Some horse are low in magnesium - so supplements like Be Calm contain magnesium to help them re-balance.
About 1.5 years ago my mare started getting worse & worse about riding. We thought it was behavioral not physical as she gets regular chiropractic, massage, dentistry and saddle adjustments (as in every 4 months or so for saddle and Chiro, twice a year for dentistry, and massage more often as she needs it).
Finally we figured out it was physical. Vet did U/S - everything in her female organs looked good - just VERY fertile and VERY LARGE follicles - so she is in pain and unable to "bend" her body before the follicles dissapate. She also started failing to pay attention and her muscles got hard as a rock.
Hops has cured the lack of attention, Raspberry leaves (prime ingredient in Mare Magic just less expensive - order at Herbalal.com http://www.herbalcom.com/?source=overture - helped a bit.
Next month will be 3 months for the hops so she'll come off the hops at that time, I have moved from Raspberry leaves to Chasteberry leaves - just started so not sure if it'll work. Mare gets rock hard in her muscles, nervous (diaherra) and frantic when her buddy is not there - all symptoms of her issues.
Beware that SOME herbs are illegal in recognized competitions - I think Vervain and Valerian are illegal - at least one definitely is illegal, just not sure which one...but read that 3 months should fix the imbalance - so time the dosage to complete before she starts competing.
Also - be careful as you can overdose on some herbs (not Hops or Raspberry or Chasteberry leaves) and many places on line will not give dosage. With the hops I started on a very low dose once a day and within 2 days saw a small improvement, so I doublesd the dose. Saw more improvement with that so I gave same dose twice a day and have held at that dosage since it has resolved the issue (that issue - she has others).