Magnesium in the equine diet

Magnesium is an important mineral in the horse, activating over 300 different biochemical reactions all necessary for the body to function properly.  Magnesium supplementation has been shown to improve performance and allow human athletes to reach exhaustion later in their exercise routine. It increases oxygen delivery to muscle tissue; it promotes muscle strength, endurance and relaxation. Magnesium also activates enzymes necessary for the metabolism of carbohydrates and amino acids which lead to protein synthesis.

 Magnesium is often the most neglected mineral in horse feeds. Calcium and magnesium work closely with each other, each requiring the other for balance.   Calcium is in charge of contracting the muscle and magnesium looks after the relaxation or release of the muscle, so when there is not enough magnesium in the cell, calcium can leak back in causing a stimulatory effect and the muscle cannot completely relax. This can put the body into a continually stressed state. Low magnesium makes nerve endings hypersensitive thus intensifying both pain and noise.  Magnesium is required for proper nerve and muscle function.

 Only around 1% of magnesium is stored in the blood; the rest is stored in soft tissue and bone and the body is very efficient at maintaining that level in the blood stream to facilitate organ function.  This is why blood level magnesium tests are rarely indicative of an animal’s true magnesium status. 

 Horses with magnesium deficiency may have all or only a few of these signs so it is important be aware of them.  They may be borderline and only exhibit signs during competition or stress.  For instance, horses with magnesium deficiency often have very sore tight backs in spite of excellent saddles and pad, proper fit, conditioning and training.  They don’t respond well to chiropractic adjustments and massages or these treatments don’t last more than a couple days and the tension and soreness return.  They often resent or even act afraid of being touched leading the owner to ask themselves, ‘Is someone abusing this horse when I am not around?’  Their response to outside stimuli is over reactive and they tend to become fractious, worried, fearful or resistant to training.

Magnesium is assimilated quickly in times of stress, such as travelling or heavy training.  Horses lose magnesium through sweat and urine.  Many performance and race horses can become deficient as the season progresses as they are using the available magnesium more rapidly due to stress, travel and competition. Horses with low magnesium status will often crave salt, which exacerbates the shortfall.

The daily magnesium requirement for maintenance has been estimated to be 13mg of elemental magnesium per kilogram of body weight.  There are 1000mg to a gram, so the total daily magnesium requirements of a 500kg horse would be 15.5 grams of magnesium for a non-working horse, but many horses require more than 15 g daily.  There are many factors that affect magnesium absorption and utilisation. Working horses require 10-30% more magnesium for light to moderate exercise, respectively, due to sweat losses.  Horses who sweat heavily will lose magnesium at a more rapid rate.

Magnesium should be split between morning and evening feedings to increase absorption and decrease its occasional laxative effects. Once a horse becomes low on magnesium, it is very difficult for them to catch up without supplementation. Magnesium toxicity is rare because excess is naturally excreted. 

 First scientific evidence to support the calming effect of magnesium

 The long-awaited proof that magnesium can potentially help calm horses has been found. New research, conducted by the WALTHAM® Equine Studies Group and Australian collaborators, has shown that magnesium can significantly slow reaction speed responses.

 Evolutionarily the horse is a prey animal and as such is a creature of flight. Today when the flight reaction is felt to be excessive some owners opt to use calming supplements, usually containing magnesium. However, to date there has been no published evidence to show that magnesium can have a calmative effect in horses.

 The study was conducted at Charles Sturt University in New South Wales, Australia by Jessica Dodd, as part of her PhD programme. It was supervised by Dr Glenys Noble in collaboration with the WALTHAM® Equine Studies Group, headed by Professor Pat Harris MA PhD, VetMB DipECVCN MRCVS. It investigated the effects of magnesium aspartate supplementation on the reaction speeds of six Standardbred geldings. The addition of 10g of magnesium to a roughage (clover/ryegrass hay) diet, which already provided the recommended daily intake of magnesium, reduced the average reaction speed response in the horses by more than a third. Without the supplement the mean response time was 5.3 metres per second and with the supplement it slowed to 3.1 metres per second.


 1 Jessica Dodd, Greg Doran, Patricia Harris, Glenys Noble (2015) Magnesium aspartate supplementation and reaction speed response in horses - to be presented at ESS, Florida in May 2015. 

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