You're correct in thinking that your mare is not receiving all of the nutrients she requires from the lite feed you've been giving her. In order for that to happen, you'd have to feed the minimal amount listed on the feeding directions. Though I cannot be sure, the feeding rate is probably 4 or 5 pounds daily. In feeding that much, you're upping the calorie-density of her diet, exactly what you'd like to avoid.

Kentucky Equine Research (KER) developed a supplement called I.R. Pellet that was specifically designed for situations like yours. Your mare would receive 60 g of I.R. Pellet each day and that would provide all the vitamins and minerals she needs without the additional calories that come from using a feed. I.R. Pellet is packaged in a 4.5-kg bucket and lasts 75 days. It works out to be about $1.00/day.

If you're looking for a slightly less expensive option, you can switch her to a well-formulated balancer pellet that would provide her with essential nutrients at a feeding rate of about 2 pounds per day. One such product is Stamm 30, manufactured by KER Team Member Hallway Feeds. Of course, this will increase her caloric intake.