I think, as with many things horsie, there are a lot of different variables. Things shoiuld be looked at with flexibility depending on the horses body weight, temperment, the weather they live in, age, thier daily exercise and type of feed/hay they are fed that is in their area. Each horse is an individual, even within those same limitations.
You can take all of the good advice given here that works for each individual horse, and apply it to your own horse's needs and go from there. Trial and error with the best knowledge possible. Good for you for noticing how your horse is fed and trying to figure out the best possible solutions.