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Dietary tryptophan effects on plasma and salivary cortisol and meat quality in pigs^1,2

A. C. Guzik^*, J. O. Matthews, B. J. Kerr^,3, T. D. Bidner^* and L. L. Southern^*

^* Department of Animal Sciences, Louisiana State University Agricultural Center, Baton Rouge 70803; and Premium Standard Farms, Kansas City, MO 64105; and USDA-ARS, Swine Odor and Manure Management Research Unit, Ames, IA 50011

	Abstract

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Four experiments were conducted to determine the effects of supplemental Trp on meat quality, plasma and salivary cortisol, and plasma lactate. Experiment 1 was a preliminary study to measure plasma cortisol concentrations in 4 barrows (50 kg of BW) that were snared for 30 s at time 0 min. Pigs were bled at –60, –30, –15, 2, 4, 6, 8, 10, 15, 20, 25, 30, 45, 60, 90, and 120 min. Plasma cortisol was near maximum 10 min after the pigs were snared. In Exp. 2, 20 barrows (50 kg of BW) were allotted to a basal corn-soybean meal diet or the basal diet with 0.5% supplemental L-Trp for 5 d. After the 5-d feeding period, pigs were snared for 30 s and bled at –10, 0, 2, 4, 6, 8, 10, 15, 20, 25, 30, 45, 60, 90, and 120 min after snaring. Pigs fed the diet with supplemental Trp had a lower (P < 0.01) mean plasma cortisol than pigs fed the basal diet. Plasma lactate also was decreased (P < 0.07) by supplemental Trp. In Exp. 3, the same pigs and treatments were used as in Exp. 2, but 5 pigs were snared and 15 pigs adjacent to those being snared were bled to determine if pigs are stressed when they are adjacent to pigs being snared. For pigs adjacent to snared pigs, the area under the curve (P < 0.06) and mean for plasma cortisol was lower (P < 0.01) in pigs fed Trp relative to those fed the basal diet. In Exp. 4, 90 barrows (initial BW of 106 kg) were allotted to 6 treatments in a 3 x 2 factorial arrangement. Three diets with Trp (basal diet, basal supplemented with 0.5% Trp for 5 d, or pigs fed the basal diet with a 0.1 g/kg of BW Trp bolus given 2 h before slaughter) were combined with 2 handling methods (minimal and normal handling). Dressing percent, 24-h pH, and 24-h temperature were reduced in the minimally handled pigs (P < 0.10) compared with the normally handled pigs. Pigs fed Trp in the diet relative to those fed the basal diet had increased 45-min temperature, Commission Internationale de l’Eclairage (CIE) redness (a*) and yellowness (b*) values, and drip and total losses (P < 0.10). Tryptophan in bolus form decreased 45-min pH in the minimally handled pigs but increased 45-min pH in the normally handled pigs (handling x Trp bolus interaction, P = 0.08). Tryptophan in the diet increased CIE lightness (L*) in minimally handled pigs but decreased CIE L* in the normally handled pigs (handling x Trp diet interaction, P = 06). No other response variables were affected by handling method or Trp. Results indicate that Trp decreases plasma cortisol but has no positive effect on meat quality.

Key Words: cortisol • meat quality • swine • tryptophan

	INTRODUCTION

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Stressing pigs at weaning, mixing, and transport can play an important role in animal well-being, immune function, and growth performance. Meat quality may also be impaired, appearing as PSE pork. In addition to these effects, the stress response of a pig (such as stress-induced vocalization to snaring) may influence other pigs that are in close proximity to the pig being stressed.

Certain neurotransmitters (epinephrine, dopamine, and norepinephrine) are responsible for the animal quickly responding to stress and returning the body to a state of homeostasis (Henry et al., 1992, 1996; Adeola et al., 1993). However, neurotransmitters are often difficult to measure because they are found in various parts of the brain, and animals must often be slaughtered to assess changes in concentration (Piekarzewska et al., 2000). To evaluate stress without slaughtering animals, it may be possible to measure plasma or salivary cortisol (Ruis et al., 1997) because cortisol also responds to stress.

