retail display
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INTRODUCTION
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The low Mn requirements for growing-finishing swine were established using research conducted over 35 yr ago, and until recently, little was known about the effect of dietary Mn on pork carcass characteristics or fresh pork quality, especially during retail display. In the first study conducted by this laboratory, supplementing swine diets with 20 to 320 ppm of Mn from AvailaMn-80 (AvMn; Zinpro Corp., Eden Prairie, MN) did not affect subjective color scores, but Japanese color scores were increased and L* values decreased by feeding diets supplemented with 350 ppm of Mn from AvMn (Apple et al., 2004
). Additionally, LM chops from pigs fed 350 ppm of Mn from AvMn were less discolored after 4 d of simulated retail display than chops from pigs fed the control diets or diets supplemented with 350 or 700 ppm of Mn from MnSO4 (Apple et al., 2005). In another study, Roberts et al. (2003) reported that LM chops from pigs fed 80 ppm of Mn from AvMn received greater color scores than chops from pigs fed 0 or 40 ppm of Mn. Moreover, after 4 d of retail display, chops from pigs fed 80 ppm of Mn had the greatest reflectance ratios (indicating a high percentage of oxymyoglobin), whereas chops from pigs fed 40 ppm of Mn had the lowest reflectance ratios (indicating greater metmyoglobin formation).
In the aforementioned studies, Mn was excluded from the mineral premix in the control and supplemented diets (Apple et al., 2004); thus, the only Mn included in those diets originated from other feedstuffs in the diets. Therefore, the objectives of this study were to test the effects of basal dietary Mn (included or excluded from the mineral premix) and the supplementation of 350 ppm of Mn from an inorganic (MnSO4) or organic (AvMn) source of Mn on performance and carcass traits of growing-finishing swine, as well as pork quality traits during 7 d of retail display.
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MATERIALS AND METHODS
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Animals and Diets
The University of Arkansas Interdepartmental Animal Care and Use Committee approved animal care and all experimental procedures involving animals before initiation of this experiment. Crossbred barrows and gilts (n = 168) from the mating of line 348 females to EB boars (Monsanto Choice Genetics, St. Louis, MO) were sorted into 6 BW blocks (2 blocks of 24 pigs/block and 4 blocks of 30 pigs/block) based on initial BW (22.2 ± 0.6 kg). Within each block, pigs were randomly allotted to pens (5 pigs/pen in blocks 1, 2, 5, and 6, and 4 pigs/pen in blocks 3 and 4), with stratification across pens based on sex and litter origin. Within blocks, pens were randomly assigned to 1 of 6 dietary treatments: 1) a negative control (NC) corn-soybean meal grower-finisher diets devoid of Mn in the mineral premix; 2) NC diets supplemented with 350 ppm (as-fed basis) of Mn from MnSO4; 3) NC diets supplemented with 350 ppm (as-fed basis) of Mn from AvMn, a Mn-AA complex; 4) a positive control (PC) corn-soybean meal grower-finisher diets with Mn included in the mineral premix to meet NRC (1998) requirements; 5) PC diets supplemented with 350 ppm (as-fed basis) of Mn from MnSO4; or 6) PC diets supplemented with 350 ppm (as-fed basis) of Mn from AvMn.
The pigs were fed a 4-phase dietary program, with transition from grower I to grower II, grower II to finisher I, and finisher I to finisher II when the average BW of each block reached 36.4, 68.2, and 90.9 kg, respectively. The experimental diets (Table 1) were formulated to be isolysinic (lysine content of grower I, grower II, finisher I, and finisher II diets was 1.16, 0.95, 0.66, and 0.53%, respectively; as-fed basis) and isocaloric (ME content of grower I, grower II, finisher I, and finisher II diets was 3.46, 3.37, 3.36, and 3.34 Mcal/kg, respectively; as-fed basis). Supplemental Mn (MnSO4 or AvMn) was added to the diets at the expense of cornstarch (Table 1), and all diets were formulated to meet, or exceed, the NRC (1998) AA, energy, and other nutrient requirements for growing-finishing swine.
