At weaning, piglets are suddenly weaned and mixed often resulting in stress behaviour, such as, depression, neuroendocrine hormone disturbance and aggressive behaviour, which is known to have a negative impact on feed intake, gastrointestinal health, growth performance and immune function. Moreover, dietary tryptophan (Trp) may reduce stress. Tryptophan has already been proved to alleviate depression and aggression in humans and animals. Tryptophan crosses the blood–brain barrier and acts as the precursor for 5-hydroxytryptamine (5-HT) synthesis in the serotonergic neurons. The cerebral neurotransmitter 5-HT is essential for aggression inhibition and also regulates sleeping, appetite, mood and behavioral activity. In this study, one hundred and forty-four piglets were selected from 28 d old weaning piglets and assigned randomly to three groups (Control group, Treatment group I, Treatment group II). The Control group was fed the basal diet with 0.21% tryptophan level, the treatment groups (Treatment groups I and II) with basal diets plus an additional 0.1% and 0.2% tryptophan respectively. Piglets were fed the diets for 28 d, during which observations were divided into three phases: prophase (4 – 6 d), metaphase (15 – 17 d), anaphase (26 – 28 d). Daily behavioral observations were made 14:00 – 16:00 during each phase afternoon. Saliva was collected from multiple pigs in each pen from 16:00 to 17:00, and body weight of each piglet was individually recorded at d 1 and 28. Aggressive behaviours of piglets were recorded for 8 h immediately after remixing, and saliva per pen was collected from 16:00 to 17:00. Subsequently, 2 pigs per pen were sampled for blood via the jugular vein and subsequently killed to obtain hypothalamus. Salivary cortisol, plasma norepinephrine and epinephrine, hypothalamic 5-hydroxytryptamine concentrations were measured using enzyme linked immunosorbent assay (ELISA) kit.
High-Trp diet signiﬁcantly raised hypothalamic 5-hydroxytryptamine concentration (P<0.05), and diminished salivary cortisol, plasma norepinephrine and epinephrine concentrations (P<0.05). Behavioral categories (eating, sitting) during daily behavioural observations were not affected by dietary treatment. However, supplemental tryptophan group induced more lying and less standing (P<0.05) in metaphase and anaphase. However, there were no signiﬁcant differences in lying and standing in prophase. After remixing, compared with pigs fed the basal diet, those fed Trp showed reductions in frequency of ﬁght (P<0.05) and time spent ﬁghting (P<0.05), with a bigger decrease in 0.2% supplemental tryptophan group (P<0.05). There were no differences between dietary treatments for growth performance of piglets (P>0.05). Consequently, excessively supplemental Trp may modify the behaviour reactivity of piglets during weaning and mixing, increase hypothalamic 5-hydroxytryptamine concentration, and diminish salivary cortisol, plasma norepinephrine and epinephrine concentrations.
In conclusion, diets containing high Trp levels may be helpful in lowering stress and increasing welfare in pigs during speciﬁc periods of increased stress such as mixing, weaning or transport.
Hua-Wei Liu, Bao-Ming Shi, Da-Sen Liu, An-Shan Shan. Supplemental dietary tryptophan modifies behavior, concentrations of salivary cortisol, plasma epinephrine, norepinephrine and hypothalamic 5-hydroxytryptamine in weaning piglets. Livestock Science (151) 213–218. http://dx.doi.org/10.1016/j.livsci.2012.11.003