Tryptophan (a precursor of serotonin) has been shown to increase concentrations of serotonin in pigs (Adeola and Ball, 1992; Pethick et al., 1997), rats (Leathwood, 1987), and chickens (Laycock and Ball, 1990). Consequently, supplemental Trp may affect the response an animal has to stress through its ability to increase serotonin. Furthermore, Trp has been used in finishing pigs to improve meat quality by lowering stress at slaughter (Adeola and Ball, 1992; Pethick et al., 1997).

The current experiments were conducted to evaluate effects of Trp on cortisol and lactate concentrations after introduction of an acute stressor and to determine the effect of increased dietary Trp on meat quality of finishing pigs.

	MATERIALS AND METHODS

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The materials and methods used in these experiments were approved by the Louisiana State University Agricultural Center Animal Care and Use Committee.

General
Four experiments were conducted to evaluate the effect of Trp on plasma and salivary cortisol concentrations and meat quality in finishing pigs. Yorkshire, Yorkshire x Landrace, and Yorkshire x Landrace x Duroc barrows from the Louisiana State University Agricultural Center Swine Unit were used in each experiment. Pigs and their environment were monitored daily. Barrows were allotted to treatments on the basis of BW, and ancestry was equalized across treatments in randomized complete block designs.

Experiment 1
A preliminary experiment was conducted to determine the time at which cortisol reached a maximum after introduction of an acute stressor (snaring). Four barrows (50 kg of BW) were housed individually in 0.6 x 1.2 m polyvinyl chloride metabolism crates for the duration of the experiment. Pigs were allowed a 2-d adjustment period before catheters were surgically inserted into the anterior vena cava using the technique described by Amoikon et al. (1995).

After the adjustment period and 2 d after catheter insertion, pigs were bled for 3 h, with blood collection times of –60, –30, –15, 2, 4, 6, 8, 10, 15, 20, 25, 30, 45, 60, 90, and 120 min. At time 0, pigs were snared for 30 s, and blood collection resumed with the 2 min sample. Blood samples were placed in tubes containing 10 mg of sodium fluoride and 8 mg of potassium oxalate. To reduce stress, the same technician was responsible for all blood collections for each pig. Blood samples were placed on ice for 2 h and then centrifuged for 15 min at 1,500 x g at 4°C. Plasma was then frozen (–20°C) until subsequent analyses for cortisol and lactate. This experiment was designed to provide the basis to determine blood collection times and duration of bleeding for subsequent experiments.

Experiment 2
A second experiment was conducted to determine the effects of snaring on plasma cortisol concentrations in pigs fed supplemental Trp. Twenty barrows (initial BW of 50 kg) were allotted to 1) a cornsoybean meal basal diet (Table 1), or 2) the basal diet plus 0.5% supplemental L-Trp (as-fed basis). The basal diet was formulated to meet or exceed 105% of the requirements for all amino acids in growing pigs at 20 to 50 kg of BW (NRC, 1998). Pigs were fed twice daily to approximate ad libitum intake. After catheterization as described in Exp. 1, pigs were fed the experimental diets for 5 d. The 5-d feeding period was based on the results of Adeola and Ball (1992), who reported increased serotonin production after feeding supplemental Trp for 5 d.

Experiment 3
On the third day, using the same pigs as in Exp. 2, another experiment was conducted to determine if a stress response is present in pigs adjacent to those being snared. At 0800, 5 pigs were snared for 30 s. After the pigs were snared, blood collection commenced at 2 min on those pigs adjacent to those that were snared. There were 8 pigs that were previously fed the basal diet and 7 pigs that had been fed the diet containing 0.5% supplemental L-Trp. Collection times were as in Exp. 2.

