Managing Dietary Phosphorus for Livestock and Poultry

Animal Manure Management February 17, 2015 Print Friendly and PDF

Phytate Phosphorus

Phosphorus is required in the diet of animals, but if overfed or wasted, can contaminate the environment and water supplies. Cereal grains fed to livestock contain phytate-bound phosphorus. Phytate-bound phosphorous is digestible by ruminant animals such as cows, sheep, and goats, but it cannot be digested by single-stomached animals, such as pigs and chickens. Phytate consists of a carbon ring structure with balanced phosphate groups surrounding the ring. Since horses are a hind gut fermenter, they are able to process the phosphorus much like ruminant animals.

Since phytate-bound phosphorous is unavailable to pigs, chickens, and other single stomached animals, phosphorous from other sources is supplemented to meet the needs of the animal. The extra phytate-bound phosphorus will be unavailable and excreted in the manure.

Reducing the proportion of cereal grains in the diet will usually reduce the amount of phosphorus fed. However, for pigs and chickens, and there are few economic alternatives to cereal grains. Plant breeders are working to develop feed grains lower in phytate content and higher in available phosphorus.

Phytase in the Feed

An enzyme called phytase can be included with the diet. Phytase will break down phytate and release digestible phosphorus. Mixing phytase (commercially available) in the diet will reduce the phosphorus required in supplements.

Interactions Between Nutrients

Another factor affecting phosphorus availability is the presence of other nutrients in the diet. Overfeeding calcium can limit the availability of phosphorus. Calcium and other nutrients should be fed in balance so as not to disrupt the availability of phosphorus.

Calcium:Phosphorus Ratio for Horses

Horses are a bit unique; they require calcium and phosphorus to be in a specific ratio in the diet. Young growing horses, as well as lactating mares, should receive a Ca:P ratio of 2:1, while mature horses not reproducing can get by with a 1:1 ratio. Calcium should never be fed at a level lower than phosphorus because phosphorus will tend to interfere with calcium absorption into bone. Horses at maintenance require .17% phosphorus in the diet and .24% Ca. The highest levels of phosphorus are needed in reproducing mares (.34%). Typical horse diets approach 2 to 3 times the required level of phosphorus, which can be detrimental to the environment. This high phosphorus level is partially due to the estimated Ca:P ratio in alfalfa hay being 6:1. Many horse owners try to counteract this by adding more phosphorus to the diets. Many equine supplements already contain more phosphorus than is necessary. There are also phosphorus concerns for ruminant animals such as cows, sheep, goats, and etc.

Ruminants and Phosphorus

Ruminant animals have a phytate enzyme produced naturally within the rumen that breaks down phytate-bound phosphorus and makes it available to the animal. According to the National Research Council, a lactating dairy cow requires between .35 and .40% phosphorus in the diet. Previous dairy feeding practices included as high as .55% or .60% phosphorus in the diet. This would mean an excess of 25 to 30 pounds fed to a cow in a normal lactation. If you multiply this over a dairy herd with 100 cows, then nearly 3,000 pounds extra phosphorous would be fed over the course of a year. Some dairy farmers think that phosphorus is a mineral required for proper reproductive function. While phosphorous is indeed important for normal bodily functions and is important for reproduction just as all nutrients are important for reproduction, there is no special link between phosphorus and reproduction in a cow. Most dairy farmers have already reduced phosphorus in their diets to levels given by the National Research Council.

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.