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  • Q:

    I have an 11-year-old Arabian mare. She has access at all times to pasture and is fed a complete feed twice a day. Her body condition score is 5. She has been, since I got her, on the nervous side, but I noticed recently that her muscles tremble at times. It happens occasionally when she is just standing, but I notice it more if she is in a stressful situation, after trailering or socializing with new horses. I've read about magnesium deficiency in horses and wonder if this may be my mare’s problem. I do not want to supplement her if magnesium deficiency is not the cause. Any ideas would be helpful.

  • A:

    Clinical signs of magnesium deficiency include nervousness, muscle tremors, and ataxia. Though magnesium deficiency is considered rare in horses that have access to pasture and fed good-quality hay, it may be more common for horses to have marginal dietary intakes, due to several factors including pasture fertility and competitive mineral interactions, which result in magnesium intakes or availability below optimal levels.

    Nervous, hot horses may benefit from receiving supplemental nutrients that are thought to have calming or quieting properties, including magnesium. Much of the evidence for calming supplements and their components, however, is anecdotal. The successful supplementation of magnesium and thiamine as calming agents is thought to be due to the improvement of nutritional deficiencies of these nutrients and the role of magnesium in muscle relaxation. Recent research in Canada and Australia has shown that magnesium supplementation may reduce anxiety in horses.

    A detailed diet evaluation is a cost-effective and practical way to calculate total intakes and assess nutritional imbalances and deficiencies. Having a baseline evaluation can be helpful when considering supplementation, especially when trying to determine a therapeutic dose. Your veterinarian can also assess your mare’s magnesium status with blood tests, and a spot-sample of fractional clearance of magnesium (creatinine/magnesium quotients) can be used to identify insufficient dietary supply.

    The amount of added magnesium in commercial feed formulas varies between manufacturers. Selecting a feed that provides elevated levels of magnesium may be beneficial for your mare. It is important to select a feed that has a recommended feeding rate that matches your mare’s caloric needs.

    Dietary energy sources can also impact horse behavior. Reducing starch and sugar (nonstructural carbohydrate, NSC) and increasing fat can help reduce excitability and responsiveness in certain horses. Stress can have a negative impact on digestive health. For horses experiencing repeated episodes of stress, I recommend a daily digestive health supplement, such as KER’s RiteTrac or EquiShure to support health and function. Subclinical digestive issues such as hindgut acidosis or gastric ulcers can affect the horse’s behavior, performance, and overall health. RiteTrac and EquiShure offer superior ingredients and research-proven value.

    In addition to a diet evaluation and supplementation, there are behavioral reasons that might be contributing to the mare’s nervousness. Horses often become more comfortable in situations when they are exposed to them often. As an example, “nervous haulers” will often become less anxious as they are hauled more and more, especially if a second horse acts as a comforting tagalong. Exposing horses to different environments and training scenarios as often as possible helps create calm, usable mounts.

    B-Quiet and B-Quiet Paste, available in Australia, contain thiamine and magnesium, and these supplements can be used in horses showing anxiety or nervousness, as this mare does.

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