Experiment 4
Ninety barrows (average initial and final BW of 106 ± 2.3 and 111 ± 2.4 kg, respectively) were allotted to 1 of 6 treatments on the basis of BW and ancestry in a randomized complete block design. Four replicates of 3 or 4 pigs per replicate were used. Feed and water were provided on an ad libitum basis, and pigs were housed in an open-sided finishing barn in 1.52 x 4.27 m pens with cement-slatted floors. A 3 x 2 factorial arrangement of treatments was used; there were 3 diets [cornsoybean meal basal diet, the basal diet with 0.5% supplemental L-Trp on an as-fed basis, or pigs fed the basal diet with a bolus of Trp (0.1 g/kg of BW) given 2 h before slaughter] and 2 handling methods at slaughter (normal and minimal).

The diets were fed for 5 d before slaughter. Pigs were weighed at the beginning and end of the 5-d experimental period. To achieve minimal handling before slaughter, pigs were transported to the slaughtering facility, were not mixed during transport or at the slaughter facility, and were moved with minimal force to the stunning area. Pigs receiving normal slaughter handling were mixed before transportation and slaughtered within 2 h of mixing, which resulted in fighting among pigs that continued until the time of slaughter (Matthews et al., 2001). Pigs receiving the bolus (0.1 g of L-Trp/kg of BW in 60 mL of H₂O) were administered the dose orally 2 h before slaughter. Pigs that did not receive the bolus received 60 mL of H₂O orally 2 h before slaughter. On the day of slaughter, after the minimally handled pigs were slaughtered, the other half of the pigs were mixed, transported, and handled at the slaughter facility to induce stress (Matthews et al., 2001). All pigs were held without feed for 16 h before slaughter. In all pigs, blood and saliva were collected at the time of slaughter during exsanguination for cortisol analysis.

Carcass Evaluation
Pigs were slaughtered by exsanguination after electrical stunning. Hot carcass weights and rectal temperatures (rectal glass probe) were collected at the time of slaughter, after exsanguination but before entering the scalding tank. Carcass measurements were collected on the left side of the carcass after a 24-h chill at 2°C. Linear measurements included dressing percent, LM muscle area (10th rib), 10th rib backfat, average back-fat, and carcass length. Pork quality measurements between the 10th and 11th ribs included 45-min and 24-h pH (hand-held pH meter, Model 2000, VWR Scientific Products Co., South Plainfield, NJ), 45-min and 24-h temperature, and Commission Internationale de l’Eclairage (CIE) lightness (L*), redness (a*), and yellowness (b*) values (determined using the CIELAB color space with a D65 illuminant and a Minolta spectrophotometer, Model CM-508d, Minolta, Ramsey, NJ). After collection of carcass data, two 2.54-cm chops were collected between the ninth and 10th ribs for pork quality (color and marbling; NPPC, 1991), drip and 48-h cook loss, and Warner Bratzler shear force (Instron Model 4501 Universal Testing Instrument, Canton, MA). Drip loss, cook loss, and shear force were conducted as described by Matthews et al. (2001).

Salivary Cortisol
The relationship between plasma and salivary cortisol was of interest for practical purposes of collecting samples without stressing (i.e., snaring) the pigs. On the day of slaughter (after electrical stunning), saliva was collected by inserting a cotton-tip applicator into the mouth until the cotton was fully moistened. The sample was placed in a tube, centrifuged, and saliva was removed, frozen, and stored for later cortisol analysis. The technique followed that previously used in other studies (Ruis et al., 1997; Gaverink et al., 1998).

Plasma Cortisol and Lactate
Cortisol concentrations (µg/dL) were determined by using an ImmunChem coated tube (Cat No. 07-221102, ICN Biochemicals, Costa Mesa, CA). The intraassay CV for cortisol determined with a porcine plasma pool was 6.0%. Lactate concentrations (µg/dL) were determined using a commercial kit (Sigma Tech. Bulletin #826-UV, Sigma, St. Louis, MO).

Statistical Analysis
Data were analyzed by ANOVA procedures (Steel and Torrie, 1980) appropriate for randomized complete block designs using the GLM procedures of SAS (SAS Inst. Inc., Cary, NC). In Exp. 2 and 3, all hormone data were analyzed with the individual pig representing the experimental unit. Area under the response curve for cortisol was determined using trapezoidal geometry (0 to 120 min), and the –10- and 0-min samples were used to establish a baseline. In Exp. 4, treatment differences were determined using orthogonal contrast statements for a 3 x 2 factorial arrangement of treatments, with the pen of pigs serving as the experimental unit.

