Phosphorous (P) is the second most abundant mineral in the body. Besides bone mineralization, phosphorous is involved in protein accretion and energy metabolism. Deficient levels can inhibit skeletal development and growth. Phosphorous is often the third most expensive diet component and excess levels can lead to increased diet costs. Exogenous phytase is commonly added to grow-finish diets to increase the availability of phosphorous which decreases the amount of mono or dicalcium phosphorus in the diet.

Here are three things to consider about P in diet formulation:

1. Determining Phosphorous values in diet formulation.
Phosphorous values can be assigned to ingredients as either total, available or digestible. Total P values are rarely used in diet formulation as it does not accurately represent the amount available for use by the pig and can cause deficiencies.

Available P is determined through the use of a slope-ratio assay method, in which a standard source of phosphorus is given a value of 100% and the availability of P in an ingredient is estimated as a relative percentage to the standard source. Although the slope ratio method seems to be more accurate to estimate availability of P, there is a concern about assuming 100% availability in the standard source. Also, determining the available P value for each ingredient is generally expensive.

Therefore, digestible P provides a better estimate of the amount of P being digested and absorbed. Digestible P can be expressed several ways. Two common ways are apparent digestible and standardized total tract digestibility. The standardized value is preferred because it accounts for endogenous loss of P.

2. Take formulation into consideration.
A digestible P:calorie ratio should be utilized when formulating diets as dietary energy concentration will influence the amount of feed consumed. Because of this, when using a standard ratio, the dietary level of P can be adjusted to account for changes in feed intake. In addition, as feed efficiency and lean growth improve, the dietary P requirement concentration will increase. When using phytase, P release values should be used to ensure accurate P levels are being formulated.

3. Understand the Ca:P relationship.
Both P and Ca are required for bone mineralization. Since Ca and P share an absorption pathway, excess dietary Ca can have an antagonizing effect on P digestion and absorption. A Ca:P ratio is commonly used in diet formulation to ensure excess Ca does not interfere with phosphorous absorption.

Calcium supplementation is low-cost and if not limited in diets, least-cost formulation will allow excess Ca into the diet to lower cost. This can lead to a wide Ca:P ratio that may lead to reductions in grow-finish pig performance especially when P is below the requirement. When diet P concentrations are adequate, increasing the Ca:P ratio can improve growth performance of grow-finish pigs, so Ca:P ratios should be formulated on a digestible basis. If digestible Ca values are not available, the ratio should be formulated on analyzed Ca:analyzed P ratio.

Digestible P values should be utilized when formulating a Ca:P ratio in diets and the amount of digestible P being provided should be adequate to meet P requirements.

Farm Journal Pork | Joel DeRouchey, Hayden Williams, Mike Tokach, Jason Woodworth and Bob Goodband | August