Ditto what desert said.
Oats are very high in NSC (50%+) and have very little as far as nutrition.
Is the mare an easy keeper? If so, then a RB (ration balancer) would probably work for her. RB's are highly concentrated and made for horses who are easy-med keepers who maintain weight on limited calories. I use Buckeye Gro N Win RB. Triple Crown 30% is a great RB and so is the Glo line by ADM. All three of these brands have fixed formula and not low cost formulas. This means that every bag is the same no matter what and they do not replace ingredients with whatever is the cheapest at the time like brands such as Nutrena and Purina.
If she needs a few more calories to hold weight while on the RB, you can add extra calories in the form of alfalfa pellets, rice bran pellets, beet pulp, etc.
A doet low in NSC (starch+sugar) is also important. High NSC diets have been proven to cause issues when used long term. Issues such as ulcers, hindgust acidosis, laminitis, founder, IR, obesity, poor hoof health, etc. When choosing a feed, I choose one that has an NSC level lower than 15%. Whole grains such as oats, corn, barley, etc are very high in NSC. Read the label. Stay away from feeds with these ingredients. Distilled grains are ok as the distilling process cooks out more of the starch and sugar.
If she is a med-hard keeper and it's holding weight on a RB, then a different feed such as Triple Crown Lite, Triple Crown Sr or Buckeye Safe N Easy may be more cost efficient for you.
Follow the feeding instructions for your horses age/weight. This is very important. The directions are there for a reason.
Hope this helped.
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