My 15-hand Standardbred can be spooky at times, especially when we go to adult riding club. I have found that injecting him with magnesium the day before we go anywhere calms him down, and he behaves a lot better. He is housed in a paddock but the grass is not very lush at the moment and he gets fed lucerne (alfalfa) chaff because he is an easy keeper. What is your opinion of the injections, as they are quite expensive?
A horse’s behavior and response to different situations is related to many factors including breed, age, training, proximity to horses, previous experience, disease, environment, skill of handler, and in some cases nutrition. Certain behaviors can indicate a nutrient deficiency, and nervousness or spookiness has been linked to magnesium deficiency.
Lack of magnesium in the diet and resulting magnesium deficiency can have varying symptoms which include nervousness, muscle tremors, hypersensitivity of the skin, increased body temperature during exercise, and ataxia. Magnesium is stored in the bone and muscle, so serum or blood magnesium is a poor indicator of body magnesium levels. Diagnosing deficiency is best done with muscle biopsy or dietary analysis, though deficiency would be unlikely in any horse with day-to-day access to good-quality forage.
Studies have shown that horses normally excrete much dietary magnesium in their urine, and it took 13 days of feeding a magnesium-deficient diet before horses began to conserve magnesium or excrete less in their urine. It is unlikely that much, if any, of the injected magnesium is still present the day after injection. In clinical situations, intravenous magnesium is used to treat heart arrhythmias and hypoxic/ischemic encephalopathy (dummy foals). Concentrated magnesium solutions have been used for euthanasia.
There is a significant risk for adverse effects, including death, when intravenous magnesium is administered to horses that do not have a medical need for magnesium supplementation. It is likely that the dose administered is too low to have any major effects. If concentrated magnesium solutions are given outside the vein, swelling, infection, and clotting of the jugular vein are very possible. Inadvertent injection of concentrated magnesium solutions into the carotid artery could be fatal, but have a high chance of inducing seizure-like activity and resultant secondary injuries.
There is some anecdotal evidence that magnesium supplementation improves anxious or bad behavior in horses. However, there are no scientific studies that have proven an effect. Several commercially available supplements contain magnesium to help with behavioral issues. It is more likely that dietary supplementation will have a more profound and lasting effect on magnesium status than the occasional intravenous injection.
Your Standardbred’s diet of pasture and lucerne chaff is unlikely to provide the daily recommended amount of many nutrients including magnesium. I understand the reluctance to provide additional grain to your easy-keeping horse, but there are several options you can consider that will ensure he gets his daily recommended intake of nutrients without the calories. A low-intake balancer pellet or trace mineral/vitamin supplement is recommended in addition to your horse’s pasture diet to supply daily recommended amounts of all minerals and vitamins.