Dressage is a demanding discipline for both human and horse. Just like what the human athlete eats is important, so is feeding your dressage horse. If your equine athlete has too much energy, they may have a loss of control and concentration. However, if they have too little power they may not have the oomph to give an award giving performance. Add into all the fun that every horse is an individual, and you're on the first steps of a customised diet for your horse.
The foundation of any horse's diet is fibre. Essential fibre can come from such sources as hay or grass. With naturally relaxed horses it may be tempting to cut back on the fibre and to give concentrated feed in larger doses, this is really bad for your horse. Horses evolved around turning grass into fast and we want to continue the tradition. Fibre is essential to your horse's digestive needs, especially when it comes to performance equines. Low fibre diets can lead to health issues such as ulcers and other performance impediments.
There's plenty of helpful calculators to figure out how much hay you should be feeding your dressage horse, but it's typically between 4.5 kg and 6.8 kg. (15 – 20 lbs) Once you have that in hand, you can move on to how much and what type of grain to feed. As always, you know your horse and their personality best. If your horse is a hard worker but has a habit of being excitable after a high energy feeding, you might want to try something that's high in fibre and oil content to help provide the levels of energy they'll need without the potential behavioural issues. Cereal feeds are full of starch, and you shouldn't overfeed, or you'll run into hindgut problems, but they provide sudden bursts of energy. Mixing the two gives a great combination of the upfront pop with a longer lasting stamina. It's yet another one of those experiment until you find out what's right for you and your horse things.
All horses need vitamins and minerals to keep operating at peak efficiency, and a dressage horse is certainly no different. Between forage and feed, your horse should have all the bases covered, but it's a good idea to keep track of how much they're getting and be prepared to offer supplements if necessary. And, of course, your horse will need lots of cool, fresh water.
As your horse's workload changes, as will the food they will need. When in doubt, talk to your vet!