I think what a horse is being fed depends largely on what kind of work it's doing, how good (or not good for that matter) the grazing is that is available. There are also some special requirements if a horse is prone to laminitis, gains weight easily or has other health issues.
I help out with caring for 12 horses and a donkey. Some of them aren't ridden at all (retired/have health-problems that rule out ridden work), the others are ridden a couple of times a week.
Because of that most of them are fed hay and straw and have unlimited grazing over night and a mineral lick-it thingy. There is no need for grain or any extra food. Some of them could actually lose a couple of pounds
We have a little group that have had laminitis and are therefor kept on a sand paddock with hay only. Our "senior" (34 years old) is fed a mash-like senior-pellets mix with water several times a day, since his teeth are too worn down to eat hay properly.
Another horse has been severely neglected and almost starved to death before it came to us. It is also fed on hay and grass only and is slowly but steadily gaining weight on it's way to being a healthy horse again.
As purplefdu said said, feeding a ready-made grain/pellets mix does save time. Just make sure to get the right kind and amount for your horse. Most feed-suppliers (at least in the UK) have hotlins that you can call to get info about the different types they carry.
Also your vet might be able to help with picking out the right food.
Always remember though to introduce food changes slowly so you won't upset your horses digestive system.
Mefiant