Most horse owners know their horses need dietary calcium (Ca) and phosphorus (P) in the right amounts to maintain a healthy skeleton. Phosphorus in bones not only provides structural support for the skeleton, but it also acts as a reserve of P for other bodily functions. Phosphorus is important in cell membranes and in reactions requiring cellular energy. Phosphorus also helps form the backbone of DNA and contributes to the pH and electrolyte balance in body fluids. In a nutshell, P is an essential nutrient that animals cannot live without.

Dietary P comes from many common feed ingredients, including forages, oats, corn, and soybean meal. The P found naturally in grains and forages is considered organic; feed manufacturers might also add inorganic P to commercial horse feeds. Inorganic P sources are most often listed on a feed label as monosodium phosphate; mono-, di-, and tri-calcium phosphate; and defluorinated phosphate. These inorganic phosphates come from mining and processing rock phosphate to make them acceptable for animal consumption.

Adding inorganic phosphate to feeds to ensure adequate P intake might seem good for the horse, but it’s potentially harmful for the environment. Currently, only a handful of rock phosphate mines remain in the world, and the raw supply of phosphate is dwindling. Because P reserves are decreasing, more attention is being placed on conserving this nonrenewable resource. Further, P excreted in animal manure can be an environmental issue. Phosphorus from animal manure can run off or leach into nearby water bodies where P-hungry algae consume it and grow excessively.

Consequences of these algae “blooms,” or eutrophication, include reduced oxygen available for aquatic life, the death of oxygen-requiring species, and ecological disruption. To reduce eutrophication due to P runoff, farmers are being encouraged to reduce their animals’ P excretion and implement a variety of other best management practices. Most of the concern about P in animal manure has focused on cattle and swine operations, but the potential for P runoff exists in areas of dense horse population, as well