Equine America Magnitude Powder 908g. Equine America Magnitude is a Magnesium powder supplement that can help to relax nervous, excitable and tense horses. Magnesium has many roles within the body including the health of nerve endings and fibres and a horse deficient in magnesium may show signs of tension or nervousness. Magnitude is a highly absorbable form of Magnesium that should be fed daily to help maintain healthy levels of Magnesium in the horse, promote healthy nerve tissue and to reduce nervous tension. Read more… 908g - 6 month supply.