If she foundered from being overweight and eating grain, then she's most likely to be a "metabolic" founder than a "mechanical" founder. Unfortunately this means that she may require a restricted diet for the rest of her life to reduce the risk of further episodes of laminitis and/or founder.
Most horses who have experienced metabolic founder will do best on little to no grain. Weight can be maintained via safer feeds like beet pulp, low sugar hays, and some feeds that are specifically engineered to be low in starch and carbohydrates. Grass may need to be limited as well. Each horse is an individual in terms of their tolerances for all of these food items and you will need to be particularly vigilant in terms of what causes her to become foot sore. As soon as you notice any indication of lameness, it will be important to link that with a specific "trigger" such as: she was out on grass longer, a change of hay, a change in weather/dry/wet conditions, a change in season (especially Fall/Winter). Any one of these may affect her -- or none may. However, in order to reduce her risk for pain and further complications of her condition, it's best to play it safe and use any food in moderation.
A good start would be NO sweet feed whatsoever, or food that is heavy in molasses. It is important to reduce sugars and starches in the diet. Many of the products that are being advertised as low starch and safe for metabolic horses will be pelletized in some form or another to reduce/eliminate grains (which are high in starch and carbohydrates). Do be aware, however, that not all of these "low starch", "safe" feeds are low ENOUGH in starch and sugars for every horse. While one horse may do well on a product, another laminitis horse may suffer miserably on that same feed.
Basically our idle, mature horses do not need any type of "feed", and will do well on an all hay diet (and hopefully some grass if available and tolerated!) along with a vitamin and mineral supplement. Beet pulp is a great substitute for the "feed in a bucket" scenario and is a good place to hide powdery or liquid vitamin/mineral supplements. Another option is a pelletized vitamin product that provides your horse's "seal of approval" clink in a bucket. If your founder horse is not acutely sensitive and sore/lame frequently, you may be able to get away with "over-the-counter" grass or grass/mix hay and a general vitamin/mineral supplement such as Farrier's Formula, Select I or Select II, Vita-Key antioxident, Accel, etc.. However, there are going to be those individual horses (worst case scenario) which need much more intense management than this to stay sound, including tight control of the diet requiring scientific balancing of forage, vitamins and minerals. Under those extreme circumstances your veterinarian could be of assistance and probably just as helpful would be the Yahoo Equine Cushings group which is a veterinarian-guided support group dealing exclusively in the management of equine metabolic disorders (including founder and laminitis). The group discussions can be a bit overwhelming at first; however, the group FILES section would be a great place to start in order to better educate yourself about how important diet is to your mare's health.
Equine Cushing's Group
From my personal perspective, I have a late pregnant mare who is prone to laminitis, especially from pasture grass. I have had good luck feeding her a ration balancer which contains all the important vitamins and minerals as well as being a good source of protein. Because ration balancers are highly concentrated, they generally only require 4-12 oz per day to provide all the daily nutrients. They are low in starch and sugar because they contain no grains. Basically they are just a vitamin-mineral supplement with protein added in as a bonus. I use Purina Born to Win at 32% protein because I also feed grass hay to this particular horse. Purina also makes a lower protein version -- Mare & Maintenance -- at 12% protein which could be fed alongside alfalfa or legume hay. Most of the major feed manufacturers have a ration balancer (I believe Nutrena may be one of the few that does not) and these are definitely worth looking into. But, as mentioned above, a metabolic horse's tolerance for starch and sugar will be highly individual. Some may be able to tolerate oats and not have a reaction, where even a handful of oats may be enough to tilt the scale for a sensitive horse. It will take a little bit of experimentation on your part to determine how sensitive your mare is for feeds, hay and grazing. Just remember -- at the FIRST SIGN of soreness, withdraw anything that may be a trigger and be prepared to nail down the culprit or start over at square one!
Robin C
Looks like Liz and I were typing at the same time
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Robin C