Credit: Thinkstock Studies conducted by Kentucky Equine Research (KER) indicate that horses lose about 4.7 g of phosphorus each day and that the true digestibility of ingested phosphorus is only about 25%, meaning that 75% of ingested phosphorus is not used by the body.

Phosphorus is an essential mineral for horses, making up 14-17% of the equine skeleton. It plays an important role in enzyme systems, is an integral component of genetic material, and helps produce energy in the form of adenosine triphosphate (ATP).

According to Nutrient Requirements of Horses, published by the National Research Council, an average 500-kg (1,100-lb) horse needs 14.3 g of phosphorus per day. Studies conducted by Kentucky Equine Research (KER) indicate that horses lose about 4.7 g of phosphorus each day and that the true digestibility of ingested phosphorus is only about 25%, meaning that 75% of ingested phosphorus is not used by the body.

“Although phosphorus is required in the diets of horses, excessive amounts in the manure can become a water pollutant. Further, inorganic phosphorus, which is often used to supplement horses, is a dwindling nonrenewable resource,” warned Kathleen Crandell, PhD, an equine nutritionist at KER.

Therefore, scientists are investigating alternative sources of phosphorus. One option is organic phosphorus in the form of phytate-P.

“Phytate-P is found in seeds or grains consumed by horses,” said Crandell. “If the horse can liberate that bound phosphorus, it is possible that less inorganic phosphorus needs to be supplemented to fulfill dietary phosphorus requirements. Use of less inorganic phosphorus reduces pressure on the natural resources of phosphorus.”

With these facts in mind, researchers from the University of Kentucky evaluated phosphorus digestibility and the breakdown of organic phosphorus (phytate-P) by both young and mature horses.

“The goal of the study was to examine phosphorus digestibility and phytate degradation in horses fed a low-phosphorus diet with no added inorganic phosphorus while still meeting the horse’s daily requirements,” explained Crandell.

To do this, four mature geldings and four yearling Thoroughbreds were offered a concentrate feed containing an organic form of phosphorus (phytate) and no added inorganic phosphate. They were also fed timothy hay, alfalfa hay cubes, and water containing other organic orthophosphates. Feces was collected for four days to calculate the digestibility of phosphorus and phytate-P degradation.

The overall diet contained 0.28% phosphorus, of which 17.4% was phytate-P. According to the researchers, that total amount of phosphorus in the diet was equivalent to 25 g/day and 16 g/day in yearlings and mature horses, respectively. The overall digestibility of phosphorus was higher in yearlings than mature geldings (42% vs. 27%), and phytate-P disappearance in both groups of horses was 94.8%, meaning that most of the phosphorus in phytate-P was released and potentially available for the horses to absorb and utilize.

“Based on these results, the researchers concluded that organic phosphorus, including phytate-P, could help meet the phosphorus requirements of young and mature horses; therefore, less inorganic phosphorus is needed. Decreased inorganic phosphorus supplementation slows depletion of inorganic phosphorus sources and helps minimize excretion of excess phosphorus in the environment,” Crandell noted. “Further research would be beneficial in this field.”

*Fowler, A.L., T.L. Hansen, L.A. Strasinger, et al. Phosphorus digestibility and phytate degradation by yearlings and mature horses. Journal of Animal Science. In Press.

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