	RESULTS

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Experiments 1 and 2
In Exp. 1, plasma cortisol was 3.6-fold greater at 8 to 15 min after snaring than the average of the 4 samples before and immediately after (–30, –15, 2, and 4 min) the pigs were snared (Figure 1). In Exp. 2 as in Exp. 1, plasma cortisol was greatest approximately 10 min after snaring. The mean cortisol concentration for pigs fed Trp was lower (P < 0.01) than those fed the basal diet, indicating that Trp decreased the cortisol response to snaring stress (Figure 2). Plasma lactate was at its greatest concentration 2 min after snaring. Plasma lactate also was decreased (P < 0.07) in pigs fed Trp relative to those fed the basal diet (Figure 3). Both of these metabolites suggest that dietary Trp affected the stress response and that Trp may be beneficial for reducing stress.

View larger version (8K): [in this window] [in a new window]   Figure 1. Plasma cortisol concentration after snaring (Exp. 1). Data are means of 4 pigs (SEM = 0.15).

View larger version (8K): [in this window] [in a new window]   Figure 2. Plasma cortisol concentrations of pigs fed a basal diet or the basal diet with 0.5% supplemental Trp for 5 d (Exp. 2). Data are means of 10 pigs each (Trp, P < 0.01, SEM = 0.22).

View larger version (8K): [in this window] [in a new window]   Figure 3. Plasma lactate concentrations of pigs fed a basal diet or the basal diet with 0.5% supplemental Trp for 5 d (Exp. 2). Data are means of 10 pigs each (Trp, P < 0.07, SEM = 1.18).

View larger version (8K): [in this window] [in a new window]   Figure 4. Plasma cortisol concentration of pigs adjacent to snared pigs when fed a basal diet or the basal diet with 0.5% supplemental Trp for 5 d (Exp. 3). Data are means of 8 pigs for the control diet and 7 pigs for the Trp diet (area under the curve, P < 0.06, SEM = 1.15; Trp, P < 0.01, SEM = 0.13).

	DISCUSSION

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Tryptophan also has been shown to increase protein synthesis in livers of pigs (Cortamira et al., 1991), but feeding crystalline Trp to pigs above their perceived nutritional requirement has not been shown to have a beneficial effect on growth performance or meat quality (Sève et al., 1991; Adeola and Ball, 1992; Page et al., 1993; Geesink et al., 2004). Results of Exp. 4 agree with results from previous experiments indicating that the use of dietary Trp is not beneficial for improving meat quality. In our results, a greater CIE L* value and lower color score were observed, which both indicate a poorer meat quality in pigs fed Trp that were subjected to minimal handling. Lower CIE a* and CIE b* values were obtained and are also indicators of meat color but are less conclusive. With the increase in both salivary and plasma cortisol and the negative effects on meat quality, these results may indicate that, in this experiment, Trp had a stimulatory effect instead of a sedentary effect.

Adeola and Ball (1992) also evaluated the use of supplemental Trp in finishing pigs (92 kg) on stress responses and the reduction of PSE pork. Pigs received a corn-barley-soybean meal diet supplemented with 5 g of Trp/kg in the diet 5 d before slaughter. Tryptophan supplementation had little effect on muscle and fat depth, but there was a lower incidence of PSE pork in pigs fed excess Trp than in pigs fed the control diet (27 vs. 33%, respectively). The pH, color, and structure of the loins and hams were similar regardless of diet fed. Pethick et al. (1997) also reported a lower incidence and severity of PSE pork when 5 g of Trp was supplemented per kilogram of diet.

