Dietary phosphorus in poultry: how low can we go?

Phosphorus − one of the most expensive nutrients in poultry feed − is an essential element for bodily functions, bone and egg shell formation. However, excessive faecal excretion of phosphorus has increasingly become an environmental issue due to the oversupply of phosphorus in layer rations.

Supplementing with microbial phytase helps to minimise excessive excretion by unlocking the plant-bound phosphorus in the feed ingredients. When poultry can use more phosphorus from the feed itself, less phosphorus needs to be supplemented in the diet (in theory). This will decrease the risk of oversupply and associated feed costs, and will put less pressure on the environment.

Reducing dietary non-phytate phosphorus

A group of researchers affiliated with the University of Manitoba in Canada hypothesised that phytase supplementation, along with properly reducing dietary inorganic phosphorus levels, would reduce phosphorus excretion and achieve comparable performance compared to birds fed high-phosphorus diets devoid of phytase.

So, a study was conducted to determine the effects of microbial phytase supplementation to diets containing levels of dietary non-phytate phosphorus (NPP) lower than the NRC requirement (2.5 g/kg) on production performance, plasma biochemistry, egg and bone quality and phosphorus excretion in laying hens.

A 12-week feeding trial (22-34 weeks of age) was set up with 48 Lohmann white laying hens. The birds were randomly allocated to one of 6 maize-soybean meal-oat-based diets: containing 2.0, 2.5 or 3.0 g/kg NPP without phytase, or containing 1.0, 1.5 or 2.0 g/kg NPP with phytase (1,000 U/kg diet) where phytase inclusion was expected to provide 1.0 g/kg NPP to laying hens, thus making the phytase-unsupplemented treatment serve as a control for the phytase-supplemented treatment.

Results with and without phytase

​Egg weight and egg production, feed consumption, bodyweight and the feed conversion ratio of laying hens fed lower NPP diets supplemented with phytase were comparable to those of hens fed high NPP phytase-unsupplemented controls.

Eggshell thickness, specific gravity, Haugh units, tibia bone mineral density, tibia ash percent, plasma phosphorus and other biochemical parameters were not significantly different between the dietary treatments. Total phosphorus intake, excretion and retention were affected by diet (P<0.001) but its deposition in eggs was not significantly different.

Contrast analysis further showed that total phosphorus excretion of phytase present vs. phytase absent was on average reduced by 40.4 mg/hen per day (P<0.01). Moreover, total phosphorus excretion was linearly (P<0.01) reduced by reducing dietary NPP and this relationship was similar regardless of whether phytase was supplemented or not.

Conclusion

The results from this study indicated that NPP levels in laying hen diets could be reduced to 1.0 g/kg (excluding the portion of NPP released by phytase) with the inclusion of phytase without negative effects on the production performance and health of the hens, thus reducing phosphorus excretion into environment.