Drugs not always the answer for keeping horses calm

Researchers looking at the effectiveness of tryptophan as a calming supplement in horses found that while it has some effects, it did not change reactive behaviour.

Tryptophan is often marketed in the horse industry as a calmative supplement, claiming to encourage focus and relaxation while reducing tense, nervous, and spooky behavior. Horse owners frequently use calming products in situations including transportation and competition.

Tryptophan is the amino acid precursor to serotonin (also known as 5 hydroxytryptamine, or 5-HT), a neurotransmitter involved in mood, appetite, sleep, memory, and learning in many species, including humans. Psychiatrists utilize serotonin-enhancing drugs in people to treat conditions such as depression, anxiety, and obsessive-compulsive disorder.

Brittany Davis and Colorado State University colleagues Terry Engle, Jason Ransom, and Temple Grandin carried out a small study using 11 horses (9 geldings and two mares) at the university’s Equine Teaching and Research Center. Three of the horses were Quarter Horse type and the other nine were draft horse crosses, and ranged in age from two and a half to 16 years.

The horses were assigned a series of four treatments: low (20mgTryptophan/kg bodyweight); medium (40mgTrp/kg BW) and high (60mgTrp/kg BW), and no supplementation (control).

Horses received each treatment for three days, followed by a four-day wash out period during which they received no supplementation.

The researchers assessed the horses’ behaviour, and physiological response, using a reactivity or startle test, in which they recorded the time taken for the horse to move away from a stimulus (such as an alarm sound accompanied by an opening umbrella).

They saw no significant behavioral effects following tryptophan supplementation.

Reporting the findings in the March edition of the journal Applied Animal Behaviour Science, the team said they did find different physiological effects on Day 1 and Day 3 of supplementation. On Day 1, all treatment groups showed at least one change associated with sedation. Low dose tryptophan treatment appeared to have a sedative effect in terms of changes in heart rate and serum lactate levels, whereas the medium and high doses reduced cortisol levels. In contrast, on Day 3, they found either no change or changes associated with excitation (significant increase in the time for heart rate to return to normal after startling in the medium treatment group).

They conclude: “Supplementing tryptophan to horses at higher doses (40 mg/kgBW and 60 mg/kg BW) may be an effective way to reduce cortisol levels in stressful situations. We also saw some evidence of calming effects of tryptophan at a lower dose (20 mg/kgBW). However, these findings were seen in only a few of the results variables measured in this experiment. The horses we used responded to tryptophan supplementation more favorably on the first day they received the supplement.

“It appears that supplementing horses with tryptophan may produce desired results only a few hours after administration and that longer-term use may provide no additional benefit or may even have unwanted effects.”

The researchers suggested that looking into the potential causes of unwanted behaviors should be the first step before owners turn to calming drugs or supplements.

“Providing more training, turnout time, or treatment for an underlying disease or condition could result in a more sustainable way to reduce a horse’s unwanted behaviors and could improve welfare for the animal.”

Preliminary evaluation on the effectiveness of varying doses of supplemental tryptophan as a calmative in horses. Brittany P. Davis, Terry E. Engle, Jason I. Ransom, Temple Grandin. Applied Animal Behaviour Science (2017) 188, pp34–41. Doi:10.1016/j.applanim.2016.12.006

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