"Consider the difference between valerian and chamomile. Both have calming effects. The difference is that valerian can actual alter a horse's state beyond what is normal while chamomile could only restore a horse to its normal state of calmness. Valerian can cause a horse that is normally not very calm to become calm or even sedated. On the other hand, giving a chamomile to a horse that is normally tense or high-strung would not have much of a sedating effect. Chamomile is effective in restoring a normally calm horse to its regular state under stressful conditions such as a show"
Upnover - I'm guessing that chamomile works the same as Tryptophan. It doesn't alter your horse's natural disposition like Valerian, it just soothes them a bit and helps the refocus.
I've spent most of my life riding horses. The rest I've just wasted.