Nutrition is truly all about balance. When a balanced diet is discussed for horses, you must ensure that all the required nutrients are provided to the animal in adequate amounts. The NRC 2007 (Nutrient Requirements of Horses) provides recommendations on the minimum amounts of nutrients required to prevent nutritional deficiency. However, there can be interactions between various nutrients, particularly the minerals, and therefore, it is imperative to pay attention to the ratios as well.

The Roles of Calcium and Phosphorus

Minerals are one of the six required nutrient classes for horses, and calcium and phosphorus are two of the required macrominerals. Minerals are essential for health as they contribute to a variety of cellular functions within the body. Calcium is the most abundant mineral in bone. Additionally, calcium is involved in neuromuscular function, blood clotting, enzymatic activity, and brain function. Phosphorus also plays a role in the structure and strength of bones and is important for the regulation of muscle contraction, cellular integrity, and energy conversion.

Calcium and phosphorus are required macrominerals; this means that they are required in grams per kg body weight. The number of grams of each mineral that a horse requires will depend on their physiological state and workload. For example, growing, lactating, and pregnant horses will have a higher requirement than a light riding horse.

Deficiency Symptoms

Both inadequate and excess intake of calcium and phosphorus can have negative health consequences. When calcium is deficient in the body, or if excessive phosphorus is impairing the body’s ability to absorb calcium, the horse will mobilize calcium from the bones to maintain an adequate blood calcium content. Over time, this leads to a weakened skeleton and orthopedic diseases.

The most commonly-seen orthopedic disease in mature horses with a calcium deficiency is osteomalacia, which is also referred to as ‘big head disease’. When a horse is suffering from osteomalacia their bones will be softer and deformed. The term ‘big head’ stems from the clinical symptoms often being easily visible in the head region with the softening of facial bones. If there is an acute deficiency of calcium, the horse will often present with muscle trembling, neurological issues and have decreased intestinal motility.

When there is an inadequate intake of phosphorus in the diet, similar to calcium, the horse will mobilize bone stores of phosphorus. This negatively impacts bone strength and when a phosphorus deficiency is left unaddressed for a prolonged period of time, muscle weakness and lameness are also common deficiency symptoms. When the bones are weaker, the horse will be more prone to fractures.

Young and growing horses, as well as broodmares, are most at risk for calcium and phosphorus deficiency. When phosphorus is deficient there will be slower growth and bone formation issues. For broodmares, calcium is critical as tissue development and milk production require a significant amount of the mineral. Therefore, these horses will have much greater requirements to prevent deficiency.

The Ratio

The calcium-to-phosphorus ratio in the diet is one of the most crucial for horses, as when the amount of phosphorus exceeds the amount of calcium, serious health issues present. This is because where there is too much phosphorus in relation to calcium the body’s ability to absorb calcium is impaired. The minimum ratio is 1.1 parts calcium to 1 part phosphorus. If the ratio falls below this value, that is when health issues will likely arise. The safe range is generally regarded as 1.5:1 to 6:1, however, ideally a ratio of 2 parts calcium to 1 part phosphorus is sought after.

Most of the calcium that your horse requires will often come from the hay and pasture they are consuming, especially if they are fed a legume hay. Feed ingredients such as alfalfa are rich in calcium, and raw cereal grains such as oats or corn tend to be higher in phosphorus.

If you have a forage analysis done, the laboratory will include a calculated calcium-to-phosphorus ratio for you. This is valuable information, as you will be able to acknowledge if the ratio is skewed in the forage, at which point you should consult an equine nutritionist for advice.

However, evaluating the ratio in the overall diet is what is important. When evaluating your horse’s ration, it can get complicated quickly if you are adding many individual ingredients to a diet to balance it. This tactic also involves a significant number of calculations, as you are taking over the responsibility that each required nutrient is being fed and provided in adequate amounts. It doesn’t have to be that complex; the easiest way to ensure that your horse has an adequate calcium-to-phosphorus ratio is to feed a pre-formulated product.

Choosing a feed product that is from a reputable company and matches your horse’s energy requirements will have been formulated by an equine nutritionist to provide adequate minerals in balanced ratios as long as it is being fed at the manufacturer’s recommended rate for that animal’s body weight.

To conclude, if the ratio is not skewed in your hay, and you are feeding a balanced commercial feed product at the recommended amount, you likely do not need to worry about your horse’s calcium-to-phosphorus ratio. However, if your hay has a skewed ratio, or if you are noticing potential deficiency symptoms, you should consult a qualified equine nutritionist who can calculate the ratio in your horse’s overall diet.