Magnesium is a staple of horse calmers and certainly the most supported calming ingredient, both scientifically and anecdotally. There is a well-established link between magnesium and behaviour. It supports against stress and helps to maintain normal nerve transmission.

Because magnesium is an essential dietary component and very safe to feed (one of the few calming ingredients that has been well tested in horses), magnesium would usually be the first port of call when looking to help your horse with any stressful behaviour.

Products vary a great deal in quality; magnesium oxide being the cheapest but least well absorbed. For the best result, ensure that your magnesium horse calmer uses high quality, bio-available magnesium from a trusted manufacturer.

There are many plant extracts used and it is difficult to highlight any as the best horse calmer. Considering the lack of equine research it is reasonable to question whether you believe that the extract could help you (consider also that your horse will not receive the same placebo benefit). Then check the extract quality and that the feeding rate is sufficient for a horse.

Valerian is the most likely to help calm your horse but it is banned under competition rules. It may be useful outside of competition, when a drowsy action is acceptable, but not all horses will be responsive to supplementation.

You should also consider potential side effects. Confusion, dizziness, nausea and gastro-intestinal irritation are listed (in humans) as possible side effects of some the herbal extracts often found in horse calmers.

L-tryptophan is an essential amino-acid that contributes to the synthesis of serotonin, a neurotransmitter often referred to as a ‘mood hormone’. L-Tryptophan lost some favour after a research paper suggested that it may actually increase excitability in horses.  Horse calmers are now more likely to boast of L-tyrosine or L-theanine, although they have actually been used for many years.

L-tyrosine is similar to L-tryptophan in the respect that it is an amino acid associated with the synthesis of neurotransmitter substances.  It is present in the diet and can also be synthesised by the body, so there seems little benefit to daily supplementation. The appeal is that, during a time of acute stress, supplementing L-tyrosine may aid cognitive function (memory and learning) by delaying the depletion of neurotransmitters. It is a bit of a leap to apply this to behavioural problems in horses, particularly as feeding levels tend to be very low in horse calmers.

L-theanine is different in that it is non-dietary; it is not a required nutrient. It is actually an extract of green and black tea, therefore belonging with the herbal extracts.  L-theanine is reputed to aid relaxation. Some claims are made about cognition but these are more likely to be attributable to the combination with caffeine (as naturally exists in tea).

Found in very few horse calmers, casein is a protein rich substance derived from cows’ milk. Certain peptides found in casein are reputed to reduce anxiety and improve sleep; advertisers will often liken this to the effects of suckling. The amount contained in horse calmers is typically very low compared to that recommended for human supplementation.

Many other vitamins and minerals may be present but they are unlikely to contribute meaningfully as a calmer. Keep this in mind when looking for the best calmer for your horse, it is easy to waste money looking at inconsequential differences.

B Vitamins

B-Vitamins are nutrients often associated with functions of the nerves and energy metabolism. The best candidates for your horse calmer are B6 (pyridoxine) which works in synergy with magnesium, and B1 (thiamine) which is thought to help with premenstrual syndrome.

Unlike us, horses gain their B-Vitamins from bacterial fermentation in the hindgut. They may still be supplemented as generally healthful, but are unlikely to calm.

Yeast and Digestive Additives

Many horse calmers contain yeast for hindgut stability. This is generally intended to help protect the hindgut from disturbance caused by stress, rather than with the expectation of calming. Some horse calmers do make claims about reducing butterflies but there is no logic to this. In any instance, a high quality digestive supplement would be needed in order to help stabilise the hindgut. This could also help reduce behaviours associated with discomfort and irritability (but is unlikely to help generally sharp or excitable behaviour).

Ant-Acids (e.g. Calcium Carbonate) and Gastric Additives

Gastric discomfort is certainly associated with ill behaviour and should be considered. However, your horse calmer will not contain appropriate levels of quality ingredients. A better approach would be to choose a high quality gastric supplement and assess any improvement independently. You may need to also consider veterinary treatment and management changes.