Testing for magnesium deficiency - how accurate?

Short story: How accurate are Magnesium tests when running a blood panel?

Long story: Moved mare to a new facility and the previously steady calm schoolie type turned into an hyper-alert, hyper-reactive firebreathing dragon. Treated her for ulcers, tested for Lyme (neg) and had the chiro out (who noted muscle tension).
(and it’s not the “Spring sillies” brought on from lush grass since we’re still under feet of snow…)
The main change (in addition to the move obviously) has been her going from a mainly alfalfa diet at the previous barn to free choice grass hay at the new barn.
We’re in an area with an acidic soil - which may negatively impact the availability of Mag in the soil (we’ll need to test our hay for sure as well).
When chatting to our cattle farmer neighbor, he mentioned that he had seen similar flighty/nervous behavior in his dairy herd - which was connected to Magnesium deficiency.
So running a blood panel on the mare sounds like a good idea but I’m reading conflicting information online about how accurate the test are. Anything else we could test for at the same time (Calcium?) to get a better picture?

Why not just get some magnesium and feed it? If it works, great.

What kind of hard feed, ration balancer, or vitamin mineral supplement is she on? Most horses do well on a fairly comprehensive geberal one. Most horses are not fed enough bagged feed to get their full VM needs, because of the calories.


Way easier to just feed it and see if it helps. If you feed too much her manure will get loose and you’ll know to back off.

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Agree that it’s worthwhile to just try adding magnesium.

If you do want to run bloodwork, throw in a vit e and selenium level. Vitamin e degrades so quickly in cut hay. Especially in a long winter, horses can get awfully low if not supplemented.

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Thanks! She’s an easy keeper - so no bagged feed for her. She’s on grass hay, soaked beet pulp and Mega-Mag pelleted multi-vit and mineral supplement.

Thanks - the chiro was indeed mentioning low Selenium levels in other horses in our area and that would make sense about Vit. E and long winters!

That is helpful to know - I always worry about “over-supplementing” without knowing if the horse needs it - but it’s good to know they wouldn’t pass the extra.

Selenium is the only one that’s really dangerous in overdose and also I think just about the only one that has a legitimate blood test.

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My understanding is that magnesium oxide, in particular amongst the magnesium forms, is not absorbed by the body if there is no deficiency. KER has a summary that supports this. For that reason my vet is also on the “just feed it and observe” bandwagon.

Vitamin E and selenium are worth blood testing before you supplement.

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I would echo just trying her on a good mag supplement and see if you notice a change in her behaviour.

My vet said, short of doing an invasive and painful muscle biopsy, there’s no effective way to test mag deficiency. So people who suspect it, just supplement.

Pls note that not all magnesium supplements are created equal. Magnesium malate is one of the most easily absorbed forms, and it’s not cheap. If it’s cheap, chances are it’s not good and won’t work.

After I started my mare (then filly) on MagRestore, it took less than two weeks for her reactivity/tension/flightiness to subside by about 50%. She is still a high-energy, high-strung horse in general, but it was to the point where you’d go into her stall while she was eating and she’d spook. Go over and touch her flank and she’d react like you poked her with an electric cattle prod. Sneeze while leading her and she’d be hanging from the rafters.
She is now back to being a normal, alert, slightly looky horse - the Magnesium did wonders for her. :blush:


I also agree that starting her on a high quality magnesium supplement will be the best way to determine if she is deficient. She will either benefit from it, or she won’t.

But also I wanted to mention that MVP Mega-Mag is designed to balance alfalfa and/or grain based diets. If she is only getting grass hay and beet pulp then you should consider switching to MVP Mega-Cell or another comprehensive V/M supplement or ration balancer that is made to complement grass forage diets.

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Thank you! That is what I had been reading as well.

That is where I am still a bit confused as I see quite a bit of confusing info on this. Then again, this doesn’t seem to reference Mag Malate: https://feedxl.com/36-which-form-of-magnesium-is-best/

Yes - typo! She was on Mega-Mag at her previous place (since on alfalfa hay there) and she’s be switched to Mega-Cell here.

another vote for MagRestore. They have a sample size. https://shop.performanceequinenutrition.com/horse-calming-supplements-c29.aspx

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FYI, most forms of minerals are more bioavailable if they end in -ate. Easy to remember, because they get ate!


Yes, thanks! Can I ask which studies support this?

By looking through journals at the local university library, all I can find in terms of studies of the bioavailability of different forms of magnesium in a monogastric animal (a rat) is the 2005 Coudray & al. study.
And they’re saying that they found no “major differences” in terms of intestinal absorption between the 10 forms of magnesium they tested; and that orgnanic and inorganic forms are equally efficient (for rats anyway).
They do seem to mention that magnesium gluconate has the highest magnesium absorption and retention rate but magnesium sulphate (Epsom salts) has the lowest absorption and retention rates.

I understand the palatability issue of Mag. oxide but I am unclear on the bioavailability argument - based on this study at least. Anyone has a more recent study I could look at?

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I don’t know about studies with magnesuim, but here is one article on bioavailability for sheep.

I haven’t come across a lot of studies for horses, as they don’t bring the money like other livestock.

I did find this article about magnesium absorption generally, and why it’s hard to test for. It’s from 2017.

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Not really relevant but kind of funny-- Recently I was given a partial bag of MagRestore so, just to be sure, went through the website’s questionnaire. Answered “never” to almost every question and was still advised that my horse definitely needs this supplement. :smile: Hmm!