Background information

Magnesium along with calcium and phosphorus are known as the major minerals. Copper, zinc, iron, manganese, selenium and iodine are trace minerals. In a horse’s nutrient intake, either magnesium can be at too low a level compared to the NRC (National Research Council) daily recommendations AND/OR the calcium to magnesium ratio can be too high. Whichever one applies, the diet won’t be optimal.

Magnesium is involved in so many body processes it is difficult to think of anything that doesn't involve magnesium. For an excellent review, see: Linus Pauling Institute, Oregon State University http://lpi.oregonstate.edu/infocenter/minerals/magnesium/
Some examples of studies

Magnesium and the oxidative stress http://www.ncbi.nlm.nih.gov/pubmed/23074840

Magnesium and the inflammatory response: potential pathophysiological implications in the management of patients with aneurysmal subarachnoid hemorrhage? http://www.ncbi.nlm.nih.gov/pubmed/22966500

http://www.ncbi.nlm.nih.gov/pubmed/22907037

http://www.ncbi.nlm.nih.gov/pubmed/22379366

http://www.ncbi.nlm.nih.gov/pubmed/21677120

http://www.ncbi.nlm.nih.gov/pubmed/21199787

http://www.ncbi.nlm.nih.gov/pubmed/20536778

http://www.ncbi.nlm.nih.gov/pubmed/20513641
Comparing magnesium mineral sources

Magnesium oxide (MgO) vs Magnesium chloride MgCl2 – When it comes to magnesium in horses, there is no authoritative study that has been done comparing magnesium absorption for all magnesium sources. Dr Eleanor Kellon VMD recommends either the best quality (lowest contaminants/iron) magnesium oxide or magnesium carbonate (MgCO3). In the USA, magnesium carbonate is the better as their magnesium oxide can be high in iron whereas we are a lot luckier here in Australia, our MgO is a lot purer. Another source is magnesium chloride which is a comparatively poor source of magnesium if measured by %. A high quality MgO is 57% Mg, magnesium carbonatte is 25% Mg and magnesium chloride is about 12% Mg. There is no evidence in horses that it is better assimilated than other sources.

DD Harrington and JJ Walsh published a study that documented 70% absorption for magnesium oxide in the horse, same for magnesium carbonate and magnesium sulfate. Equine magnesium supplements: Evaluation of magnesium oxide, magnesium sulfate and magnesium carbonate in foals fed purified diets. Equine veterinary journal 12(1): 32-33. http://onlinelibrary.wiley.com/doi/10.1111/j.2042-3306.1980.tb02298.x/abstract

Unfortunately the study is not available free online. Quote from study: "Results of all parameters evaluated indicate that MgO, MgSO4 and MgCO3 are essentially equivalent as supplementary dietary sources of Mg for growing foals. Mean apparent Mg absorption was approximately 70 per cent for all supplements compared to 40 to 60 per cent for Mg in natural feeds fed to mature ponies (Hintz and Schryver 1972)".

Note: Best not to use magnesium sulfate (MgSO4 – Epsom salts) as the sulfate can give horses diarrhoea.

In rats, it was found to be about the same when comparing MgO and MgCl2. This factsheet has a description of the study: http://www.ahuroa-feed.co.nz/uploads/8/8/3/4/8834018/__newsletter_36_-_which_form_of_magnesium_is_best.pdf