Arando, I moved your thread from "Horse Talk" to "Nutrition" as you may get more answers there.
Grazing may or may not be nutritious depending on grass.
On nice rich pasture surely they have enough not just keep, but in fact gain weight, sometime a lot (so you have to be careful). On other hand you may have a dry pasture, and in this case you may want to supplement with the hay.
Both my horses are off-work for the moment. They get 1 lbs of pellets twice/day (about equal to your 1 kg), hay, and I let them out to the pasture for couple hours (unless we have a rain like today). I know many barns keep horses on pastures during the summer (no hay, or just to supplement if they are partially stabled), and they do just great.
How does your horse look? Overweight, skinny, or normal? Could you post some pics may be?