The delicate balance between two key minerals in horses’ diets

More than 2,000 mineral species exist in nature, ranging from the sodium and chloride that make up your table salt to expensive diamonds. In the context of nutrition, minerals contribute to hundreds of vital functions in the body—everything from brain activity to heartbeat. Minerals are essential for life, growth, and metabolism. Therefore, it is important to understand not only the amounts your horse requires but also their intricate relationships with one another. Due to these complex interactions, an excess or deficiency of one mineral can impair the absorption and utilization of others.

In horses this notoriously applies to two very important macrominerals: calcium and phosphorus. An inverted ratio (more phosphorus than calcium) in the diet can cause serious health problems. The good news is if your horse is currently eating adequate amounts of forage, as well as a ration balancer or concentrate appropriate for his life stage and activity level, his calcium-to-phosphorus ratio is most likely just fine. But let’s find out.

The Dynamic Duo

Calcium and phosphorus both play important roles in your horse’s health. His bones and teeth contain almost 99% of the calcium in his body. This mineral also contributes to brain and neuromuscular function, blood clotting, and enzyme activity. Phosphorus helps regulate muscle contraction, maintain cell integrity, and optimize glucose use needed for energy conversion. While calcium makes up 35% of the average bone, phosphorus accounts for 14-17% of its composition. This ratio of approximately 2:1 between calcium and phosphorus in the skeleton is reflective of the ratio most horses need in their diet.

Different Horses, Different Amounts

While the Ca:P ratio is undeniably important, so are the amounts of each mineral in the horse’s diet. “Much attention is paid to the ratio but, before all else, the absolute intake of both minerals must be adequate,” says Anna Pesta, PhD, an equine nutritionist for Purina Animal Nutrition who lives in Aiken, South Carolina.

Both calcium and phosphorus are macrominerals, meaning all horses must consume several grams per kilogram of body weight (as opposed to the milligrams-per-kilogram requirements of microminerals). But exactly how many grams a horse needs depends on a couple of factors.

Growing, heavily pregnant, and lactating animals need the most. This is because requirements are greatest during the times in a horse’s life when tissues are developing, Pesta explains, whether inside or outside the womb. Naturally, young growing horses (≤ 2 years) have increased calcium and phosphorus requirements.

Sarah Ralston, PhD, VMD, Dipl. ACVN, has dedicated her career to equine nutrition research and academia at The University of Pennsylvania School of Veterinary Medicine, Colorado State University School of Veterinary Medicine, and the Department of Animal Science at Rutgers University in New Jersey, and has a longtime interest in young, growing horses. For these individuals, she says, “calcium should constitute at least 0.6% and phosphorus about 0.4% of the total ration, as both minerals play a crucial role in the ossification process that forms the skeleton when bone replaces ­cartilage.”

At the opposite end of the spectrum, horses that need the lowest amounts are adults at maintenance. These animals are not breeding, working, or performing and only need enough of the minerals to fuel everyday bodily functions. 

In the middle are the horses in some level of work. “The effect of exercise on requirements for calcium and phosphorus is a bit unclear,” says Pesta, but scientists do know that horses undergoing intense exercise experience an increase in bone mass and, thus, have greater calcium needs. For horses in heavy work, “the increased needs due to bone/tissue repair and losses through sweat appear to be well-covered by an intake of 1.5-2 times the maintenance-level amounts,” she says.

Ralston adds that hard-working immature horses, such young racehorses (< 3 years) and other juvenile athletes, require calcium and phosphorus beyond what is necessary for either growth or performance alone, although nutritionists don’t agree on exact amounts. While more research is warranted for this subcategory of performance horses, the recommended daily values for calcium and phosphorus are readily available for other categories. Table 1 provides these estimates, which were calculated using tables from the National Research Council’s (NRC) Nutrient Requirements of Horses Sixth Revised Edition, published in 2007, and assume a body weight of 500 kilograms (1,100 pounds) unless otherwise specified. 

Adequate Ratios

Nutrition is all about balance. Now that we’ve established necessary amounts of both minerals, let’s look at appropriate Ca:P ratios. Horses should never consume more phosphorus than calcium. The absolute minimum ratio is 1.1:1 (one-and-one-tenth part calcium to one-part phosphorus). Feeding more phosphorus than calcium impairs the body’s ability to absorb the latter mineral, which in turn causes an array of musculoskeletal issues. Most horses need a ratio between 1.5:1 and 2:1. However, scientists have established a ratio of up to 6:1 to be safe, provided you’re still feeding the minimum required amount of phosphorus.

Excessive calcium intake, while rare, has consequences, as well. It can interfere with the absorption of certain trace minerals (iron, zinc, and copper) and unnecessarily tax the kidneys, which must work harder to eliminate the surplus calcium.

