Skewing the balance of calcium to phosphorus in a broodmare’s or young horse’s diet can cause many issues in the development of that horse. The calcium to phosphorous ratio in the diet of a young, growing horse is critical to his short- and long-term health. The grave nature of that balance of nutrients starts with the pregnant mare and continues until the young horse quits growing!
Normal adult horses, of course, need proper nutrients in the proper balance, but the calcium to phosphorus ratio isn’t as critical in the normal adult horse.
Many times, you will see the calcium to phosphorus ratio expressed as Ca:P. Horse owners need to consider not only the ratio, but the minimum amounts of calcium and phosphorus in the diet. You also must ensure that the amount of phosphorus in the diet is not too high. A phosphorus deficiency is rare in horses.
What Is the Calcium to Phosphorus Ratio for Horses?
Calcium and phosphorus are called macro minerals. These are minerals needed in the largest quantities in the horse’s diet. But “large” is all relative. Calcium requirements range from 0.3% to 0.8% of a horse’s total diet. Phosphorus requirements range from 0.2% to 0.5% of a horse’s total diet.
The ideal ratio of calcium to phosphorus in the young, growing horse’s diet is one part calcium to one part phosphorus. Two parts calcium to one part phosphorus is also quite acceptable. While there is a little play in the ratio, it’s important that the phosphorus amounts don’t exceed the available calcium in the diet. That is critical because the improper ratio can affect how other minerals are absorbed by the horse’s body and could lead to a variety of issues.
According to research, prolonged excessive amounts of phosphorus in the diet can cause secondary nutritional hyperparathyroidism, or big head disease (see below). Getting too much phosphorus in the diet usually isn’t an issue unless horses are receiving a high “cereal” diet that contains such feedstuffs as wheat bran, corn, or oats.
That’s not to say those are “bad” ingredients. But it is critical that the diet of any horse—especially the young horse—be balanced for the needs of that animal. Horse breeders must keep the dietary ratio of calcium to phosphorus in line with breeding and lactating mares.
Why Is Supplementing Calcium and Phosphorus Essential for Growing Horses?
While some breeds of horses grow until they are five or more years old, the calcium to phosphorus ratio is most important in rapidly growing young horses. This rapid growth happens during gestation and in the first year of life.
It’s recommended to keep some of our draft and warmblood breeds on a “growth-type diet” longer to ensure they are getting the proper balance of nutrients. Horse owners want to make sure that those horses have a “quality” of growth.
If owners feed a grass-based hay and lots of cereal grains, there is the possibility of causing a calcium deficiency. For example, if your broodmare has a calcium-deficient diet, she actually will leach calcium from her bones to make sure she provides enough to her fetus.
A functional calcium deficiency in a young, growing horse can cause issues in bone development. Even if you are feeding the proper amount of calcium in the diet, you can skew the ratio if you feed too many cereal grains high in phosphorus. In that case, bone could be replaced with connective tissue, and that would, of course, cause severe developmental issues.
Big head disease was commonly seen in decades past when mature draft horses were used for hauling. They would have diets high in grain with little pasture that resulted in an oversupply of phosphorus in the diet. This would cause a loss of minerals in the bone, which was especially noticeable in the head. The result was fiber filling in instead of bone, which is called fibrous osteodystrophy.
Dr. Rambo recalls investigating an issue with a herd of mature horses having thyroid issues. It wasn’t big head disease. In her sleuthing for answers, she found that the herd’s hay was very high in phosphorus and low in calcium, which is unusual. Further investigations found that the hay was fertilized with poultry litter, which is what caused the calcium/phosphorus changes in the hay.
Can Too Much Calcium Harm Your Horse?
Horse owners should not think that the only problems that can occur are with too-high phosphorus levels. Sometimes, owners are feeding far too much calcium.
