Feeding young female pigs the amino acid tryptophan reduces aggression and eases management, according to a study by USDA’s Agricultural Research Service (ARS) scientists and cooperators.

The tryptophan-enhanced diet reduced aggression and improved overall behavior among young female pigs during the eight-month study. Tryptophan is only available through diet.

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

The supplemented diet raised blood concentrations of tryptophan in 3-month-old females by 180%, and by 85% in 6-month-old females. The result was calmer animals, mainly at the younger age. Prolonged aggression causes chronic stress, leading to poorer welfare, increased disease susceptibility and reduced growth and efficiency.

The study diet was 2.5 times the normal amount of tryptophan and was fed for one week to grower pigs and finisher pigs. Another group of pigs was fed a normal diet. Behavior activity and aggressiveness were measured before and after the seven days of supplementation.

To test for aggression, researchers placed an “intruder” pig in the pen until an aggressive event occurred or five minutes had elapsed. Pigs receiving the high-tryptophan diet showed less aggression in fewer attacks of the intruder, and those that did attack were slower to do so, compared with the non-supplemented pigs.

A tryptophan-enriched diet may help reduce aggression when pigs are mixed together and form a hierarchial order, thus lowering physical injury and acute stress.

For more ARS research, go to http://www.ars.usda.gov.