Understanding Ca and P Ratios for Horse Health

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. This article continues in the April 2021 issue of The Horse: Your Guide to Equine Health Care. Subscribe now and get an immediate download of the issue to continue reading. Current magazine subscribers can access the digital edition here. 

The Horse: Your Guide to Equine Health Care, April 2021

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