The pigs were housed in a curtain-sided building with 1.52 x 3.05-m pens (0.77 m2/pig) on completely slatted concrete floors. Within each pen, pigs had ad libitum access to feed and water via single-opening feeders and cup waterers. Individual pig BW and feed disappearance were recorded weekly during each feeding phase to calculate ADG, ADFI, and G:F on an as-fed basis.
Carcass Data Collection and Loin Fabrication
When the lightest block of pigs averaged 113.6 kg, all pigs were transported approximately 760 km from the University of Arkansas Swine Research Farm to a commercial pork packing plant (Bryan Foods Inc., West Point, MS). After a 12-h rest period, the pigs were slaughtered according to industry-accepted procedures, and 10th-rib fat and LM depths were measured on-line with a Fat-O-Meter automated probe (SFK Technology A/S, Cedar Rapids, IA), and HCW was recorded. Carcasses were subsequently subjected to a conventional spray-chilling system for 24 h. Before carcass fabrication, mid-line backfat depths were measured at the last rib and last lumbar vertebra, and loins were marked with their respective tattoo number. During carcass fabrication, boneless pork loins were collected, vacuum-packaged, boxed, and loaded onto a refrigerated truck for transportation to the Oklahoma State University Food and Agricultural Product Center (Stillwater, OK) for pork quality data collection.
Approximately 48 h postmortem, pork loins were removed from the vacuum packages and cut into 2.5-cm-thick LM chops. Two chops from each loin were vacuum-packaged and frozen at –20° C until completion of the 7-d retail display for determination of initial 2-thiobarbituric acid reactive substances (TBARS). After a 15-min bloom period at 4° C, 2 additional chops were visually evaluated by a 5-person trained (Hunt et al., 1991) panel for initial (d 0) color (1 = pale pinkish gray to 6 = dark purplish red; NPPC, 1999), discoloration [1 = total (100%)] discoloration to 8 = no (0%) discoloration; Hunt et al., 1991], and firmness (1 = very soft-very watery to 5 = very firm-very dry; NPPC, 1991) scores, as well as marbling (1 = 1% i.m. fat to 10 = 10% i.m. fat; NPPC, 1999). Furthermore, initial (d 0) L*, a*, and b* values were determined from a mean of 8 random readings (4/chop) made with a Minolta Chromameter (CR 300, Minolta Corp., Ramsey, NJ) using illuminate D65 and an 8-mm viewing area (0° viewing angle).
After collection of the subjective and objective pork quality measurements, the chops were weighed, placed in modified-atmosphere trays (0.6 EVOH, Rock-Tenn Co., Norcross, GA) flushed with an 80% O2/20% CO2 atmosphere in a Mondini modified atmosphere packager (CVS 0.1-S, Harpack, Easton, MA), and sealed with a high barrier film (Film No. 1050, Cyrovac Sealed Air Corp., Duncan, SC). Packages were then placed in open-topped, coffin-chest display cases (M1-8EB, Hussman, Bridgeton, MO) maintained between 2 and 4° C, and the chops were displayed under continuous, 1,600 lx of cool-white, fluorescent lighting (Bulb No. F40 T12, Promolux, BC, Canada) for 7 d. The chops were rotated within the display cases daily to decrease potential environmental effects associated with the defrost cycle and case location.
Retail Display Data Collection
On each day of retail display, the chops were visually evaluated for color, discoloration, and firmness by a 5-member panel at 0800 and 1700. At the conclusion of the 7-d retail display period, the packages were opened, the chops were weighed, and the difference between the initial (d 0) and final (d 7) chop weight was divided by the initial chop weight and then multiplied by 100 to calculate the moisture loss percentage. Also, L*, a*, and b* values were measured as previously described and used to calculate hue angle (representing a change from the true red axis), as: tan–1 (b*/a*); chroma (representing the total color or vividness of color), as (a*2 + b*2)1/2; and the change in total color (
E) from d 0, as 
(Minolta, 1998). After the final color measurements were collected, the chops were vacuum-packaged and frozen at –20° C for TBARS analysis.
