by Dr Derek Cuddeford, Royal (Dick) School of Veterinary Studies, University of Edinburgh
Without magnesium (Mg) we and our horses would fall apart because it makes up about 0.05% of body mass which is equivalent to 0.25kg in a 500kg horse. Sixty per cent is located in the skeleton (0.15kg) and without this mineral the bones would collapse. Around 30% is in the muscle and the rest is present in enzymes, blood, etc. Apart from being crucially important for skeletal integrity it is involved in many biochemical processes within the body. The absorption of this mineral from horse feeds has been measured to be in the range of 40 to 60% and the lower figure is used to estimate requirements so it is likely there will be an oversupply.
The daily dietary requirement is considered to be 15mg Mg/kg BW; equivalent to 7.5g/day for a 500kg horse at maintenance. A magnesium:energy ratio at maintenance (110mgMg/MJ digestible energy) is used to determine work magnesium requirements so that a horse in hard work consuming 140MJ would require 15.4g magnesium daily. It will be appreciated that racehorses of different body weight (say 450 to 525kg) will probably be fed the same amount of dietary energy and thus would receive the same amount of magnesium daily based on the above. This would seem somewhat anachronistic in view of the fact that requirement is supposedly related to body weight! It should also be noted that there are differences in opinion over the magnesium requirement for horses in work.
It has been estimated for growing horses that 0.85 to 1.25g of magnesium is required per kg weight gain per day (in addition to maintenance needs) so that a 200kg foal gaining 1kg/day would need ~6g of magnesium daily. Milk magnesium levels are low, averaging 90mg Mg/kg milk in early lactation and falling to half this value in late lactation. In view of the fact that a mare in early lactation will be consuming ~2xmaintenance energy levels, background levels of magnesium in the dietary components used should ensure dietary adequacy.
Thus, based on the foregoing, it is clear that magnesium is an essential nutrient for horses and, taking the example of a 500kg animal, the requirement will never exceed 15g/day irrespective of physiological status. The magnesium content of forages varies and is mostly between 0.2 and 0.3%DM so our 500kg horse fed these forages would receive 12.5kg dry matter supplying between 25 and 37g magnesium daily which would be way in excess of need! Magnesium deficiency is extremely rare in the horse because plant sources are excellent suppliers of magnesium. Furthermore, magnesium toxicity is virtually unknown in the horse apart from that which may arise through the incorrect use of magnesium sulphate in cases of impaction colic. Thus, one must then ask the question why would a horse require supplementary magnesium and what possible benefit could it be to the animal?
Head-shaking can be a serious problem for some horses. Professor John Madigan from the University of California, Davis recommends feeding 4 ounces of a commercial supplement (a calmer) that provides 20g magnesium as part of a therapeutic protocol for head shakers to raise the threshold for “firing” of the trigeminal nerve. The syndrome involves abnormal “firing” of this nerve. He uses this product in conjunction with melatonin and a homeopathic product. Apparently it can take 4 to 8 weeks before any improvement is noted. The product used by Professor Madigan was primarily a magnesium supplement that also claims to do several other things. Essentially it is meant for nervous/tense horses that have difficulty relaxing based on the fact that the mineral is involved in both nerve and muscle function. The rationale behind this is what is known about the effects of severe magnesium deficiency evidenced by general nervousness, excitability, muscle tremors, and convulsions but no normal horse diet is magnesium deficient! The product is also believed to play a role in horses with Equine Metabolic Syndrome and insulin resistance. Chronically overweight horses and those prone to laminitis are said to benefit since it is considered that the mineral helps to maintain horses at an appropriate weight. Finally, the product is also supposed to provide relief to animals with sore muscles. However, I fail to understand how the provision of ~2x the basic maintenance magnesium requirement on top of a diet already adequate in magnesium can achieve these claimed functions. There is no scientific evidence to support a role for magnesium in treating insulin-resistant horses nor can it have any role in regulating body weight which as we all know is controlled by the amount of food energy consumed!
A study reported in the Equine Veterinary Journal in 2013 which was conducted at the University of Liverpool Veterinary School and funded by a supplement company assessed the efficacy of an unnamed feed supplement in alleviating the clinical signs of headshaking in 32 horses. The results showed that the supplement offered no benefit over a placebo in reducing the clinical signs of headshaking. What is particularly interesting is that the magnesium (quantity undisclosed) was a component of the supplement and that there was a proxy placebo effect (owner’s beliefs based on knowing animal is receiving a treatment) when based on subjective owner perception of clinical signs. This means that owners reported a significant improvement during all activities for both placebo and supplement compared with pretreatment scores. This shows that reliance on anecdotal evidence (owner’s views) is flawed. The significant placebo effect measured, stresses the need for properly conducted, randomized controlled trials, with blinding to assess true treatment effects. When horse owners “invest” in a treatment they expect a positive result and thus “see” an improvement albeit that objective measures do not always support such a conclusion.
Very little is known about the potential benefits (and risks come to that!) of supplements marketed as calming agents. There have been very few scientific studies and the reported benefits are mostly anecdotal. These products usually contain several ingredients and one of the most common apart from magnesium is thiamine (B1), a member of the B complex of vitamins. The rationale for its inclusion is similar to that of magnesium in that B1 deficiency is associated with brain dysfunction (convulsions) and thus people seem to think that large doses might reduce excitability and anxiety in horses. Fortunately large doses of B1 are not harmful to the animal although they may have a negative effect on the bank balance.
The use of calming supplements irrespective of what they contain appeals to horse owners who want a quick fix for an excitable/difficult to manage horse. However, I would propose that owners should re-evaluate their feeding programme and management of their horse. Our removal of horses from a natural diet and normal way of life are causal of the behaviours that we often witness. We regard the resultant activities as aberrant but in fact they are the responses of the animal to the situation we place them in. Most obviously horses should be allowed to consume fibre-rich feed ad libitum in an environment where they can interact with other horses and exercise freely. Reducing quantities of starch fed and increasing fat intake may contribute to a more contented horse. Furthermore, your horse may be hot simply as a result of overfeeding, under work and isolation that no calmer can possibly reverse.
Finally, Dr Kathleen Crandell of Kentucky Equine Research has stated that “People are feeding magnesium in therapeutic doses to calm a horse, and some say it reduces the thick, cresty neck and the risk of foundering in insulin-resistant horses but so far, there are no scientific studies supporting these claims, and results are largely anecdotal.” I agree! Fortunately horses seem to have a high tolerance to excessive levels of dietary magnesium. It is indisputable that magnesium is a vital nutrient for all horses and ponies but, there is no evidence to show that supplementing magnesium beyond requirement has any measurable benefit.