Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
Welcome to my forum.
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All the best,
Dr. Getty
Hi JS,
Yes, you can certainly feed Glanzen in place of Nutra Flax and Accel. And, regarding the selenium issue, 3 pounds of Purina Ultium provides less than 1 mg of selenium. So, since you are in a Se-deficient area of the country, I would recommend the Glanzen-SE. Give your 4 year old three scoops per day and your 7 year old, 2 scoops per day. This will meet their vitamin/mineral need, as well as provide the essential fatty acids in their diet.
And, one other suggestion -- cut back on the Ultium for your 7 year old -- she doesn't a pound of Ultium each day since she holds her weight so well. 
Instead, just give her a handful with her hay stretcher, along with access to hay and/or pasture 24/7. And, I would also discontinue feeding the sunflower seeds. They add too many omega 6s to her diet which are not only more calories than she needs (since you'll be feeding Glanzen) but the omega 6s increase inflammation. You might also consider reducing them for your younger mare.
Hope this is helpful! Keep up the fine work!
Dr. Getty 
Where are you from? Bayfield, CO
Hi JS,
Since the sunflower seeds are so high in fat, they are not advisable for a horse that needs to lose a bit of weight.
For normal weight horses, they can be part of a nutritious diet, but in moderation since too many will provide excessive amounts of omega 6 fatty acids.
So, your 7 year old would be better off without them.
Dr. Getty
Where are you from? Bayfield, CO