Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/2/2005
Publication Date: 8/2/2005
Citation: Li, Y.Z., Kerr, B.J., Kidd, M.T., Gonyou, H.W. 2005. Use of supplementary tryptophan to modify the behavior of pigs. Journal of Animal Science. 84:212-220.
Interpretive Summary: As a limiting essential amino acid, tryptophan is typically supplied in the diet at levels required for maximum animal growth. When supplemental tryptophan is supplied in the diet above that required for protein synthesis, it has been shown to alter brain tryptophan levels which subsequently influence the synthesis of serotonin, an inhibitory neurotransmitter in the central nervous system. In domestic animals, therapeutic functions of tryptophan include reducing feed intake, modifying aggression, suppressing hysteria, and inhibiting the response to stress. In swine production and marketing there are potentially high levels of stress as animals are moved from their home pens, regrouped, transported, and handled in the packing plant. Suppression of the stress response and associated aggression may be beneficial in terms of animal welfare and meat quality. These experiments were conducted to determine the effect of short-term supplementation of tryptophan on behavioral and physiological responses, and meat quality indices of pigs within a stable social group, when re-grouped, and during handling. Data from these experiments reported behavioral effects within a few days of pigs being fed diets containing tryptophan at two to four times above suggested growth requirements. These behavior differences included a greater time spent lying and less time eating. Aggression among unfamiliar pigs was reduced in duration and intensity, but not frequency, when pigs were fed high tryptophan diets. The responses of pigs to the stressors of handling, including electric shock, were unaffected by tryptophan treatment. Dietary tryptophan had no effect on the color, pH, or drip loss of the longissimus dorsi. 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 can be used to reduce aggression, but has little effect on the response to handling.
Technical Abstract: Three experiments were conducted to investigate the short-term use of supplementary Trp on the behavior of grow/finish pigs. Three levels of dietary Trp were used, representing the standard requirement for growth (Control), twice (2X), and four (4X) times the control amount. In Exp. 1, pigs were fed the diets for 7 d, during which observations were made of their general behavior (time budget), aggression within the group of familiar pigs, and response to a startling auditory stimulus. Behavior effects were evident during the period of supplementation for both the 2X and 4X diets. Pigs fed supplemental Trp spent more time lying and less time eating. Similar behavioral effects were evident after the startling stimulus. Based on these observations, the subsequent studies retained the same dietary levels of Trp and incorporated a 3 d feeding of diets prior to behavior testing. In Exp. 2, pigs were fed the experimental diets for 3 d before being regrouped with unfamiliar pigs on the same diet. The subsequent aggression was affected by Trp supplementation. Higher levels of Trp resulted in similar numbers of fights, but the total duration of aggression was reduced by approximately one third, particularly the inverse parallel pressing component of the fight. In Exp. 3, pigs were exposed to specific handling tests on the farm and meat quality assessments after being fed the experimental diets for 3 d. The only behavioral or physiologic difference observed among the treatments was a slower movement of high Trp treatment pigs in a minimal-forced situation. Response to confinement on a scale, an electric prod, and movement in general were similar among treatments. There were no differences among dietary treatments for any of the meat quality characteristic parameters measured. It is suggested that high levels of Trp may result in animals avoiding stressful situations if possible, but have no effect on response to stressors that are forced upon the animal.