All great replies
Much depends on:
1. If the horse has healthy issues and if so what they are.
2. How much pasture and the quality of that pasture.
3. If the horse is in/out or in most of the day, or out 24/7.
4. What feeds are available in your immediate area.
5. How hard the horse is worked on a weekly basis. While "hard" is by whose definition, hard to me means a performance working horse, such as a reiner, endurance, ramping up for the show circuit, etc.
I have a 20 yo TWH that was diagnosed with EMS last spring (Equine Metabolic Syndrome).
I have three other horses. In terms of feed only one is normal. I am currently in the process of switching everyone over to the feed diet the EMS horse is on because it won't hurt anyone. Supplements won't quite be the same, but at least I will only be feeding out of one feed bag and one vitamin/mineral bag - lol lol
Everyone will soon be eating EquiJewel pelleted rice bran. It is high in calcium and is 20% healthy fat. They will also all be getting Burkmann's vitamin/mineral supplement as opposed to Purina's "Born-to-Win".
The BtW is 32% protein and I can see just a tch of difference in the horse that gets it to tell me he is soon going to be eating Burkmann's because it is 25% protein. He is a very compliant, laid back horse, but I have noticed a teentsy bit of sass since he's been on the BtW - lol lol
Three are getting Omega-3 horseshine. The EMS horse gets BOSS instead, because his Royal Highness has gotten pickey over the winter and no longer likes water in his food (to help keep everything together).
No one, including the EMS horse, gets much grain. They each average 8oz of feed 2X/day, plus 8 oz of vitamin/mineral supplement 2X/day.
This time of year they all go out on 14 acres by 7:00AM and come in between 6:00 PM & 7:00PM depending on the weather.
They are all on quality grass hay, which they get morning and night. The chemistry of the hay can also play a big part as to what grain you decide to feed your horse. We had poor hay this year, but it turns out that was the better thing where my EMS horse is concerned. It still smells like tea and I would think about still put ranch dressing on it and eating it myself, but I can tell the quality isn't what it has been in the past from the same man.
This is the time of year I start using a lot of hay because there isn't much pasture. This year my pasture is REALLY thread bare because my area of Middle Tennessee finished the year in an approximately 15" inch water deficit:(
Again, much depends on the horse and it's environment, and what quality feeds/supplements are available close by. Hope this helps