The objective of this research was to determine the impact of feeding grow-finish pigs super-dosed levels of phytase on the digestibility of nutrients, and pig growth performance. The enzyme phytase is included in pig diets to improve phosphorus digestibility through the breakdown of phytate, thus decreasing the amount of inorganic phosphorus needed to be added to the pig’s diet to meet its phosphorus requirements. This allows producers to decrease ingredient costs as well as improve environmental sustainability, by lessening the amount of phosphorus excreted into the environment. In addition to these benefits, there has been considerable interest in super-dosing phytase, meaning adding phytase to diets at much higher levels than required to meet the phosphorus requirement, with the hope of further enhancing pig performance and nutrient utilization. The logic is that phytate binds more than just phosphorus, and adding very high levels of phytase could release these other nutrients to enhance pig performance. There have been previous studies on this subject, but the results have been highly inconsistent, making it difficult for producers to determine whether or not it is beneficial to include phytase in their diets at super-dosing levels.

In this study, 4 different levels of phytase, 3 of which were considered super-dosed, were used in 2 separate experiments in order to help determine whether it is biologically and financially practical to super-dose phytase. The first experiment was an intensive metabolism study that helped to further understand the mode of action of super-dosing phytase by looking at nutrient digestibility, at both the ileal and total tract levels. The second experiment was a large-scale commercial study to further determine whether there are growth performance benefits that may be gained from super-dosing phytase. This study was done to further evaluate the impact of super-dosing phytase at a commercial production level, and give producers further confidence in the results they can expect at their facilities. The combination of the two different experiments provides a deeper understanding of the mechanism of phytase when super-dosed.

However, at the completion of the two experiments, it was found that under the conditions of these experiments, super-dosing phytase provided no improvement in growth performance to the pig or in nutrient digestibility. Therefore, under the conditions of this study, super-dosing phytase is not recommended for the grow-finish phase of production. However, previous research in our laboratory found a definite response in the starter period.

Key Findings
• Superdosing phytase, up to 2,500 FTU/kg, did not improve either apparent ileal or apparent total tract digestibility of dry matter, energy, crude protein or fat.
• Superdosing phytase, up to 2,500 FTU/kg did not improve apparent ileal digestibility of most essential amino acids. However, there was a trend for phytase to improve ileal digestion of threonine, valine and tryptophan up to 1,750 FTU phytase/kg.
• The results of this experiment confirm that only about 92 to 95% of starch in corn is digested in the small intestine. Thus, about 5% of that starch is treated like fiber and is fermented in the large intestine, meaning it is used with less energy efficiency. Interestingly, larger animals digest starch in the small intestine more effectively than smaller pigs.
• The most noticeable impact of superdosed phytase was its breakdown of phytase into the inositol ring; in other words, it removed phosphorus from phytate in increasing amounts as the concentration of phytase increased.