Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
Welcome to my forum.
Here you will find more than 6 years of questions and my answers. It is searchable and offers a great deal of information.
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All the best,
Dr. Getty
Hi Jann,
Since we don't know exactly how much magnesium is in the pasture, it is safe to assume that it is, on average, .20% Dry Matter, which provides approximately 10 grams of Magnesium if her total grass and hay intake is approximately 14 lbs of dry matter.
So, to answer your question, give her 2 scoops of Quiessence per day (this will provide 10 grams of magnesium and the forage, another 10).
This is going on averages, and the only way to know if your hay has this much magnesium is to have it tested.
All the best,
Dr. Getty 
Where are you from? Bayfield, CO