Microbial phytase in poultry nutrition

During the past decade, the inclusion of microbial phytase in poultry diets has increased remarkably, mainly in response to heightened concerns over phosphorus (P) pollution of the environment. The capacity of this feed enzyme to release phytate-bound P and reduce P excretion is now well documented. Effectively, phytase is an alternative, economical P source and, as global phosphate reserves are not renewable, this is beneficial to their preservation. Based on limited studies, it appears that exogenous phytase hydrolyses less than 0.35 of dietary phytate in broilers at the ileal level. If so, there is considerable scope to enhance phytate degradation by the introduction of more effective phytate-degrading enzymes or enzyme combinations, and facilitative nutritional and management strategies. Alternatively, dietary phytate concentrations may be reduced by the inclusion of selected, low-phytate feedstuffs or dephytinised feed ingredients. There is a distinct possibility that phytate negatively influences protein and energy utilisation in poultry and, as these influences would be ameliorated by phytase, there are substantial, practical implications. Nevertheless, there is still no consensus as to the extent that phytase enhances protein and energy utilisation. Responses in amino acid digestibilities following phytase supplementation are variable and the underlying mechanisms have not been completely understood; consequently, these two aspects are considered in detail in this review. The impact of phytase on protein and energy utilisation may be more positive than generally realised, but this should become increasingly evident if greater phytate degradation rates can be achieved. The experimental use of dephytinised feed ingredients may define the negative impact of phytate on protein and energy utilisation and facilitate the identification of the contributing factors, particularly in relation to energy utilisation. Some recent studies suggest that phytate increases, and phytase decreases, endogenous sodium losses. Although the basis for this phytate-induced shift of sodium into the gut lumen is not clear, it may have implications for acid-base homeostasis and intestinal uptakes of glucose and amino acids. If the momentum in the practical acceptance of microbial phytase in poultry diets continues, it is likely that phytase feed enzymes will re-define nutrient requirements, particularly in relation to P and calcium, and increasingly contribute to ecologically sustainable poultry production in the future. This would be facilitated by a more fundamental research focus, which, arguably, has been lacking in the past. (c) 2006 Elsevier B.V. All rights reserved.