Microbial phytase is often used in diets fed to pigs because it is well established that the digestibility of phosphorus in many ingredients is increased if microbial phytase is used in the diet.
The reason for this increase in digestibility is that microbial phytase has the ability to release some of the phosphorus that is bound in the phytate complex in many feed ingredients.
Without phytase in the diet, most of the phytate-bound phosphorus is not digested by the pigs and is instead excreted in the manure.