Feeding Magnesium to Horses – Why, What, How


Why Feed Your Horse Magnesium?

It is an important micromineral that is utilised by a horse to protect cells from inflammation and free radical damage. Magnesium has a role in nerve triggers and muscle contraction.

In horses, severe magnesium deficiency is rare. However, there are times and diets when the levels of magnesium within the physiology of the horse are low. This seems to show up as a horse being more nervous, easily excited sometimes hyper-active.

With a horse, whose main feed intake is pasture, the levels can drop in the springtime as new grass will be lower in magnesium. Often though the behaviour issues are resulting from higher levels of sugar in this pasture. While that can be true, (and it is a problem of digestion) it may be compounded by less available magnesium.

Bringing the levels up to the norm is easily done with a magnesium supplement. A horse needs 10g of magnesium a day, most is provided in pasture, premix feeds and hay. So ideally a top up of 5gms of actual magnesium is ideal in a supplement.

However, overfeeding a supplement is never beneficial and in this case the excess will be immediately excreted, which is an expensive waste of money. Excessive feeding may result in decreased calcium and phosphorous uptake, compromised intestinal integrity, even real difficulties.

Recent studies have been showing that supplementing magnesium may be beneficial for overweight horses, demonstrated by the visual reduction of cresty necks. At this stage the results are mainly anecdotal that it assists horses with insulin resistance and EMS.

Which Magnesium is Best for Horses?

Magnesium, by itself, cannot be fed – it has to be in a compound. There are many compounds available. Once ingested the magnesium separates and it is important to understand just how much magnesium is released from the compound. It is called the bioavailability, not all products show the actual magnesium released on the label, so it will pay to ask the supplier.  

Compound                                            Actual Available Magnesium            Comment 

Magnesium Oxide                                 60.3 %                                                  Palatable and economic

Magnesium Gluconate                          5.39%                                                  Need a very high amount

Magnesium Citrate                               16.2%                                                    Expensive

Magnesium Chloride                            11.98%                                                  Unpalatable

Liquid Magnesium Pidolate                  8.6%                                                     Expensive need a high amount

Magnesium Asparate                             7.49%                                                  Expensive need a high amount

Magnesium Sulphate                              9.9%                                                   Strong Laxative effect, unpalatable

Regarding the actual absorption of different magnesium’s, scientific tests (Coudray et Al) have shown that over 10 different forms, no matter whether organic or inorganic, the magnesium will be absorbed in the intestine at the same rate. All were equally efficient in restoring blood magnesium levels.

How Much Magnesium to Give a Horse?

Using a blood test to assess magnesium levels is not possible as the available magnesium does not show up in such a test. So, it is important to add up how much magnesium the horse is already ingesting when assessing the amount to supplement in the feed. A 500Kg average horse (16hh-16.1hh) needs 10gm per day of actual magnesium. Larger horses, or those in hard work need a little more, e.g., 12gms.

No more than 30gms should be in the daily diet of a horse. If using magnesium to help an overweight horse and one with EMS, high levels may be given but should be done in consultation with a veterinarian. Chromium is often given with magnesium to these horses.

Magnesium is found in hay, and many feedstuffs, particularly lucerne and beet feeds. It is usually added as a supplement in premix feeds but remember if you are not giving the recommended quantity of the premix feed then the supplements will be proportionally lower.

Typically, a horse on some pasture, and or hay will only need about 5gms of supplemented magnesium per day to ensure that they receive the required 10g daily intake. Those on a hard feed and a mineral mix may need no supplementation. Add up all the total intake and limit the total – i.e. don’t add extra if the daily requirement has been met, that will cause complications with the ratios of minerals  and will not further quieten down a horse. Look for other reasons why he is showing signs of being hyper and tricky to work with. The sugars in the pasture are likely to be the culprit. There is an important ratio to take note of between calcium and magnesium, the total calcium intake needs to be about 2.5 times higher than magnesium. So overfeeding magnesium may disrupt this ratio.

So, taking the most available and economic source as magnesium oxide and its availability of 60%, your horse may need 8gms of magnesium oxide in the Spring. Checking the label for the amount in a premix feed or other feedstuffs, it is likely that as little as 5gms is sufficient. Remember more is not necessarily better, but a slight increase in Springtime will cover the lower levels of newly growing pasture.

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