Phosphorus in My Horse's Diet: What is it Good For?

Mares and foals have relatively high P requirements compared to a mature sedentary horse.

Photo: University of Kentucky's College of Agriculture, Food, and Environment

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.

Why is inorganic P added to horse feeds?

Research conducted in the 1970s concluded that horses could not absorb the P from grains and grain byproducts as effectively as they could P from inorganic sources (Hintz et al., 1973). Consequently, feed manufacturers began adding inorganic (rock) phosphate to concentrates to make sure horse received enough dietary P. However, if a horse has too much P in its diet, it will just excrete it into the environment. Researchers at the University of Kentucky (UK) are focused on finding a balance between feeding enough P for optimal health and production (growth, lactation, reproduction, performance, etc.) without overfeeding P, thus conserving this resource while reducing the environmental footprint of horse operations.

Can horses effectively utilize P from diets not containing inorganic P?

Results from a recent study conducted at UK has challenged the belief that horses cannot efficiently utilize organic sources of P (Fowler et al., 2015). In the study, a group of yearlings and a group of mature horses consumed a diet consisting of forage and a pelleted concentrate formulated to meet daily recommended P intakes with only the organic P found in the feed and no added inorganic P. The researchers collected feces from these horses to calculate digestibility of P. They found that horses were able to digest and absorb enough P to meet their requirements, even without the addition of dietary inorganic P.

The study also measured the degradability of a compound called phytate that binds P (phytate-P). Phytate-P is a common organic form of P found in many grains and grain byproducts that cannot be digested by simple-stomached animals. However, some gut bacteria are capable of releasing P from phytate for absorption. In this study, phytate-P was almost completely degraded (95% disappearance) in the feces, indicating that horses and their microbial communities can liberate much of the P associated with this molecule. Researchers concluded that both growing and mature horses can effectively utilize P from organic sources and might not need inorganic P added to their feeds to meet their requirements

Based on this research, supplementing horses with inorganic P might be unnecessary if other feed ingredients already contain adequate concentrations. However, it is important to realize that some horses, such as breeding and performance horses, have relatively high P requirements and that the P content of common equine feeds can vary greatly. For example, beet pulp and soybean hulls, common feed ingredients found in low-starch feeds, contain relatively low P (~0.1%). On the other hand, wheat bran and rice bran can have P concentrations upwards of 1.2%.

The diet’s forage component can also provide varying amounts of P. Certain areas of the United States, such as the northern and northwestern states, have low soil P, and forage grown in these areas also contain low P. Conversely, forage grown in areas with high soil P, such as states in the Southeast, will have greater P concentrations. The P concentration of the diet components ideally should match the P requirement of the horse. Mature nonbreeding horses have relatively low P requirements compared to lactating mares and growing horses. The P requirements of many mature nonbreeding horses can be met with the appropriate forage; additional concentrate is often not necessary. However, horses with high P requirements should receive feed concentrates with the appropriate P concentration to ensure they receive adequate phosphorus.

Currently, it appears that mature and growing horses can be fed diets without added inorganic P. However, more research is needed to examine whether horses with relatively high requirements truly need inorganic P added to their diets. If P needs can be met with organic sources of P, we can minimize the use of diminishing rock phosphate reserves and reduce excessive excretion of P into horse manure.


Ashley Fowler, MS, is a graduate student working with Laurie Lawrence, PhD, in the Department of Animal and Food Sciences at the University of Kentucky.

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