Just wondering what the different opinions were and what brands seemed most effective etc.
*silly question alert* Do you feed the supplement all the time or only when they're in season?
I have a LOT of mares - both those in work and brood mares/retired mares in the field. Apart from an occasional 'silly season', NONE have needed anything different because they're mares. There are a lot of 'problems' I would want ruled out before wasting money on supplements other than the normal mineral licks that both sexes get. I would start with a uterine scan by a vet who does a LOT of this sort of scanning - preferably at the about to come into season stage. If you can't spot when she's coming into season (or going out) much more reason for a scan.
Unless behavior was extreme/dangerous I wouldn't want to supplement really. Mares are entire, and subject to hormonal fluctuations. It's what makes them mares and wonderful
Mind you, I'm saying this because my mare is the most unshow-y mare ever, I can never tell when she's in season.
There is another mare on the yard who is a grump 365 days a year, the level of grumpiness just increases and decreased. She would be an ideal candidate for an ovarian scan, and it wouldn't surprise me if she had over developed/perpetual follicles.