Short communication A note on the effects of a commercial tryptophan product on horse reactivity Jens Malmkvist * , Janne Winther Christensen Danish Institute of Agricultural Sciences, Department of Animal Health, Welfare and Nutrition, P.O. Box 50, DK-8830 Tjele, Denmark Accepted 5 October 2006 Available online 7 November 2006 Abstract 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. # 2006 Elsevier B.V. All rights reserved. Keywords: Equus caballus; Behaviour; Reactivity; L-Tryptophan; Heart rate 1. Introduction Reactivity and flight reactions in horses used for sports, leisure and breeding can be a problem and a safety risk, e.g. during handling, riding, loading and transport in trailers because reactive horses may be more difficult to control. Several non-prescriptive feed additives containing tryptophan are marketed worldwide, claiming to calm fearful or excitable horses on days when they are exposed to fear-eliciting stimuli. To date very few scientific studies have been conducted using the dose ranges provided in commercial preparations. As tryptophan continues to be used www.elsevier.com/locate/applanim Applied Animal Behaviour Science 107 (2007) 361–366 * Corresponding author. Tel.: +45 89991314; fax: +45 89991500. E-mail address: Jens.Malmkvist@agrsci.dk (J. Malmkvist). 0168-1591/$ – see front matter # 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.applanim.2006.10.004