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 (Létourneau-Montminy et al., 2012). Therefore, 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 (Stein, 2016). When diet P concentrations are adequate, increasing the Ca:P ratio can improve growth performance of grow-finish pigs (Vier et al., 2017). Furthermore, 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.
In summary, 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.