Calcium is the most abundant mineral in your horse's body, with roughly 99 percent stored in bones and teeth. It is essential for skeletal strength, proper muscle contraction, nerve signaling, blood clotting, and hoof wall integrity. The calcium-to-phosphorus ratio in the total diet is critically important for horses — ideally between 1.5:1 and 2:1 (Ca:P). An inverted ratio (more phosphorus than calcium) is genuinely dangerous, as it causes the body to leach calcium from bones to maintain blood calcium levels, leading to a condition called nutritional secondary hyperparathyroidism, historically known as big head disease because the weakened facial bones swell and become distorted. Most grass hays provide a naturally favorable Ca:P ratio, while grain-heavy diets tend to push the ratio toward phosphorus, which is why balancing forage with concentrates matters.
A 500kg horse at maintenance needs about 20 grams of calcium per day — roughly the weight of four teaspoons of sugar. Ten kilograms of average grass hay provides about 30 to 50 grams, which comfortably exceeds the requirement. Alfalfa hay is much higher in calcium (10 to 15g per kg), making it an excellent calcium source but one that should be balanced with the overall diet to avoid mineral imbalances.
1.12% of daily nutrient intake
Calcium makes up 1.12% of your horse's total daily nutritional requirements by weight.
Shifting lameness, stiff gait, swollen facial bones (big head disease in severe cases), weakened bones prone to fracture, poor hoof quality, muscle tremors, and in growing horses, developmental orthopedic disease including physitis and angular limb deformities.
Moderate calcium excess is generally well tolerated by horses with healthy kidneys. Very high calcium intake can interfere with the absorption of other minerals like zinc, copper, and magnesium, and may contribute to enterolith formation (intestinal stones) in predisposed breeds and regions.
| Life Stage | Size | Min | Max | Unit | Notes |
|---|---|---|---|---|---|
| Adult | — | 20 | 40 | g | For a 500kg horse. Most grass hays provide 30-50g per 10kg, easily meeting the requirement. Maintain Ca:P ratio of 1.5:1 to 2:1. |
| Juvenile | — | 30 | 50 | g | Growing foals need more calcium per kilogram of body weight for skeletal development. Ensure adequate vitamin D for calcium absorption. |
| Pregnant / Nursing | — | 30 | 55 | g | Late-gestation and lactating mares have significantly increased calcium needs for fetal skeletal development and milk production. Alfalfa hay helps meet these demands. |
| Senior | — | 20 | 35 | g | Same as adult requirements. Ensure the Ca:P ratio remains favorable, especially if the horse has difficulty chewing and eats less hay. |
| Working / Active | — | 25 | 45 | g | Working horses lose some calcium in sweat and have slightly increased needs. Most diets with adequate forage meet this without supplementation. |
Source: NRC 2007
The calcium-to-phosphorus ratio is one of the most critical nutritional parameters in equine nutrition. A ratio of 1.5:1 to 2:1 (Ca:P) supports proper bone mineralization. An inverted ratio (more P than Ca) causes the body to leach calcium from bones, leading to nutritional secondary hyperparathyroidism (big head disease) with swollen facial bones and weakened skeleton.
What this means: Always ensure your horse's total diet has more calcium than phosphorus. Grass hay naturally has a favorable ratio, but adding large amounts of grain (which is phosphorus-heavy) can invert it. If feeding significant concentrates, balance with alfalfa hay or a calcium supplement.
Vitamin D regulates calcium absorption from the gut and directs it into bones where it belongs. Without adequate vitamin D, calcium passes through the gut without being properly absorbed, even when dietary levels are sufficient.
What this means: Ensure your horse gets regular outdoor turnout for natural vitamin D synthesis through the skin. Indoor horses on hay-only diets may need a vitamin D supplement, especially in winter or at high latitudes, to ensure their calcium intake actually reaches their bones.
Calcium and magnesium compete for absorption pathways and work in opposition in muscle tissue — calcium triggers contraction while magnesium promotes relaxation. Excess calcium can interfere with magnesium absorption, potentially contributing to muscle tension and anxiety.
What this means: If your horse is on a high-calcium diet (heavy alfalfa) and seems tense or spooky, consider whether magnesium is being crowded out. Adding a magnesium supplement at 5-10g daily may help restore the balance. The ideal Ca:Mg ratio in the total diet is roughly 2:1 to 3:1.
High calcium intake can reduce iron absorption, though this is rarely a concern in horses since dietary iron is typically far in excess of requirements. In practice, this interaction is more beneficial than harmful, as it may slightly reduce the absorption of excessive dietary iron.
What this means: This interaction is generally not a concern for horse owners since iron excess, not deficiency, is the typical equine scenario. The high calcium in alfalfa hay may actually provide a modest benefit by limiting iron absorption from iron-rich forages.