Perhaps increased concentrations of Trp intake immediately before slaughter may improve meat quality by increasing serotonin and reducing stress, and allowing the glycogen stores to remain high. However, because of the fasting that commonly occurs before slaughter, the diet may not play an important role in affecting the stress response. That hypothesis is the basis for use of the bolus in our experiment. The reason there was not a decrease in cortisol after the administration of the bolus is not understood, but this response may be due to a palatability problem or taste of the bolus that had a negative impact on the pigs. The contradiction in the cortisol response of Exp. 2 and 3 compared with Exp. 4 is not well understood but may be due to the many environmental changes that the finishing pigs had to contend with before slaughter.

Although our results using dietary Trp agree with previous research, handling method did not affect the response variables measured in our study as it has previously. Matthews et al. (2001) evaluated the handling method on carcass traits and observed a decrease in rectal temperature and plasma cortisol in minimally handled pigs. In addition, a lower CIE L* was observed in minimally handled pigs. Cortisol and rectal temperature are valid indicators of stress, and it is not known why we did not see a decrease in either in minimally handled pigs. Differential responses to potential stressors are not uncommon. Warriss et al. (1998) indicated that a 1 to 3 h rest in lairage was optimal for meat quality, whereas Geverink et al. (1996) indicated that lairage time should be kept at a minimum to avoid agonistic behavior.

As previously discussed, Trp may play a role in affecting the stress response through its role in regulation of serotonin. However, the effect of dietary Trp on meat quality may not be beneficial because the pigs are fasted for a long duration before slaughter. On the other hand, increased dietary Trp concentrations may play a role in decreasing stress during the transport to the slaughter facility and thus reduce the incidence of PSE pork. Consequently, greater concentrations of dietary Trp may be necessary to induce the sedative effect that is needed for improved meat quality as reported by Pethick et al. (1997).

Data from the first 3 experiments suggest there is a time lapse between the introduction of the stressor and the response. Therefore, these data indicate that blood samples, if taken quickly, can be analyzed for cortisol without the effect of snaring becoming a factor. This is supported by McGlone et al. (1993), who indicated that plasma cortisol increases after a pen of pigs is aroused. However, because pigs adjacent to those that have been snared exhibit an increase in plasma cortisol, only the number of pigs that can be bled within 2 to 3 min should be bled if plasma cortisol, and perhaps other metabolites, are the intended response variable. In contrast, McGlone et al. (1993) indicated that bleeding one pen of pigs in a room did not cause elevated plasma cortisol among pigs housed in other pens.

	IMPLICATIONS

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Increased dietary tryptophan decreased cortisol after an acute stress, indicating that tryptophan can reduce the stress response. Tryptophan supplementation did not improve meat quality or give any indication of affecting stress at the slaughter facility. Further research is needed to elucidate any beneficial effects of tryptophan on meat quality in finishing pigs before any definite conclusions can be drawn.

	Footnotes

 
¹ Approved for publication by the director of the Louisiana Agric. Exp. Sta. as manuscript No. 2005-18-0258.

² Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the USDA or the Louisiana State University Agricultural Center and does not imply approval to the exclusion of other products that may be suitable. Amino acids and financial support were provided in part by Nutri-Quest Inc., Chesterfield, MO 63017.

³ Corresponding author: kerr{at}nsric.ars.usda.gov

Received for publication June 2, 2005. Accepted for publication February 9, 2006.

	LITERATURE CITED

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Adeola, O., and R. O. Ball. 1992. Hypothalamic neurotransmitter concentrations and meat quality in stressed pigs offered excess dietary tryptophan and tyrosine. J. Anim. Sci. 70:1888–1894.[Abstract]

Adeola, O., R. O. Ball, J. D. House, and P. J. O’Brien. 1993. Regional brain neurotransmitter concentrations in stress-susceptible pigs. J. Anim. Sci. 71:968–974.[Abstract]

Amoikon, E. K., J. M. Fernandez, L. L. Southern, D. L. Thompson Jr., T. L. Ward, and B. M. Olcott. 1995. Effect of chromium tripicolinate on growth, glucose tolerance, insulin sensitivity, plasma metabolites, and growth hormone in pigs. J. Anim. Sci. 73:1123–1130.[Abstract]

