Horses require 2% - 2.5% of their body weight as dry matter intake per day - 2.5kg per 100kg of body weight, with an absolute minimum of 1% of body weight as forage.
Because ulcer issues are becoming so common, you should aim for as much of that 2.5% as possible to be forage. This can be helped by using a balancer, such as Equilibra 500, alongside your chosen forage.
Horses naturally graze for around 17 hours per day* so for optimum health we need to mimic these patterns as much as we can. The average 500kg horse should be consuming around 12.5kg of dry matter per day.
1kg of dry matter forage would be approximately 1.22kg of hay, 2.5kg of haylage and 4kg of grass (from grazing, not a bag). So a 500kg horse would need to eat, per day, roughly: 15.25kg of hay 31.25kg of haylage 50kg of grass*
*The average grazing rate of grass is 0.5-0.6kg dry matter per hour, assuming sufficient growth is present. Split the total forage amount into portions spaced equally throughout the day/night, leaving no longer than 5 hours without forage to allow your horse the optimum feeding and grazing time. The less time your horse is without forage the better, and a horse with or recovering from ulcers should always have a source of forage available. If like most of us you have a busy schedule, multiple haynets can be a convenient way to spread the feeds as needed (or a haynet inside a haynet for the greedy ones!). If the overnight haynets (for those stabled) are empty, you need to provide more.