(This is an excerpt from my book, Equine Nutrition: From a Species Appropriate Perspective; please note this applies to both genders even though I use the masculine here for the biggest part.)
Senior status is variable among breeds and even individual horses within breeds, though most people consider a horse 20+/- to be senior. But that doesn’t mean the horse is “old” with regard to capability. There are plenty of 30+ year olds that can run circles around the human; and in fact I have one living with me! We should go by the signs of aging rather than the actual age. With regard to nutritional requirements, the senior horse has no physiological differences from the younger adult. A horse 15-20 years old may begin to lose some teeth; as well, the horse that is 25+ years may begin to show issues with masticating food properly and we may see signs of “quidding”. The evidence of quidding of forage will be boluses of food dropped from the horse’s mouth. The longer the horse has spent his life allowed a species appropriate diet, meaning free access to forage only, the less dental issues the horse will have going into advancing age, given no other health issues. But because of the horse’s hypsodont dentition, it is highly recommended that the senior horse continue to have at least an annual dental checkup and may need to be done twice yearly in some situations.
There is never any need to remove the older horse from pasture, even if he starts to quid the grass. He will still be getting some nutrients, although he will need to be supplemented; additionally the action of masticating is a biological requirement, just as it is with any horse. There will come a point that the older horse will not be able to masticate long-stem dry hay; at this time (preferably before he starts to lose condition) weaning him onto a chopped haylage product is an excellent way for the horse to obtain nutrients that he would not get otherwise. Soaked hay cubes are another excellent feed for the senior horse, although some owners are reluctant to feed soaked hay cubes because of the perceived time “cost”. I personally have done so and found that if I simply put the cubes into soak while doing barn chores, the “extra” time required is virtually nil. There is no reason to not feed both soaked hay cubes and chopped haylage; the haylage can be fed free choice and the soaked hay cubes can become a “meal”. In this way, between the pasture and the haylage, the horse is still allowed to trickle feed as desired and the forage “meal” should present no digestive issues; it is also an excellent opportunity to provide appropriate nutrient supplementation to assist the naturally decreasing digestive functions of the aging horse. As with any forage, the hay cubes should be of high quality. Typically one finds hay cubes in either timothy or alfalfa, or a mix of both. Which to feed will depend upon the individual horse and the availability in the local area. Some people will recommend soaked beet pulp over hay cubes. In a perfect world, I have no adversity to recommending beet pulp. However, the unavoidable fact is that most commercially grown sugar beets (the type of beet that is generally used for beet pulp; and no, beet pulp is not loaded with sugar) are GMO crops. Even if they are not genetically modified, the method for kill-down on the beet tops (the greens) in order to harvest the beets (the root of the plant) is to use herbicide spray. Exactly how much herbicide is going to be taken up by the root – the beet – is a variable factor that will depend upon the exact chemical compound used and its half-life. Finding organically grown beet pulp is practically impossible (if someone has a source, this author would be very interested in knowing it). This simply is not a risk I personally am willing to take. This is not to say that the hay grown for the hay cubes has not had herbicide applied to the field, although it is generally not applied as often to hay fields as it is to commercial beet crops. It may be quite possible to find a hay cube product that is at least somewhat sustainably grown, at least easier than finding organic beet pulp!