Cortamira, N. O., B. Sève, Y. Lebreton, and P. Ganier. 1991. Effect of dietary trytophan on muscle, liver, and whole-body protein synthesis in weaned pigs: Relationship to plasma insulin. Br. J. Nutr. 66:423–435.[CrossRef][Medline]

DeNapoli, J. S., N. H. Dodman, L. Shuster, W. M. Rand, and K. L. Gross. 2000. Effect of dietary protein content and tryptophan supplementation on dominance aggression, territorial aggression, and hyperactivity in dogs. J. Am. Vet. Med. Assoc. 217:504–508.[CrossRef][Medline]

Denbow, D. M., F. C. Hobbs, R. M. Hulet, P. P. Graham, and L. M. Potter. 1993. Supplemental dietary L-tryptophan effects on growth, meat quality, and brain catecholamine and indolamine concentrations in turkeys. Br. Poult. Sci. 34:715–724.[Medline]

Geesink, G. H., R. G. C. van Buren, B. Savenije, M. W. A. Verstegen, B. J. Ducro, J. G. P. van der Palen, and G. Hemke. 2004. Short-term feeding strategies and pork quality. Meat Sci. 67:1–6.[CrossRef]

Geverink, N. A., A. Kappers, J. A. van de Burgwal, E. Lambooij, H. J. Blokhuis, and V. M. Wiegant. 1998. Effects of regular moving and handling on the behavioral and physiological responses of pigs to preslaughter treatment and consequences for subsequent meat quality. J. Anim. Sci. 76:2080–2085.[Abstract/Free Full Text]

Geverink, N. A., B. Engel, E. Lambooij, and V. M. Wiegant. 1996. Observations on behavior and skin damage of slaughter pigs and treatment during lairage. Appl. Anim. Behav. Sci. 50:1–13.

Hedrick, H. B., E. D. Aberle, J. C. Forrest, M. D. Judge, and R. A. Merkel. 1989. Principles of Meat Science. 3rd ed. Kendall/Hunt, Dubuque, IA.

Heine, W., M. Radke, and K. D. Wutzke. 1995. The significance of tryptophan in human nutrition. Amino Acids 9:191–205.

Henry, Y., B. Sève, Y. Colléaux, P. Ganier, C. Saligaut, and P. Jego. 1992. Interactive effects of dietary levels of tryptophan and protein on voluntary feed intake and growth performance in pigs, in relation to plasma free amino acids and hypothalamic serotonin. J. Anim. Sci. 70:1873–1887.[Abstract]

Henry, Y., B. Sève, A. Mounier, and P. Ganier. 1996. Growth performance and brain neurotransmitters in pigs as affected by tryptophan, protein, and sex. J. Anim. Sci. 74:2700–2710.[Abstract]

Kerr, B. J., E. T. Moran, Jr., and M. T. Kidd. 2005. Effect of supplementary tryptophan before marketing on carcass quality in broilers. J. Appl. Poult. Sci. 14:306–314.

Laycock, S. R., and R. O. Ball. 1990. Alleviation of hysteria in laying hens with dietary tryptophan. Can. J. Vet. Res. 54:291–295.[Medline]

Leathwood, P. D. 1987. Tryptophan availability and serotonin synthesis. Proc. Nutr. Soc. 46:143–146.[CrossRef][Medline]

Lee, S. R., and W. M. Britton. 1982. Effect of elevated dietary tryptophan on avian hypothalamic serotonin, 5-hydroxyindole acetic acid and norepinephrine. Poult. Sci. 61:1500. (Abstr.)

Matthews, J. O., L. L. Southern, T. D. Bidner, and M. A. Persica. 2001. Effects of betaine, pen space, carcass traits, and pork quality of finishing pigs. J. Anim. Sci. 79:967–974.[Abstract/Free Full Text]

McGlone, J. J., J. L. Salak, E. A. Lumpkin, R. I. Nicholson, M. Gibson, and R. L. Norman. 1993. Shipping stress and social status effects on pig performance, plasma cortisol, natural killer cell activity, and leukocyte numbers. J. Anim. Sci. 71:888–896.[Abstract]

McGlone, J. J., W. F. Stansbury, and L. F. Tribble. 1985. Nursery pig performance: Effects of regrouping and high levels of tryptophan during heat stress. Page 16 in Texas Tech Univ. Swine Rep. Texas Tech Univ., Lubbock.