TBARS Determination
Determination of TBARS was conducted in triplicate according to the procedures outlined by Buege and Aust (1978), with some modifications. Briefly, 10 g of LM were homogenized with distilled, deionized water in a Waring blender (33BL79, Dynamics Corp. of America, New Hart-ford, CT) and subsequently centrifuged for 10 min at 1,850 x g (J-6M, Beckman Instruments Inc., Houston, TX) at 4° C. Then, 2 mL of the supernatant were combined with 4 mL of thiobarbituric acid reagent and heated in a boiling water bath for 15 min. The samples were cooled immediately in an ice water bath, vortexed, and centrifuged at 1,850 x g for 10 min at 4° C. Absorbance was measured at 531 nm with a Beckman UV-VIS spectrophotometer (DU 750, Beckman-Coulter Inc., Fullerton, CA), and TBARS values were reported as milligrams of malonaldehyde equivalents per kilogram of wet muscle weight.
Data Analysis
All data were analyzed as a randomized complete block design, with blocks based on initial BW. Analysis of variance was generated with the MIXED procedure (SAS Inst. Inc., Cary, NC). The experimental unit for all performance and carcass composition data was pen. However, because 1 pen was represented by only 1 observation (4 loins were mislabeled), loin was considered as the experimental unit in the ANOVA of retail display data. In the repeated measures model for pork quality, fixed effects included dietary treatment, display day, and the dietary treatment x display day interaction, whereas block was considered a random effect in the model. Least squares means were computed and statistically separated using the PDIFF option of SAS when a significant (P < 0.05) F-test was discovered. Additionally, pre-planned contrasts were used to make specific comparisons between the positive and negative control diets (PC vs. NC), Mn sources (MnSO4 vs. AvMn), and the presence or absence of Mn in the basal diet (+Mn treatments vs. –Mn treatments).
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RESULTS AND DISCUSSION
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Pig Performance
Dietary or supplemental Mn provided during the grower-I phase did not produce an (P > 0.65) effect on ADG, ADFI, or G:F (Table 2). Growth rate was not (P > 0.44) affected by supplemental Mn; however, during the grower-II phase, pigs fed basal diets including Mn consumed less (P = 0.02) feed than pigs fed basal diets devoid of supplemental Mn. Additionally, there was a tendency for pigs fed diets including Mn to be more efficient (P = 0.09) during the grower-II phase than those fed basal diets devoid of Mn. Even though pig performance was not affected (P 0.23) during the early finishing phase, pigs fed basal diets containing Mn grew faster (greater ADG; P = 0.05) and consumed more (P = 0.05) feed than pigs fed basal diets lacking supplemental Mn during the late-finishing phase. Moreover, during the finisher-II phase, ADFI tended to be decreased (P = 0.08) in AvMn-supplemented compared with MnSO4-supplemented pigs. Across the entire growing-finishing period, however, ADG, ADFI, and G:F were not (P 0.22) affected by basal diet Mn level or supplemental Mn source.
Grummer et al. (1950) concluded that providing 40 ppm of Mn in swine diets increased ADG and improved overall feed efficiency when compared with pigs fed diets absent of Mn; yet when Mn levels exceeded 55 ppm, no additional improvement in pig performance was observed. Similarly, Svajgr et al. (1969) detected a numerical improvement in feed efficiency with the inclusion of 100 ppm of Mn. Other studies documenting the performance of baby pigs and sows were not improved by the dietary supplementation of Mn (Plumlee et al., 1956; Leibholz et al. 1962). Svajgr et al. (1969) reported that including an additional 50 ppm of Mn from Mn oxide in growing-finishing diets actually depressed ADG. More recently, Apple et al. (2004) reported greater G:F in pigs fed 350 ppm of MnSO4 or 700 ppm of MnAA, which is contradictory to results of the current study.
