Is there a reason why you are feeding Suregrow to ridden horses? From memory, it is meant for growing youngsters, so will be the wrong balance of protein/carbs/amino acids for established horses.
I would stop feeding it and put them on a regular balancer it's probably just too much for them, as it's meant for youngsters that are growing really that need a bit extra of everything.
All those threads are years old the ingredients have probably really changed and got more advanced over the years, my horses go nuts on most balancer type feeds hence why I would feed a powder form with no added extras to affect them.
I feed it to youngsters that are looking well but need the nutrients, from 2-3 years old then put them on something more for adult horses. However, there are plenty of people that feed it to all ages as it's a cost effective alternative to a balancer aimed towards adult horses.
Maybe your horses were lacking in certain nutrients that the Suregrow is packed full of, and they are suddenly feeling particularly great on it!
Spillers lite and lean balancer is a good alternative maybe?.
Agree with pink… personally I wouldn’t feed over 4.5/5 with it. By that point your horse will still be growing, yea, but will be working too and the need for feed will have changed
Its something I used to feed years ago, but things have moved on and there is much better things available now. Spillers lite is the best of the commercial pelleted balancers
I used to feed it when I had youngsters and fatties. I think it used to be you fed adults 30% of the same weight for young stock. Unless they have some sort of intolerance I can not imagine the amount given, I think I used to give half a cup, I doubt if there is enough DE/kg to cause a problem.