Horses like humans lose Magnesium under stress. The more stress the more magnesium deficient a Horse can be. Stresses can include emotional stress, physical stress (racing, eventing, etc), chemical stress (toxins found on farms that may lead into food and water supply) and immune stress (infections and disease). The biggest problem is also how to supply and replenish Magnesium levels to optimal ranges. Normally in the environment, horses will get enough Magnesium from a variety of grasses as they roam and travel around. In Australia however the grasses are higher in sugars and lower in minerals that you normally get from mountainous regions in Asia or Europe. So the combination of stresses and low levels in food supply means Horses become easily Magnesium Deficient.