Pork Carcass Measurements
Main effects of dietary Mn supplementation on pork carcass traits are presented in Table 3. Pigs fed diets with Mn in the ration premix tended to produce heavier (P = 0.10) carcasses than pigs consuming diets devoid of Mn within the premix. Furthermore, LM depth of pigs fed diets including Mn in the premix was greater (P < 0.05) than that of pigs fed diets excluding Mn from the premix. Otherwise, neither dietary nor supplemental Mn affected (P 0.12) last rib and last lumbar backfat depth, 10th rib fat depth, estimated fat free-lean yield, or marbling scores.
Previous research pertaining to the dietary effects of Mn on pork carcass composition is limited; however, Grummer at al. (1950) reported increased carcass weights and dressing percentages in pigs fed diets supplemented with 160 ppm of Mn. Plumlee et al. (1956) noted that pigs fed Mn-deficient (0.5 ppm) diets produced visually fatter carcasses than pigs fed diets containing 40 ppm of Mn, whereas Christianson et al. (1989) reported that backfat of gestating/lactating sows increased as the level of dietary Mn increased from 5 to 20 ppm. Yet results of this study, as well as those of Apple et al. (2004), indicated that dietary Mn had no appreciable effects on HCW, backfat, 10th rib fat depths, LM depth or area, and fat-free lean yield. Moreover, there is no evidence to suggest that marbling scores in pork can be altered by feeding pigs supplemental Mn (Apple et al., 2004, 2005).
Pork Quality
Moisture loss percentages did not (P 0.28) differ among dietary treatments (Table 4). This is consistent with previous results from this laboratory demonstrating that feeding 20 to 360 ppm of Mn did not affect purge loss from vacuum-packaged pork loins (Roberts et al., 2003), nor was LM drip loss percentages affected by feeding swine diets supplemented with 350 or 700 ppm of Mn from MnSO4 or AvMn (Apple et al., 2004). Additionally, feeding grower-finisher diets containing 20 to 360 ppm of Mn from AvMn did not alter moisture loss percentages during 7 d of retail display (Roberts et al., 2003). Conversely, Apple et al. (2005) reported that LM chops from pigs fed 350 ppm of MnSO4 lost less moisture after 2 d of retail display than chops from pigs fed control diets or 700 ppm AvMn, but moisture loss percentages did not differ among dietary treatments for the remainder of the 6-d display period.
Chops received greater (P < 0.05) overall color scores on d 0 and 1 than on d 7 of retail display (results not shown), whereas LM chops from pigs fed Mn in the basal diet and supplemented with MnSO4 received greater (P < 0.05) color scores than all other treatment combinations (Table 4
). Roberts et al. (2003) reported greater American and Japanese color scores for LM chops from pigs fed 80 ppm of Mn than chops from pigs fed 0 or 40 ppm of Mn. However, Apple et al. (2005) reported that LM chops from pigs fed 350 ppm of Mn from AvMn received greater Japanese color scores than those from pigs fed 700 ppm of Mn from AvMn and 350 or 700 ppm of Mn from MnSO4, and greater American color scores than chops from pigs fed MnSO4, regardless of dietary inclusion level. It is apparent that supplementing swine diets with Mn improves visual color scores of pork, even though this laboratory has reported differences between dietary Mn sources (MnSO4 and AvMn).
Objective color (L*, a*, b*, and chroma) values were greater (P < 0.05) at the beginning of retail display than after 7 d of retail display (results not shown). Moreover, the LM of pigs consuming diets with Mn in the basal diet and supplemented with MnSO4 were also darker (lower L* values; P < 0.05) than the LM from pigs fed NC and PC diets, as well as from pigs fed basal diets containing Mn and supplemented with AvMn (Table 4). Additionally, chops of pigs fed supplemental MnSO4 were redder (greater a* and lower hue angle values; P < 0.05) and more vivid (P < 0.05) than pigs fed supplemental AvMn. Even though chops from pigs fed diets supplemented with MnSO4 tended to be firmer (P = 0.10) than chops from pigs fed diets supplemented with AvMn, neither b* nor E values were altered (P 0.11) by adding dietary or supplemental Mn.
