DES MOINES, IOWA – Amino acid tryptophan fed to young female pigs as part of their regular diet makes them less aggressive and easier to manage, according to a study by Agricultural Research Service (A.R.S.) scientists and cooperators.

The agency said 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 critical since aggressive behavior can harm them and increase feed and medical costs for producers.

A.R.S. doctoral student Rosangela Poletto and animal scientist Jeremy Marchant-Forde at the A.R.S. Livestock Behavior Research Unit in West Lafayette, Ind. conducted the study. Collaborators included biologist Heng-Wei Cheng at the A.R.S. lab in West Lafayette, and Purdue University scientists Robert L. Meisel and Brian T. Richert.

Based on study results, the supplemented diet raised blood concentrations of tryptophan in three-month-old females by 180%, and by 85% in 6-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.

During 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. Behavioral activity and aggressiveness were measured before and after the seven days of diet supplementation.

Researchers put an “intruder” pig in the pen to test aggression 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 didn’t get the supplement.

Pigs, which form social groups, over time also form stable hierarchies or “pecking orders.” But 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.

The research was published in the journal Applied Animal Behaviour Science.