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Tryptophan-Enriched Diet Reduces Pig Aggression

by 5m Editor
19 March 2010, at 9:09am

US - Feeding the amino acid, tryptophan, to young female pigs as part of their diet makes them less aggressive and easier to manage, according to a study by Agricultural Research Service (ARS) scientists and cooperators.


Feeding the amino acid tryptophan to young female pigs makes them less aggressive and easier to manage, according to a study by ARS scientists and cooperators.

The tryptophan-enhanced diet reduced aggression and overall behavioral activity among young female pigs during the eight-month study. Tryptophan, which is only acquired through diet, is the precursor for the calming cerebral neurotransmitter, serotonin. Keeping swine calm is important, because aggressive behaviour can harm them and increase feed and medical costs for producers.

The study was done by ARS doctoral student Rosangela Poletto and animal scientist Jeremy Marchant-Forde at the ARS Livestock Behavior Research Unit in West Lafayette, Indiana. Collaborators included biologist Heng-Wei Cheng at the ARS lab in West Lafayette and Purdue University scientists, Robert L. Meisel and Brian T. Richert.

The supplemented diet raised blood concentrations of tryptophan in three-month-old females by 180 per cent, and by 85 per cent in six-month-old females, resulting in calmer animals, mainly at the younger age. Persistent aggression in pigs can cause chronic stress, leading to poorer welfare, increased disease susceptibility and reduced growth and efficiency.

In the study, a diet with 2.5 times the normal amount of tryptophan was fed for one week to grower pigs (three months old) and finisher pigs (six months old). Another group of pigs received a normal diet. Behavioural activity and aggressiveness were measured before and after the seven days of diet supplementation.

To test aggression, researchers put an ‘intruder’ pig in the pen until an aggressive interaction was triggered or for a maximum of five minutes. Pigs receiving the high-tryptophan diet showed less aggression – fewer attacked the intruder, and those that did attack were slower to do so – compared with the animals that did not receive the supplement.

Pigs form social groups that, over time, form stable hierarchies or ‘pecking orders’. However, when new individuals are introduced, aggression is used to re-establish a new hierarchical order. If repeated changes in group composition occur, persistent aggression may arise, sometimes leading to physical injury and acute stress. A tryptophan-enriched diet may help producers avoid these problems, especially when groups of pigs are mixed together.