Roberts et al. (2003) noted that LM chops from pigs fed 80, 160, and 320 ppm of Mn from AvMn had lower L* values than chops of pigs fed 40 ppm of Mn, whereas Apple et al. (2005) demonstrated that chops from pigs fed 350 ppm of Mn from AvMn were darker than chops from pigs fed the control diets and diets supplemented with 350 ppm of Mn from MnSO4 or 700 ppm of Mn from AvMn. In contrast to results of this study, Apple et al. (2005) reported that feeding 350 ppm of Mn from AvMn produced LM chops with lower b* values than chops from pigs fed control diets or diets supplemented with 700 ppm of Mn from AvMn. Additionally, hue angles of LM chops from pigs fed diets supplemented with AvMn were less (indicating a truer red color) than chops from pigs consuming diets supplemented with MnSO4, regardless of supplementation level (Apple et al., 2005). Even though supplementing swine diets with 320 ppm, or less, of Mn had no beneficial effects on LM yellowness (b* values), redness (a* and hue angle values), or total color (chroma), Roberts et al. (2003) observed that supplementing 0 or 40 ppm of Mn resulted in more total color change (greater E values) during retail display than feeding swine diets with 80, 160, and 320 ppm of Mn.
The dietary treatment x display period interaction (P = 0.04) for discoloration scores is presented in Figure 1. Discoloration scores were similar (P > 0.05) during the first 4 d of retail display. However, on d 5 and 6 of the display period, chops from pigs fed basal diets including Mn and supplemented with MnSO4 were less (P < 0.05) discolored than chops from pigs fed basal diets devoid of Mn or basal diets with Mn and supplemented with AvMn. Furthermore, chops from pigs fed Mn diets supplemented with AvMn were more (P < 0.05) discolored on d 6 of display than chops from pigs fed diets lacking Mn and supplemented with MnSO4 or AvMn, as well as pigs fed basal diets including Mn. By d 7, chops from pigs fed diets containing Mn and supplemented with MnSO4 were still the least (P < 0.05) discolored, whereas chops from pigs fed the basal diet with Mn and supplemented with AvMn were discolored the most (P < 0.05), to the point (40 to 59% discoloration) of total consumer discrimination (Kropf, 1980; Lynch et al., 1986).
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Figure 1. Effects of dietary Mn level (NC = negative control diets devoid of Mn in the mineral premix, and PC = positive control diets with Mn included in the mineral premix) and Mn supplementation (MnSO4 = 350 ppm of Mn from manganese sulfate, and AvMn = 350 ppm of Mn from AvailaMn-80, Zinpro Corp., Eden Prairie, MN) on discoloration scores [8 = no (0%) discoloration to 1 = total (100%) discoloration; Hunt et al., 1991] across 7 d of retail display (dietary treatment x display day, P = 0.04). There were no (P > 0.05) differences among dietary treatments on d 0, 1, 2, 3, or 4 of retail display (results not shown). a–dFor d 5, 6, and 7 of retail display, bars within a day lacking a common letter differ, P < 0.05.