Consequences of an Inverted Ratio

We place so much emphasis on feeding a correct Ca:P ratio because the consequences of an inverted one can be dire. The body’s top priority relative to these minerals is maintaining adequate calcium levels in the bloodstream. Acute calcium deficiency can cause muscle trembling, ileus (decreased/absent intestinal motility), and neurologic problems such as seizures. If a diet is deficient in calcium or if excessive phosphorus intake impairs calcium’s absorption, the body sacrifices bone integrity by pulling calcium away from the bones and into the bloodstream. Over time this can weaken the skeleton and lead to an array of orthopedic diseases, most notably osteomalacia (soft and deformed bones in the mature horse). This disease is also known as “adult rickets” or “big head disease,” because the bones of the face can be the most visibly affected, giving the horse’s head an enlarged, swollen appearance, says Pesta.

Young horses’ musculoskeletal systems are at additional risk if their Ca:P ratio is inverted. “It increases the incidence of developmental orthopedic disorders in horses younger than 2,” says Ralston.

Developmental orthopedic disorders are multivariate in origin, and the umbrella term encompasses osteochondrosis lesions, physitis (inflammation of the growth plates), cervical vertebral malformation (wobbler syndrome), angular and flexural limb deformities, and club feet. The clinical signs that accompany these disorders are often debilitating, including pain, stiffness, joint effusion (swelling), abnormal gait, and lameness.

Another problem that horses of all ages with inverted Ca:P ratios face is nutritional secondary hyperparathyroidism. This endocrine disease results in the parathyroid gland producing excessive hormones and can lead to weight loss, lameness, and soft-tissue mineralization.

Maintaining an Adequate Ratio

Thankfully, feeding horses appropriate Ca:P ratios is, in most cases, straightforward. “It can be tempting to overcomplicate things when we start looking at individual components of the diet, such as the specific balance between certain minerals,” says Pesta. “In the case of calcium and phosphorus, it doesn’t need to be. The simplest way to ensure proper quantities and ratios of both is to choose the appropriate feed for your horse’s life stage and feed it at the correct amounts. Read the fine print! Reputable feed companies have done the work for you.”

Feed manufacturers incorporate ingredients such as calcium carbonate and dicalcium phosphate to balance these two minerals when needed. If, for any reason, a horse owner formulates their own rations, Pesta says they must also assume the responsibility of knowing both what their horses need and what their feeds contain.

Immature, growing horses present an additional challenge. Adjusting the diet to accommodate their proportionally higher mineral requirement isn’t as simple as increasing feed quantity. Because weanlings and yearlings have a smaller body size, the volume of feed they can consume is limited, which means the concentration of these minerals within the feed must be greater, Pesta says. So again, relying on reputable feed manufacturers’ formulations for young horses is the best way to ensure you’ve met these animals’ needs.

Ca and P Sources in the Diet

Identifying calcium- and phosphorus-rich feeds helps owners make informed decisions about their horses’ diets. As with all mammals, mare’s milk is a pot of gold when it comes to calcium. For the weaned animal, legume hays such as alfalfa and clover are particularly rich in

On the flip side, raw, unrefined cereal grains are generally higher in phosphorus than calcium. Table 2 displays Ca and P content in feeds commonly used to formulate equine diets. Pesta cautions that there are always exceptions and individual ingredient variations, mainly because of differences in the mineral content and soil pH where the forage grew.

If, after evaluating the total diet and adding legume hay, you realize you need to correct either calcium or phosphorus content, you can do so using a couple of readily available ingredients. Nutritionists often recommend top-dressing rations with ground limestone to add calcium and incorporating dicalcium phosphate when both minerals are lacking. An overall mineral insufficiency warrants a properly formulated mineral supplement, while a ration balancer can address a generalized mineral, vitamin, and protein deficiency. Horses that don’t need concentrates to maintain adequate energy levels and body condition can thrive with a ration balancer as their only supplementation to forage.

In the absence of clinical signs associated with a nutritional imbalance, nutritionists recommend looking at the diet, rather than the animal, to determine Ca and P intake. “There are certainly exceptions, for horses with certain medical conditions or kidney issues, where biological samples must be gathered and interpreted by a knowledgeable practitioner,” says Pesta.

For the rest of the healthy equine population, read your commercial grain’s feed tag, and submit a sample of your hay for nutrient analysis. Ralston says you can also contact your local agricultural extension agent to gather the nutrient profiles of local forages.

Take-Home Message

Given the possible health consequences of feeding an inadequate Ca:P ratio, it’s not something you want to leave up to chance. Thankfully, a forage-based diet supplemented with either a concentrate or ration balancer generally offers appropriate levels of both minerals. Your role as the horse’s owner is to consider the overall diet, select the appropriate feed for his life stage and lifestyle, and ensure you are feeding the correct amounts. As always, veterinarians and nutritionists are there to help you with these decisions.