Mature horses can tolerate up to 6 parts calcium to one part phosphorus. However, too much calcium in the diet significantly increases the risk of developmental orthopedic disorders, specifically OCD (osteochondrosis dissecans). Growing horses also can experience epiphysitis or physitis, which is inflammation of the growth plates on the ends of bones. These can show up as swelling in the knees or ankles, and it's accompanied by soreness.
Many times, too much calcium in a horse’s diet is due to the hay being fed. These imbalances are why hay analysis is so important. The amount of calcium that even a relatively small amount of alfalfa or clover in hay bring into that diet can unbalance the entire diet.
How To Supplement Calcium and Phosphorus for Horses
Basically, your horses are getting the majority of their calcium and phosphorus from the forage in their diets, which is often a combination of pasture and hay, but sometimes is all hay.
We need to consider pasture. For example, an overgrazed pasture in the Midwest could grow a lot of clover and be really high in calcium.
Therefore, you need to balance the rest of your horse’s diet based on the forage. If you have a primarily grass forage diet, you need one type of feed or supplement to balance that. If you have a predominately alfalfa or clover forage, you need a different type of feed or supplement to balance that diet.
Unfortunately, many of the feed ingredients that are high in phosphorus are also high in sugars and starches. Research has shown that feeding diets high in non-structural carbohydrates during gestation or to the growing horse can significantly increase the risk of the horse developing OCD lesions.
Don’t think that just because you are feeding a hay rich in alfalfa that you can just give rice bran and fix it. You might have addressed one part of developing a healthy skeleton and created another health issue.
Talk to an Equine Nutritionist
An equine nutrition specialist can be a horse owner’s best friend when it comes to deciphering the best diet for an individual horse. For example, developmental orthopedic disorders can have a genetic component. An equine nutrition specialist can help you minimize those risks through diet.
A big takeaway from this discussion of the ratio of calcium to phosphorus is to test your hay, which will allow a nutrition specialist to help you balance the rest of your horse’s diet.
Tribute Superior Equine Nutrition offers several ways to get help with your horse’s diet. Our team would love to help you create a personalized feed plan for your horse.
Related Horse Feeds and Products
Growth Pellet
- Growth Pellet is a low NSC (15%), higher fat (8%), high fiber (18%) feed specially formulated for young, growing horses beginning at 3 months of age, as well as pregnant and lactating mares that require more calories in their diet. Because of the low sugar and starch (NSC) level, this feed is appropriate for mares with metabolic disorders and mares with histories of producing foals with Developmental Orthopedic Disorders. Growth Pellet is packed with premium, essential nutrients to support proper bone, cartilage and tissue development, as well as pre- and probiotics to support digestive health. It is ideal for horses consuming hay or pasture that is 50% or less alfalfa (or other legumes).
Alfa Growth®
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Alfa Growth® is a higher fat (8%) feed specially formulated for young, growing horses beginning at 3 months of age, as well as pregnant and lactating mares that require more calories in their diet. This highly palatable, textured feed is packed with premium, essential nutrients to support proper bone, cartilage and tissue development, as well as pre- and probiotics to support digestive health. Alfa Growth® is ideal for horses consuming hay or pasture that is 50% or more alfalfa (or other legumes).
Growth Cubes
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Growth Cubes is a high fiber (18%) feed specially formulated for young, growing horses beginning at 3 months of age, as well as pregnant and lactating mares that require more calories in their diet. This highly palatable feed is packed with premium, essential nutrients to support proper bone, cartilage and tissue development, as well as pre- and probiotics to support digestive health. Growth Cubes is ideal for horses consuming hay or pasture that is 50% or less alfalfa (or other legumes).
For Canadian horse owners:
16% Growth Textured (Canada)
- 16% Growth Textured is a higher fat (8%) feed specially formulated for young, growing horses beginning at 3 months of age, as well as pregnant and lactating mares that require more calories in their diet. This highly palatable feed is packed with premium, essential nutrients to support proper bone, cartilage and tissue development, as well as pre- and probiotics to support digestive health. 16% Growth Textured is ideal for horses consuming hay or pasture that is 50% or less alfalfa (or other legumes).
