Why is the correct ratio of calcium and phosphorus in a horse's feed ration so important?

Ing. Lucie Humeni

When I started working in the field approximately eight years ago, a common problem was an imbalance in the Ca:P ratio caused by excessive phosphorus (P) intake, particularly in horses fed large amounts of grains and bran. Over time, however, feeding trends have changed. With the rise of grain-free feeds and the increasing popularity of alfalfa, sugar beet pulp, and other calcium-rich (Ca) feedstuffs, I am now encountering the opposite extreme more and more often—namely, an excessive surplus of calcium, which can significantly shift the Ca:P ratio above recommended values. This shift has become so pronounced that it is worth revisiting why the correct balance of these two minerals is so important and how it can be maintained in modern feeding rations.

The importance of calcium and phosphorus

Calcium is the fundamental structural component of bone tissue. More than 99% of calcium in the body is stored in bones and teeth, but its role extends far beyond skeletal structure. Calcium regulates muscle contraction, affects heart function, participates in bleeding and clotting processes, and is essential for proper nerve impulse transmission. Blood calcium levels are tightly regulated by hormonal mechanisms, and if dietary calcium intake decreases, the body begins to mobilize calcium reserves from the bones.

Phosphorus is closely linked to energy metabolism. Without phosphorus, the production of ATP (adenosine triphosphate)—the molecule that supplies energy to cells exactly where it is needed—would not be possible. Phosphorus is a component of nucleic acids, which carry genetic information, and it forms a substantial part of cell membranes. It is also essential for proper bone mineralization. However, when phosphorus intake is excessive, it can become a limiting factor for calcium utilization, as these two minerals interact closely within the body.

The correct ratio

The optimal calcium-to-phosphorus ratio should range between 1.5:1 and 2:1. In young, growing horses, the ratio should be higher—approximately 2:1 to 3:1—to support proper skeletal development and ensure adequate bone mineralization.

Why is the correct Ca:P ratio in the ration important?

Horses are able to compensate for certain short-term fluctuations in calcium or phosphorus intake, but long-term imbalance leads to impaired bone mineralization. If the ratio is too low (i.e., excess phosphorus), the body begins to withdraw calcium from bone tissue. This process results in thinning of bone structure, skeletal deformities, and in young horses, delayed or impaired skeletal development.

But how are calcium and phosphorus actually assessed? Blood calcium and phosphorus values (based on blood analysis) can serve as a useful indicative marker; however, it is important to emphasize that they do not reliably reflect long-term dietary intake of these minerals. The horse’s body maintains very stable blood levels of Ca and P through hormonal regulation. Only severe or long-standing imbalances may be reflected in altered blood values. This means that normal blood results do not rule out an incorrect Ca:P ratio in the diet. A horse may have normal blood calcium and phosphorus levels while bone demineralization or suboptimal mineralization is still occurring. In the case of phosphorus, blood testing may help identify metabolic disorders or issues with absorption. Therefore, a well-formulated ration is essential, as it is the primary factor ensuring an optimal Ca:P ratio. Ideally, ration formulation would include analyses of all feedstuffs, which—especially for forage—can be quite challenging in horses due to varying forage sources, suppliers, and batches. This process is far more complex than in cattle production, where forage analysis is routinely used for ration formulation.

Sources of calcium and phosphorus in feeds

The Ca:P ratio must be correct across the entire ration—including hay, pasture, concentrates, and supplements. Many owners focus their calculations solely on concentrates or mineral supplements, but the largest proportion of mineral intake usually comes from forage.

Calcium-rich feeds:

• Pasture (especially clover and alfalfa)
• Meadow, clover, and grass hay
• Grass, clover, and grass–clover haylage
• Alfalfa and grass dried forages
• Sugar beet pulp

Phosphorus-rich feeds:

• Grains (oats, barley, maize, wheat)
• Legumes (peas, soybeans)
• Brans (wheat, rice, hemp)
• Malt sprouts
• Linseed (flaxseed)

Mineral supplements

It is not advisable to rely solely on mineral supplements or premixes to correct the Ca:P ratio, although they can certainly contribute to balancing the ration. If the diet contains excessive amounts of alfalfa, sugar beet pulp, or other high-calcium feeds, a phosphorus-containing mineral supplement may not always resolve the imbalance effectively—and this is an important point to keep in mind.

In conclusion

The correct calcium-to-phosphorus ratio is not just about two numbers in a table. Simply adding a mineral supplement is not enough; it is essential to evaluate the entire ration holistically and understand how individual feedstuffs interact. Regular monitoring of the Ca:P ratio is therefore not a mere “formality,” but a key preventive measure against long-term health issues.

References

Maier, I., & Kienzle, E. (2024). A Meta-Analysis on Quantitative Calcium, Phosphorus and Magnesium Metabolism in Horses and Ponies. Animals, 14(19), 2765. https://doi.org/10.3390/ani14192765

Etemadi, F., Tabatabaei Naeini, A., & Aminlari, M. (2023). Assessment of calcium, phosphorus, magnesium, vitamin D and PTH levels in sera of lame horses. Veterinary Medicine and Science, 9(5), 2070–2077. https://doi.org/10.1002/vms3.1198

NRC (National Research Council). (2007). Nutrient Requirements of Horses (6th rev. ed.). National Academies Press.

van Doorn, D. A., et al. (2004). The influence of high calcium intake on mineral absorption in horses. Livestock Production Science, 89(2–3), 267–272.

Poggi, H., et al. (2022). Interactions of dietary minerals in equine nutrition: A review. Animals, 12(8), 1062.

van Weyenberg, S., et al. (2006). Phosphorus bioavailability in horses: A review. Livestock Science, 100(1), 1–9.