All of the following information falls under the category, of 'good for all horses', however, dressage riders will find that certain things require more regular and detailed attention!
Every horse needs a balanced diet comprised of mostly forage, clean water that he should have access to at all times, adequate minerals, and supplements as needed given his level of work and the types of forage he receives. Not all hay is created equally, and so its always advisable to talk with ones veterinarian about what your particular horse may need.
Dressage horses, can have anywhere from a light workload (lower level horses, or mainly pleasure dressage horses) to a heavy workload as in and FEI horse with a rigorous competition schedule. Hay alone often does not have the calorie make-up to provide enough energy for horses in a higher level of work. Consultations with your vet or an equine nutritionist can help you decide how much "grain" or sweet feed supplementation your horse may need.
Minerals and vitamins may also come into play, but again care needs to be taken before running out and buying any topical feed dressing as commercially prepared 'sweet feeds' often have a mineral/vitamin component and you have to be careful not to overdose the horse or create imbalances.
Primarily in the form of joint supplements, these popular items have some scientific backing as to their efficiency or lack thereof. When looking for a joint supplement, its best to assess any current joint issues your horse may have or whether your goal is a "legacy" program attempting to stave off degenerative conditions in a younger sound animal.
Microbials in the forms of Pro and Pre- Biotics can also have benefit for horses whose digestive systems may be compromised or stressed by travel, stabling conditions, or temperment.
Not only does tooth care optimise a horse's ability to utilize the food stuffs that he is ingesting, for a dressage horse who is ridden with contact on the reins and must respond to subtle and delicate signals given by the rider through the reins and to the bit, its critically important that his teeth be checked for sharp points which would cause pain, waves or ramps in the alignment of his upper and lower teeth that affect bite and jaw mobility, and other conditions that would affect the horse's over all comfort in the mouth while carrying the bit or bits.
Young horses beginning their riding lives, should be prepared to carry the bit by having the veterinarian check that there are no abnormalities in the teeth or bite, and he can also file a "bit seat" in the first molars that somewhat smooths or rounds them off to allow the bit to be unimpeeded in its motion in the mouth.
Dressage horses should move soundly on all four limbs with freedom and regularity. All horses should do so, but animals with impurities can function soundly while remaining suitable for other activities. Dressage horses cannot have any impurity to the rhythm or footfall of the gait. Imbalances in the hoof, structural deformations, and improper trimming/shoeing can all cause problems. A skilled farrier cannot work wonders, but they can and should be able to trim and shoe a horse without creating imbalances. Some horses are able to go without shoes, but the additional weight of a rider (who is often not perfectly balanced even when working to do so!) on abrasive surfaces such as sand arenas, will wear down a hoof unevenly due to being ridden off balance (happens to the best of them to some degree) or conformational flaws that cause the horse to wear the foot in an uneven way. Therefore, its almost always necessary to shoe a ridden horse in order to protect the balanced trim. This also keeps the bony column of the limb in correct alignment so that it does not create unnecessary torque on the joints contributing to degeneration.
Shoeing and trimming should happen with regularity. Generally, every 5-8 weeks, although 8 weeks is a long time for most horses. The old adage "No hoof, No Horse" is very true. You are riding on his four feet. They need scrupulous attention!
Chiropractic and Massage
Dressage horses are athletes. Even in the most conscientious training programs, they will become sore. Sore muscles can lead to compensetory movements that create misalignments. Very serious riders who wish to give their horses every advantage and ensure the animals well-being often employ body workers for their horses. These professionals should be licensed and have appropriate credentials. Get references and even watch a person work first! I once was given a recommendation to use a woman for massage who was not comfortable around the horses. She insisted they be sedated, but in their sedated state the horses were not as reactive to her harsh hands making it impossible for her to recognize that she was actually causing bruising to the horses.
Proper Fit of Tack
Just as someone would not ask you to run a marathon in the wrong sized shoe or an ill-fitting one... say high arch when you don't need it etc. you would not ask your horse to carry the weight of a rider in a saddle that is too small, too wide, or not shaped adequately to his back. There are literally hundreds of saddles to choose from with every sort of inovation you can imagine. Simple and effective rules still apply to finding a properly fitted saddle. Unless your horse is extremely rare in its conformation, its best to stay away from fringe fads. Find a saddle fitter who is NOT a rep or dealer of one company. Get an open minded opinion from someone who at least deals in several brands of saddle or best yet, no particular brand! Ride in it several times before you purchase it. If you cannot ride in it, look elsewhere. Most companies recognize that you have to actually ride in it and now offer that possibility. If a company won't allow it, there are other saddles on the racks.
Likewise, bridle and bit should fit well too. Do check the USDF rulebook for what is legal in bits- keeping in mind that the mildest you can possibly find that fits the shape of your horse's mouth is the best place to start.
The Best Rider and Trainer Money Can Buy
Your horse needs to be ridden well- by you and your trainer. There is no substitute for direct instruction with a knowledgeable professional. Audit clinics, watch at shows, interview potential instructors by watching them ride several horses, give lessons and see who fits your style. They do not need to ride Grand Prix with a super fancy horse. If you are trying to make it to second level, then your instructor should be successful at that level and at least one level higher. Likewise, if your horse is less than traditional with very average gaits and you have no intention of getting a different horse, make sure to find an instructor that is both willing to work with what you have AND has experience in that department. Even if they accept your horse, if the instructor keeps trying to make your horse move like the latest greatest warmbloods from Europe, you and mostly your horse won't be happy.
And don't skimp on the lessons or the training. Skimp somewhere else. Don't go to as many shows, or stick with schooling shows.... or don't show at all for a while.... don't get the extra fancy bridle with the bling, or the matching pad and polos. Invest in putting your horse into training for as long as you possibly can. Its an investment in your future happiness and that of your horse. If its FUN... you will be able to find the FUNDS!