Ask the Nutritionist: Dr. Getty's Forum for Equine Nutrition
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All the best,
Dr. Getty
Hi Billy,
Most horses do not get enough magnesium, producing borderline deficiencies that create behavioral issues. I'm delighted to hear how well your mare is doing now that she is no longer deficient in this very important mineral.
The calcium content should always exceed magnesium, but it doesn't need to be 2:1 Ca to Mg. You can safely feed as much magnesium as calcium (a 1:1 ratio).
If by adding 3 g of Mg per day, the calcium level in diet still exceeds magnesium's, then you're ok. (Sounds like your hay is very high in Mg -- usually grass hay has closer to a 3:1 Ca to Mg ratio).
I hope this answers your question. If you need to add more calcium to the diet, alfalfa is a good choice, rather than adding a mineral supplement.
All the best,
Dr. Getty 
Author of Feed Your Horse Like A Horse
We have a 5-year old mare who is great under saddle, well trained and never mistreated. However, she is (was) hard to catch and once caught would turn here head away and lean away from you, almost as if she couldn't stand anywhere near you. Once released, she couldn't wait to get away, sometimes bolting.
Somewhere during my nutrition courses I read where there is nothing more unhappy than a magnesium deficient mare. The article said you could either have a blood test done or simply give the mare extra magnesium and see if it works.
It worked great! Now she is the first to the barn seems to want to be around you, and hangs around once released, so I guess she was lacking.
A customer of mine had a similar mare. More mg didn't help that mare. I think because the problem was behavioral since the mare had a questionable history.
Our mare was getting her mg requirements daily because I have my hay tested and balance it. Also the 2:1 c:mg ration was right on.
I currently add 3g of mg daily to her diet.
My question is: since the c:mg ratio is now off because of the added mg, should more calcium be added to bring the ratio back in line since this might be a long-term solution to her problem.
Thank you.
Where are you from? Bayfield, CO