Meunier-Salaün, M. C., M. Monnier, Y. Collèux, B. Sève, and Y. Henry. 1991. Impact of dietary tryptophan and behavioral type on behavior, plasma cortisol, and brain metabolites of young pigs. J. Anim. Sci. 69:3689–3698.[Abstract]

NPPC. 1991. Procedures to Evaluate Market Hogs. 3rd ed. Natl. Pork Prod. Counc., Des Moines, IA.

NRC. 1998. Nutrient Requirements of Swine. 10th rev. ed. Natl. Acad. Press, Washington, DC.

Page, T. G., L. L. Southern, and T. L. Ward. 1993. Effect of dietary tryptophan in excess of the requirement on growth and carcass characteristics of finishing pigs. Prof. Anim. Sci. 9:86–88.

Peeters, E., B. Driessen, R. Steegmans, D. Henot, and R. Geers. 2004. Effect of supplemental tryptophan, vitamin E, and a herbal product on responses by pigs to vibration. J. Anim. Sci. 82:2410–2420.[Abstract/Free Full Text]

Pethick, D. W., R. D. Warner, D. N. D’Souza, and F. D. Dunshea. 1997. Nutritional manipulation of meat quality. Pages 100–115 in Manipulating Pig Production VI. P. D. Cranwell, ed. Australasian Pig Sci. Assoc., Roseworthy, SA, Australia.

Piekarzewska, A. B., S. J. Rosochacki, and G. Sender. 2000. The effect of acute restraint stress on regional brain neurotransmitter levels in stress-susceptible Pietrain pigs. J. Vet. Med. Series APhy., Path. Clin. Med. 47:257–269.

Ruis, M. A., J. H. Te Brake, B. Engel, E. D. Ekkel, W. G. Buist, H. J. Blokhuis, and J. M. Koolhaas. 1997. The circadian rhythm of salivary cortisol in growing pigs: Effects of age, sex, and stress. Physiol. Behav. 62:623–630.[CrossRef][Medline]

Sainio, E. L., K. Pulkki, and S. N. Young. 1996. L-Tryptophan: Biochemical, nutritional, and pharmacological aspects. Amino Acids 10:21–47.

Sève, B. 1999. Physiological roles of tryptophan in pig nutrition. Pages 729–741 in Tryptophan, Serotonin, and Melatonin: Basic Aspects and Applications. G. Huether, W. Kochen, T. J. Simat, and H. Steinhart, ed. Kluwer Academic/Plenum Publishers, New York, NY.

Sève, B., M. C. Meunier-Salaün, M. Monnier, Y. Colléux, and Y. Henry. 1991. Impact of dietary tryptophan and behavioral type on growth performance and plasma amino acids of young pigs. J. Anim. Sci. 69:3679–3688.[Abstract]

Shea, M. M., L. W. Douglass, and J. A. Mench. 1991. The interaction of dominance status and supplemental tryptophan on aggression in Gallus domesticus males. Pharm. Biochem. Behav. 38:587–591.[CrossRef][Medline]

Shea, M. M., J. A. Mench, and O. P. Thomas. 1990. The effect of dietary tryptophan on aggressive behavior in developing and mature broiler breeder males. Poult. Sci. 69:1664–1669.[Medline]

Steel, R. G. D., and J. H. Torrie. 1980. Principles and Procedures of Statistics: A Biometrical Approach. 2nd ed. McGraw-Hill, New York, NY.

Warriss, P. D., S. N. Brown, J. E. Edwards, and T. G. Knowles. 1998. Effect of lairage time on levels of stress and meat quality in pigs. Anim. Sci. 66:255–261.

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