Phosphorus is the second most abundant mineral in your horse's body, working alongside calcium to build and maintain the skeleton. About 80 percent of phosphorus is in bones and teeth, while the rest plays vital roles in energy metabolism (as part of ATP), cell membrane structure, and DNA. The calcium-to-phosphorus ratio matters enormously — a ratio below 1:1 (more phosphorus than calcium) is dangerous and can cause nutritional secondary hyperparathyroidism. Grain-heavy diets are the most common cause of inverted ratios, since grains like oats and barley are higher in phosphorus than calcium. Phosphorus in cereal grains exists partly as phytate, which horses can partially digest thanks to hindgut bacterial phytase, though not as efficiently as ruminants.
A 500kg horse at maintenance needs about 14 grams of phosphorus per day — roughly the weight of a tablespoon of flour. Grass hay provides about 2 to 3 grams per kilogram, so 10kg of hay covers most of the requirement. Grain concentrates add additional phosphorus. The critical thing is maintaining the Ca:P ratio between 1.5:1 and 2:1 in the total diet — never let phosphorus exceed calcium.
0.78% of daily nutrient intake
Phosphorus makes up 0.78% of your horse's total daily nutritional requirements by weight. That's a tiny amount — but it matters.
Poor bone development in growing horses, reduced appetite, poor growth rates, weakness, and pica (eating dirt or unusual objects). True phosphorus deficiency is uncommon when horses receive adequate forage and some grain.
Excess phosphorus relative to calcium is the primary concern, disrupting the Ca:P ratio and leaching calcium from bones. This is more common than phosphorus deficiency, especially in horses fed large amounts of grain with insufficient forage. Signs mirror calcium deficiency: lameness, swollen bones, and skeletal weakness.
| Life Stage | Size | Min | Max | Unit | Notes |
|---|---|---|---|---|---|
| Adult | — | 14 | 28 | g | For a 500kg horse at maintenance. Never allow dietary phosphorus to exceed calcium. Grain-heavy diets are the most common cause of inverted Ca:P ratios. |
| Juvenile | — | 18 | 30 | g | Growing foals need phosphorus for bone development. Maintain Ca:P ratio of at least 1.5:1. |
| Pregnant / Nursing | — | 20 | 35 | g | Late-gestation and lactating mares need more phosphorus alongside increased calcium for fetal development and milk. |
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.
