A note on the effects of a commercial tryptophan product on horse reactivity

Tryptophan supplements are marketed worldwide as calmatives for fearful or excitable horses, although there are no scientific publications that support the efficiency of tryptophan on horses. Here, we investigate whether a commercial feed additive with tryptophan reduces the reactions of horses in standardised fear and handling tests. In two different experiments with 2-year-old stallions (N = 16) and mares (N = 12) we found no significant effects of a single dose feed additive with 6.0 g l-tryptophan on voluntary approach to novel stimuli, behaviour or heart rate during tests or ease of handling. We conclude that one should not rely on an acute dose of tryptophan to calm horses. Better understanding of the causes of fearful behaviour, as well as improving training methods for habituation of horses to stressful situations are at present more reliable ways of reducing reactivity.