Magnesium Overdose & Feeding in Horses

Magnesium supplementation in horses is a topic that is often asked about. It has been reported to have a calming effect on horses. However, before you go out and purchase a magnesium supplement for your horse, we need to understand how magnesium works in the equine body and the potential problems that can arise if too much is fed.

What does Magnesium do for horses?

Magnesium constitutes about 0.05% of the body mass. 60% of magnesium in the body is found in the skeleton and about 30% in the muscle. Magnesium is important in the blood and plays a role as an activator of many enzymes and participates in muscle contractions. Within the digestive system, magnesium affects absorption of other key electrolytes including calcium and potassium. Magnesium is also involved in the formation of bone.

Considering magnesium's diverse role in the body, deficiencies can have a major impact on your horse's overall health and wellbeing. 

(Photo above: Many nutrients are responsible for outer health. Magnesium plays an essential role in keeping horses healthy.)

How much Magnesium should a horse have?

The normal blood level of Magnesium in horses is 2.2-2.7mg/dl (NRC 2007). Values below 1.6mg/dl are defined as hypomagnesaemia aka magnesium deficiency. According to the NRC, an intake of 20mg of Magnesium per kilogram of bodyweight per day is necessary to maintain normal blood serum levels. Thus, for a 500kg horse in light to moderate exercise, an intake of 10g per day is required.

Magnesium deficiency and excess in horses

Deficiencies in magnesium can result in nervousness, muscle tremors, incoordination, increased respiration and even death. Many commonly used feed ingredients (e.g. Lucerne, beet pulp) contain about 0.1 to 0.3 percent magnesium with an absorption rate of approximately 40 to 60 %. Excessive magnesium will be excreted in the urine, but overdoses have been linked to decreased calcium and phosphorus uptake, compromised intestinal integrity, heart conduction problems and renal trouble, so it’s important not to over supplement.

The maximum tolerable level in the total diet is estimated at 0.8%, for a 500kg horse in light to moderate exercise consuming 2 to 2.25% BW. That equates to 80 to 90g of magnesium in the total diet.

Magnesium sulfate directly affects the heart when it is injected intravenously, forcing the heart into a state of arrhythmia. When used in larger doses, it can shut down cardiac function, resulting in subsequent collapse of the horse, and in some cases, death. This practice should never be done.

Magnesium sources for horses

The most common form of Magnesium available is Magnesium Sulfate, commonly called Epsom salts, however one side effect of regular feeding of Epsom salts is diarrhoea. The usual form used in animal feeds is Magnesium Oxide, a fine white powder. Magnesium Oxide is about 50% absorbable, depending on the relative levels of Calcium also present. Generally magnesium oxide is not absorbed if there is no deficiency present, meaning overdose is difficult when using this source. 

All HYGAIN® fully fortified feeds contain magnesium. When feeding a HYGAIN® feed according to the guidelines (alongside hay or pasture), appropriate magnesium is supplied to meet the requirements of your horse as set by the Nutrient Requirements Council in 2007. There is generally no need to add an additional magnesium supplement to your horse's feed. 

(Photo above: Magnesium Oxide has a white powdery appearance.)

Does Magnesium calm horses down? 

Horses may display anxious behaviour when they are experiencing a magnesium deficiency. However, there is no significant scientific evidence to demonstrate that providing magnesium in excess of daily requirements has a calming affect. 

(Photo above: There are many factors that contribute to a calm horse.Magnesium in your horse's diet is best discussed with an equine nutrition advisor.)  

Magnesium deficiency has been associated with insulin resistance and metabolic syndrome in rodents and humans, but has yet to be scientifically confirmed in the horse.

Although there have been anecdotal reports in horses of supplemental magnesium improving insulin resistance, an equine study conducted at the University of Tennessee found no advantage to feeding a magnesium supplement to insulin-resistant, laminitic horses. This is potentially due to equine diets not being deficient in magnesium. 

Unsure if your horse is getting enough Magnesium?

The Nutrition Advisors at Hygain are on hand to analyse your horse's diet and provide a clear picture of your horse's nutritional needs, including Magnesium. You can book a time to speak with Hygain's Nutrition advisors by clicking this link. Or, feel welcome to reach out by phone on (03) 5943 2255.