When dealing with ulcers there should be two objectives, first to treat the ulcers so they will heal and second to change the horse’s program so the ulcers won’t reoccur.
You might want to do a google search on “how to treat ulcers in horses” or “how to prevent ulcers in horses”. You’ll find a lot of information that explains why horses get ulcers and how to prevent them.
Colic can be caused by many things, but I would certainly consider ulcers and be examining your mare’s diet. An important thing to remember is the digestive system of the horse is designed for the animal to be constantly foraging and the horse’s stomach is constantly producing acid 24/7 to aid with digestion. If the horse goes too long without something in his stomach the acid can build up and cause irritation and eventually ulcers. Ulcer-related symptoms can include going off feed, sensitive mid section, change in attitude, colic, etc.
A common and effective treatment for ulcers is omeprazole. This can purchased under the brand name Gastroguard. Ulcerguard is the exact same product, and is available without a prescription. To treat ulcers, the recommended course is one full tube per day for 28 days. A lot of people then taper to half a tube for a week and then a quarter of a tube for another week. GG and UG are not cheap and run about $30 a tube if purchased on line. So a full month of treatment will run $900, plus a little more if you opt for the taper. Given the expense of treating ulcers and the discomfort they can cause, it is in your interest to also adjust your horse’s program to prevent the ulcers from recurring.
Diet changes that can help prevent ulcers or reduce ulcer flare ups include access to forage 24/7 or as close to 24/7 as is reasonably possible. Hanging hay nets and ensuring the nets are never empty is one solution for stalled horses. Concentrates (commercial grain) can irritate ulcers so a diet that is light on concentrates and heavier on forage might be useful. This is probably why your vet is advising you to cut back on the senior feed. Alfalfa is thought to have some buffering effects and is commonly recommended as part of ulcer diet overhaul.
Beside the diet overhaul, you should seek to remove any stressors from your horse’s program and provide them with as much turnout as possible.