In nature, nothing works in isolation—and the same is true when it comes to horse nutrition. Vitamins and minerals interact in complex ways, especially calcium and phosphorus. It's a mistake to treat nutritional imbalances by supplementing just one element in isolation. Understanding how these two minerals work together, and with others, is key to maintaining optimal equine health.
Why Do Horses Need Calcium and Phosphorus?
Calcium and phosphorus are vital for bone development, muscle function, and cellular energy.
- Bone structure is made up of 35% calcium and 17% phosphorus.
- Calcium supports normal muscle contractions, nerve signalling, and enzyme function. About 99% of a horse’s calcium is stored in the bones, with only 1% circulating in the blood.
- Phosphorus is essential for cellular energy (ATP production) and is a key component of cell membranes in the form of phospholipids. Around 80% of the body’s phosphorus is stored in bones.
Without the right balance, horses—especially young, pregnant, or heavily worked horses can suffer from long-term skeletal and muscular issues.
Dietary Sources of Calcium and Phosphorus
The balance of calcium and phosphorus in a horse’s diet depends heavily on forage and grain choices:
- Grass hay: Moderate calcium (~0.6%) and low phosphorus
- Cereal grains (e.g., oats, barley, corn): Low calcium, high phosphorus
- Bran: High in phosphorus, very low in calcium
- Lucerne (alfalfa): High calcium (>1%) and low phosphorus
- Balanced supplements such as Equilibrium Mineral Mix, Vitastance, Osmoplex
A diet high in lucerne can easily lead to excess calcium and protein if it makes up more than 50% of the total forage. The form of each mineral matters too: phosphorus from grains (as phytate phosphorus) is harder to absorb—only about 35% is utilised—compared to more bioavailable inorganic sources.
Older horses (20+ years) may require more phosphorus, but excess calcium—especially over 1% of the total ration—should be avoided in senior horses.
Calcium to Phosphorus Ratio: Why Balance Matters
The calcium-to-phosphorus ratio is as important as the total intake. Most balanced equine diets aim for a ratio between 1.2:1 and 1.4:1 (calcium to phosphorus).
If phosphorus exceeds calcium (ratio < 1:1), calcium absorption is impaired. This imbalance often occurs in diets high in grain or wheat bran. Conversely, too much lucerne can push the calcium levels too high.
Factors Affecting Calcium Absorption in Horses
Calcium absorption isn’t just about intake—it’s influenced by many factors:
- Oxalates in tropical grasses (e.g., Kikuyu, Buffel, Pangola, Setaria) can reduce calcium absorption by up to 60%.
- Phytates in grains also bind calcium, limiting absorption.
- Magnesium boosts calcium absorption, while excess phosphorus reduces it.
- High zinc decreases calcium and copper uptake.
- High sodium and chloride levels may increase phosphorus absorption by 30–60%.
If calcium intake is low, the gut becomes more efficient at absorbing it. If intake is high, excess is excreted in the urine.
How the Horse Regulates Calcium and Phosphorus
Despite the complex mineral interactions, a horse's body regulates calcium levels fairly well—if the diet meets basic requirements.
- Parathyroid hormone mobilises calcium from bones when blood levels are low.
- Calcitonin and parathyroid hormone reduce absorption and increase excretion when blood calcium is too high.
- Vitamin D supports calcium (and some phosphorus) absorption in the gut.
If calcium intake is insufficient, the body will draw from bone reserves—compromising skeletal strength to maintain essential nerve and muscle function.
Signs of Calcium Deficiency and Excess
Deficiencies can be subtle or severe, often mimicking other conditions. Watch for:
- Shifting lameness
- Weak or crooked bones (osteopenia)
- Enlarged joints
- Spontaneous fractures
- Tying up
- Poor performance
Severe deficiencies may lead to stress tetany, thumps, or exhausted horse syndrome all of which require urgent veterinary care.
Excess calcium, especially from overfeeding lucerne, can lead to enteroliths (intestinal stones).
Signs of Phosphorus Deficiency or Excess
- Phosphorus excess inhibits calcium absorption and can cause nutritional secondary hyperparathyroidism, also known as Big Head disease.
- Phosphorus deficiency results in soft, poorly mineralised bones.
Final Thoughts: Calcium and Phosphorus Are Only Part of the Picture
Equine nutrition is a balancing act. Calcium and phosphorus are essential, but their effectiveness depends on interactions with other minerals, dietary components, and the form in which they’re consumed.
Instead of supplementing blindly, assess the whole diet. Use Truguide as your Trusted Guide to select the right supplement. Consult a qualified equine nutritionist to ensure your horse receives a well-balanced mineral intake tailored to their life stage, workload, and forage type.