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Typically, in aerobically (polyvinyl chloride) packaged meat, oxymyoglobin is completely oxidized to metmyoglobin in less than 4 d, and the resulting brown discoloration persists thereafter (Pierson et al., 1970). When LM chops were overwrapped with an oxygen-permeable, polyvinyl chloride film, LM chops from pigs fed 350 ppm of Mn from AvMn were less discolored after 2 d of retail display than chops from pigs fed 350 ppm of Mn, and less discolored after 4 d of retail display than chops from pigs fed control diets or diets supplemented with 350 ppm of Mn from MnSO4 (Apple et al., 2005). Additionally, Roberts et al. (2003) noted that estimated oxymyoglobin content (630 to 580 nm reflectance ratio) was greater in LM chops from pigs fed 80 ppm of Mn than chops from pigs fed 40 ppm of Mn, or less, whereas chops from pigs fed 40 ppm had less oxymyoglobin than all chops from pigs fed 80 ppm, or more, of Mn after 7 d of retail display. The primary reason that discoloration was not detected until d 5 of display in this study may be attributed to the use of a high-oxygen modified atmosphere package. Research has shown that packaging pork in a high-oxygen (75 to 80%) modified atmosphere delayed discoloration 6 (Livingston et al., 2004) to 28 d (Seideman et al., 1979) after packaging, and metmyoglobin formation was curtailed in LM chops packaged in an 80% O2/20% CO2 atmosphere (Asensio et al., 1988). However, results of this study indicate that discoloration can be delayed by feeding elevated Mn levels even when chops were packaged in a high-oxygen modified atmosphere.
In general, pigment oxidation is accompanied by lipid oxidation (Kanner et al., 1987), and in this study, TBARS and discoloration values increased (P < 0.05) from d 0 to 7 of retail display (results not shown). However, excluding Mn from the mineral premix produced lower (P = 0.05) TBARS values compared with including Mn in the mineral premix (Table 4). There was a tendency for chops from pigs fed diets devoid of Mn and supplemented with 350 ppm of Mn from AvMn to have lower (P < 0.10) TBARS values than chops from pigs fed diets formulated with Mn and supplemented with 350 ppm of Mn from AvMn, but the trend may be more a response to dietary Mn than the additional 350 ppm of Mn from AvMn.
Apple et al. (2005) noted a trend for chops from pigs consuming 350 ppm of supplemental Mn to have lower TBARS values than chops from pigs fed unsupplemented diets or diets supplemented with 700 ppm of Mn from AvMn. However, Roberts et al. (2003) failed to observe an effect of supplementing diets with 20, 40, 80, 160, or 320 ppm of Mn on lipid oxidation of pork LM chops. There is evidence that Mn may directly or indirectly inhibit lipid oxidation. Ellis et al. (1971) demonstrated that lipid oxidation was reduced when elemental Mn was added to lard gels, but when added at high levels to methyl-linoleate, Tjhio and Karel (1969) observed a prooxidant effect of Mn. Additionally, production of superoxide anions increases pigment and lipid oxidation (Huang et al., 1995); however, mitocondrial superoxide dismutase, which catalyzes the dismutation of a superoxide anion radical into hydrogen peroxide and water (Hicks, 1980), requires Mn (Scrutton, 1971). Thus, it is possible that any reduction in pigment and lipid peroxidation (TBARS values) associated with feeding elevated Mn levels may be associated with enhanced superoxide dismutase activity.
In conclusion, results produced from this study indicate that pork color can be improved by supplementing growing-finishing pig diets with 350 parts per million of manganese sulfate above maintenance requirements without causing detrimental effects on pig performance and carcass composition. More importantly, basal diets containing manganese in the mineral premix and supplemented with 350 parts per million of manganese from manganese sulfate effectively delayed pork discoloration during 7 days of retail display.
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Footnotes
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1 The authors gratefully acknowledge the Zinpro Corp. for financial support of this experiment. Also, the authors express their appreciation to A. Hays for animal care and performance data collection, K. Richardson and the employees at Bryan Foods (West Point, MS) for assistance with pig slaughter and pork loin procurement, and M. E. Davis and J. Stephenson for assistance with carcass data collection. 
2 Current address: Department of Animal and Food Sciences, Texas Tech University, Lubbock 79409-2141.
3 Corresponding author: japple{at}uark.edu
Received for publication April 26, 2006.
Accepted for publication December 5, 2006.
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Abstract
INTRODUCTION
