By Suzie Middlebrook | B.Sc., Cavalor Nutrition Specialist

When formulating the perfect diet for your horse, one should look at the total intake of nutrients as well as some of the ratios of those nutrients compared to other nutrients. While there are several important nutrient ratios in the world of equine nutrition, such as iron:copper (4:1) or calcium:magnesium (2:1), the most talked about ratio in equine nutrition is the calcium:phosphorus ratio. Often times you will hear of the well-meaning horse owner calculating endlessly to get the perfect calcium:phosphorus ratio for their horse’s diet. So what is the calcium:phosphorus ratio and why is it important?
Starting at the basics, a ratio is a relationship of two numbers that helps convey the proportions of those items in a whole. As an example, if you purchase a concentrated fly spray that must be diluted by 10 parts water for every 1 part fly spray concentrate, then the water:fly spray concentrate ratio would be a ratio of 10:1. This is similar for the calcium:phosphorus ratio.

The safe calcium:phosphorus ratio in the diet of a mature horse is actually a range of 1:1 to 6:1, though the ideal is generally recognized to be 2:1. If the calcium:phosphorus ratio of your entire ration, including hay, grass, grain, and supplements, is between 1.5:1 and 2.5:1, that is a very appropriate ration (Sonntag
et al., 1996).

When the Ca:P is too low and there is too little calcium in the diet, the horse can develop enlarged joints or even crooked long bones in growing horses (Knight et al., 1985). In contrast, a diet low in phosphorus can cause the horse to develop rickets, which is a softening and weakening of the bones. This can lead to detrimental skeletal issues, such as crooked legs or a roach back. Scientists are also beginning to look at high calcium levels being linked to ulcers, though more research is needed to confirm that link (Dufner et al., 2005). High calcium levels can also impede the absorption of phosphorus in the diet. This can result in symptoms similar to a horse received a diet low in phosphorus despite the daily recommended value being met.

On the topic of osteochondritis dissecans, or OCD, there is quite a bit of misinformation. There is a perpetuated idea that a high level of calcium can cause osteochondritis dissecans (OCD) in growing horses, though this only seems to be the case if there is an insufficient level of phosphorus in the diet (Jordan et al., 1975). Interestingly, very high levels of phosphorus multiple times above the recommended daily intake causes a very significant increase of OCD incidence in growing horses as well (Savage et al., 1993). Ideally, the Ca:P for young horses should mimic that of milk, which ranges from 1.8:1 to 2.5:1, to avoid potential development issues (Sonntag et al., 1996). No current research has linked diets high in either crude protein or nonstructural carbohydrates to increased OCD incidence (NRC 248).

Dufner, M. M., P. Kirchhoff, C. Remy, P. Hafner, M. K. Muller, S. X. Cheng, L. Q. Tang, S. C. Hebert, J. P. Geibel, and C. A. Wagner. 2005. The calcium-sensing receptor acts as a modulator of gastric acid secretion in freshly isolated human gastric glands. American Journal of Physiology: Gastrointestinal and Liver Physiology. 289:G1084-G1090.

Jordan, R. M., V. S. Meers, B. Yoho, and F. A. Spurrell. 1975. Effect of calcium and phosphorus levels on growth, reproduction, and bone development of ponies. Journal of Animal Science. 40:78-85.

Knight, D.A., A. A. Gabel, S. M. Reed, L. R. Bramlage, W. J. Tyznik, and R. M. Embertson. 1985. Correlation of dietary mineral to incidence and severity of metabolic bone disease in Ohio and Kentucky. P. 445 in Proceedings of the 31st American Association of Equine Practitioners, New Orleans.

NRC (National Research Council). 2007. Nutrient Requirements of Horses, 6th edition. Washinton, DC: National Academy Press.

Savage, C. J., R. N. McCarthy, and L. N. Jeffcott. 1993. Histomorphometric assessment of bone biopsies from foals fed diets high in phosphorus and digestible energy. Equine Veterinary Journal Supplement. 16:80-83.

Sonntag, A. C., H. Enbergs, L. Ahlswede, and K. Elze. 1996. Componenets in mare’s milk in relation to stage of lactation and environment. Pferdeheikunde 12:220-222.