by Shannon Godwin BAppSc, GDTL
Magnesium (Mg) is an essential macro mineral closely associated with calcium and phosphorus. Mg is involved in numerous functions in the body with 60-70% of the mineral being found in the skeleton, with the remainder being distributed in the soft tissues and fluid (<1% in blood serum) both being of crucial importance to the well- being of the animal.
Magnesium is the most common enzyme activator in over 300 different biochemical reactions including protein synthesis, muscle and nerve function, blood pressure regulation and glucose control. Mg is required for energy production, glycolysis and oxidative phosphorylation as well as being involved in the synthesis of DNA, RNA and the antioxidant glutathione. Additionally, Mg is concerned with the active transport of calcium and potassium ions across cell membranes. This process is important to muscle contraction, nerve impulse conduction and normal heart rhythm. Consequently magnesium is a key element in cellular biochemistry and function.
Major losses of magnesium from the body occur in saliva (especially ruminants), sweat (horses), urine and mammary secretions. Complex disorders arise when the magnesium requirements of the animal are not being met. In ruminants a condition known as Grass Tetany associated with low blood levels of magnesium (hypomagnesaemia) is of particular importance. This may arise simply from a dietary deficiency or as a result of complex interactions that decrease Mg absorption from the rumen. Mg absorption may be inhibited when potassium (K) and nitrogen (N) intakes are high from heavily fertilised lush green pasture. Tropical grasses high in oxalates have also been identified in reducing uptake of Mg. Stress and stress hormones adrenalin, noradrenalin and cortisol can contribute to Mg deficiency. The adrenergic effects of psychological stress induce a shift of Mg from the intracellular to the extracellular space, increasing urinary excretion and can eventually deplete body stores (Galland, 1991-1992).
Deficiency symptoms are typically those of a neurological nature. Initially, there will be staggering, aggression, head throwing and development of stiffness in limbs. With further decreases in Mg level, grinding of the teeth, muscle tremors with shivering and convulsions followed by collapse, violent paddling of the feet and death. The most severe cases go down quickly and are found dead before signs are observed (Hungerford, 1990).
The severe form of hypomagnesemia is rarely documented in horses and usually presents in animals that are already critically ill. The following symptoms are however, quite common and horses experiencing these may benefit from Mg supplementation:
- Very tight, sore back not related to activity, fitness level or saddle fit.
- Physiotherapy, chiropractic adjustments, massage and other body work do not have lasting effects.
- Horse appears to never really relax.
- History of tying up.
- Muscle tremors or all over trembling not related to temperature.
- Show signs of irritability when brushed, rugged or being palpated especially over the back on either side of the spine.
- Does not tolerate work well.
- Requires long periods of lunging before being able to focus on work.
- Bucks shortly after workout begins, seems fine at first then bucks or balks.
- Is described as hypersensitive to touch or “think skinned”.
- Demonstrates repetitive movement, head bobbing, weaving, pacing etc.
Lush, young grass is typically deficient in magnesium and grasses and cereal crops have lower Mg levels than clovers and Lucerne. Forage low in Mg is also usually lacking adequate levels of calcium (Ca) and phosphorus (P) and is usually high in nitrogen (N) and potassium (K). (Kohnke, 1999) has listed the approximate Mg levels in common feed sources as follows: Lucerne 3.0g/Kg, grains 1.1-1.4g/Kg, Soybean Meal 2.7g/Kg, Canola Meal 5g/Kg, Milk Powder 10g/Kg, Magnesium oxide 10.6g/20mL, magnesium carbonate 5.6g/20mL, magnesium sulphate (Epsom salts) 2g/20mL, dolomite average 2g/20mL. AgSolutions® caters for specific Mg supplementation with the MegaMin® Extra Magnesium containing 100g/Kg of Mg.
Being familiar with deficiency signs and symptoms is important for livestock owners and as toxicity is extremely rare it is a safe mineral to feed. To benefit from the full nutritional value of any one mineral, adequate quantity of all other minerals must also be available. To ensure good nutritional health all the major nutrients must be provided at adequate, balanced levels to meet the animals’ needs.
Galland, L. (1991-1992). Magnesium, Stress and Neiuropsychiatric Disorders. Bethesda: US National Library of Medicine National Institues of Health.
Hungerford, T. (1990). Diseases of Livestock (Vol. 9th Edition). Roseville, NSW, Australia: McGraw-Hill Book Company Australia Pty Ltd.
Kohnke, J. (1999). Feeding Horses in Australia: A Guide for Horse Owners and Managers. Barton ACT: Rural Industries Research and Development Corporation.