Tyrosine is an amino acid that metabolizes to produce catecholamines which are the precursors to dopamine and adrenaline. Many antidepressant regimens include tyrosine supplementation. The horse should be able to produce its own and get adequate forage in order to do so however there is burn out in both humans and animals in which the stressful situation is too much or there simply is not enough precursor to continue to produce dopamine in those situations. 

By giving your horse a surplus of the precursor to dopamine it helps the horse deal with stressful workouts and training regimens.

Lysine is an essential amino acid. In animal tests chronic lysine deficiency increased stress induced anxiety. Lysine works synergistically with Arginine. Lysine has been tested and shown to block the stress inducing cortisol peptide

Lysine and Arginine supplementation has been documented to reduce the hormonal stress response in both humans and animals. The tests were performed by measuring salivary cortisol levels before and after exercise. Cortisol and the CRF peptide are considered the hormone of stress or the peptide of negative expectations. 

For your horse this works by blocking stress related hormones during a workout or long trail ride.

Magnesium supplementation has been tested in horses by studying the differences in flight response pre and post magnesium supplementation with magnesium winning. 

Betaine supplementation has been tested and shown to lower hormonal stress responses during exercise. Betaine has been tested shown to increase force production, work capacity, and recovery in human athletes. 

Vitamin b1 is a water soluble vitamin that helps convert macronutrients into glucose to be used for energy. It has also been shown to create a buffer to boost the body’s immune system. 

Ashwagandha has been used as an anti-anxiety supplement for horses. 

Rhodiola Rosea has been tested and the findings were that rhodiola rose had the potential to lessen perceived exertion. In a double blind bicycle race study rhodiola rosea actually led to faster race times and the bicyclists were able to exercise for an additional 24 seconds compared to the placebo group. 

This works for your horse if you are hunting, using horses for packing, or endurance riding.