Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/9/2006
Publication Date: 8/1/2006
Citation: Guzik, A.C., Matthews, J.O., Kerr, B.J., Bidner, T.D., Southern, L.L. Dietary tryptophan effects on plasma and salivary cortisol and meat quality in pigs. Journal of Animal Science.84:2251-2259. Interpretive Summary: Stressing pigs at weaning, mixing, and transport can play an important role in animal well-being, immune function, growth performance, and meat quality. Consequently, negative effects such as pale, soft, and exudative pork, are sometimes present in pork products. In addition to stress effects on meat quality, there may be a question regarding the stress response of a pig to situations within its ‘environment’, such as when a pig’s vocalization in response to a stress (i.e., snaring) may stress pigs that are in close proximity to the pig being stressed. Certain neurotransmitters such as epinephrine, dopamine, and norepinephrine are responsible for the animal quickly responding to stress and returning the body to a state of homeostasis. However, these neurotransmitters are difficult to measure because they are found in the brain, and animals must often be killed to assess changes in concentration. In order to evaluate stress without killing animals, it may be possible to measure plasma or salivary cortisol because cortisol responds to stress by being slowly released over time. Tryptophan, a precursor of serotonin, has been shown to increase concentrations of serotonin in pigs, rats, and chickens, to which one could conclude that supplemental tryptophan may impact the response an animal has to stress. Furthermore, tryptophan has been used in finishing pigs in efforts to improve meat quality by lowering stress at the slaughter facility. These experiments were conducted to evaluate the effects of tryptophan on cortisol and lactate concentrations after the introduction of an acute stressor, and to determine the effect of increased dietary tryptophan on meat quality in finishing pigs. Data from these experiments reported that increased dietary tryptophan decreased cortisol after an acute stress indicating that supplemental tryptophan can reduce the stress response. However, tryptophan supplementation did not improve meat quality or give any indication of affecting stress at the slaughter facility. Research results described in this report provides nutritionists at universities, feed companies, allied industries, and swine production facilities data showing that short periods of high dietary levels of tryptophan may have an affect on select stress responses, but has little to no effect on meat quality in finishing pigs.
Technical Abstract: 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 four barrows (50 kg 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 pigs were snared. In Exp. 2, 20 barrows (50 kg 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 five 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 these pigs adjacent to those pigs snared, 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 (106 kg initial BW) were allotted to six treatments in a 3 x 2 factorial arrangement. Three different diets with Trp (basal diet, basal supplemented with 0.5% Trp for 5 d, or pigs fed the basal diet with a 0.1g/kg Trp bolus given 2 h before slaughter) were combined with two handling methods (minimal and normal handling). Dressing percentage, 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, CIE a* and 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 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.