Pig Fed Additional Tryptophan are More 'Chilled'

US - A diet containing enhanced tryptophan levels reduced behavioural activity and aggressiveness of grower gilts, conclude a group of researchers from West Lafayette.
calendar icon 15 January 2010
clock icon 4 minute read

R. Poletto of the USDA-ARS Livestock Behavior Research Unit in West Lafayette and co-workers from Purdue University have investigated the effects of supplementing the diet with the amino acid, tryptophan, on the behaviour of growing pigs. Their paper is soon to be published in Applied Animal Behaviour Science.

Aggression can be a major problem for swine production as it negatively impacts the pigs' health and welfare, explain Poletto and her co-authors. Increasing tryptophan (TRP) intake to raise brain serotonin (5-HT)-key for aggression control, and long-term positive social handling can reduce stress in pigs.

The objective of their research was to feed a short-term high-TRP diet to grower (three months old) and finisher (six months old) maternal gilts that were either socially handled or not and measure their behavioural activity and aggressiveness.

Eight pens of six unrelated gilts were split into two blocks balanced for litter, social handling (non- versus handled) and dietary treatment (control versus high-TRP). Social-handling was applied three times per week, from day 45 until six months of age. At three months, two handled and two non-handled pens were assigned to control while the other four pens were assigned to the high-TRP diet fed ad libitum for seven days (days 1-7). At six months of age, pen assignment to dietary treatments was swapped.

Body weights and blood were taken at days 1 (pre-feeding) and 7. Blood samples were analysed for TRP and 5-HT concentrations using high pressure liquid chromatography. Behaviour was recorded from days 1 to 5 and scan-sampling used to determine time-budget behaviours and postures in a 12-hour period each day (06:00-18:00 h). Aggression evaluation in the home pen focused on counts of agonistic interactions, bites and head-knocks per interaction during three, 30-minute intervals (08:00, 12:00 and 16:00 h) from days 1 to 5. Resident-intruder (R-I) test was carried out for a maximum of 300 seconds at days 6 and 7 to measure aggressiveness, predicted by the latency to the first attack and attack outcomes.

A 2×2 factorial arrangement of dietary treatment and social handling within age was analysed by repeated measures of mixed models and Tukey adjustments.

The researchers found that the TRP-added diet raised blood TRP concentration of three- and six-month-old gilts by 180.7 per cent and 85.2 per cent, respectively (P<0.05), reduced behavioural activity and time spent standing, while increasing lying behaviour, mostly in grower gilts (P<0.05).

High-TRP diet reduced the number of agonistic interactions, and aggressiveness in three-month-old gilts, which took longer to attack the intruder pig, and displayed fewer attacks on the first day of testing (P<0.05).

Long-term positive social handling improved growth performance and had a slight effect on behaviour (P<0.05).

Poletto and her co-authors conclude that provision of enhanced TRP diet reduced behavioural activity and aggressiveness of grower gilts, and these results are likely mediated by activation of brain serotonergic system. Short-term high-TRP dietary supplementation may be used to reduce aggression at mixing in young pigs.


Poletto R., R.L. Meisel, B.T. Richert, H-W. Cheng and J.N. Marchant-Forde, 2009. Aggression in replacement grower and finisher gilts fed a short-term high-tryptophan diet and the effect of long-term human-animal interaction. Applied Animal Behaviour Science (in press). doi:10.1016/j.applanim.2009.11.015

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