I think it depends on the calmer, some need a loading amount to start with. I have found most of them to be rubbish tbh however the 2 that I have used recently that have seemed to take the edge off are 'placid' I think it is D&H comes in herb form but I buy the liquid form, and also the blue chip karmer. I don't think anything will make a nutty horse sane but I think that some help take the edge off. Also I useded Steady Up by Feedmark in the past with a sharp horse and that worked a treat. Good luck!!
I use placid as my horse used to be a nutter. She still has spooky moments but nothing like she used to be. Its roughly £12 and lasts 40 days and i think well worth it
well I had been having a few issues like that and I also feel for you also having a big chap to ride when they are being like that. since using the blue chip karma it seems to have taken the edge off and we went for a lovely hack on Saturday after not riding since the monday and he was nannying (sp) the 14 year old clydesdale with us and calmed him down when the clydie bolted!!
I've used Equifeast's Cool, Calm and Collected for years and found that it takes about a month for it to fully load, but if you follow the instructions (feed about one hour before 'traumatic' event) you will see a difference immediately (within reason, the instructions are very accurate about what you can expect).
Magnesium calmers will only work if your horse is magnesium deficient. If not, you need to try something that doesnt have magnesium as the main ingredient.
I have tried everything with my anglo when bringing him back into work and the only thing that worked for me is Global Herbs Thoroughbred calmer. It takes about 2 weeks to load. Others have used this on hot blooded types (TBs and arabs) and found it to work as well. It doesnt turn them into zombies it just takes the edge off and makes them think before being stupid. I have also heard good things about Coll, Calm and Collected - it was next on the list!
I have used cool, calm and collected and it is brilliant, I have a very stressy mare and it totally changed her. I have never found a herbal calmer that has worked for any of my horses. I have used it a few times on other horses before a stressfull situation, ie vet or competition as it will work for up to an hour after approx 45 mins of feeding.