Drs Hardy M. Edwards, Jr. and Gene M. Pesti of the Poultry Science Department of the University of Georgia have investigated possible causes of the variable responses to phytase supplementation of the diet on energy and protein utilisation in a research project sponsored by the US Poultry and Egg Association.

Almost all broiler feeds in the United States are supplemented with phytase. Phytase's primary function is to liberate phosphate from phytic acid but it has other properties as well. Under the right conditions, phytase may increase amino acid digestibility and therefore the energy birds can derive from the feed. There may be other positive effects of phytase on carbohydrate and fat absorption as well. For instance, if phytase increases calcium absorption, then fat absorption should increase (reduced calcium soap formation).

The value of phytase remains controversial because the conditions to get the maximum benefit from phytase are unknown. There is considerable published information indicating that phytase supplementation may increase the amount of energy that is utilised from the feed by the chicken. However, there are also reports of studies where phytase supplementation had no effect on energy utilisation.

The first objective of the Georgia scientists was to determine if there are differences in amino acid digestibility in soybeans processed by different methods. Three samples were obtained to investigate the influence of soybean processing method on the chicks' response to phytase supplementation. Corn (53.61 per cent), soybean meal (37.47 cent), and soybean oil (5.49 cent) based diets (with 0.1 per cent chromic oxide) were fed to three replicate pens of 10 broiler chicks each in battery brooders for 16 days. Solvent extracted, expeller or extruded soybean meals were substituted for 50 per cent of the entire diet. When the amino acid digestibilities were considered together in one ANOVA (66 error degrees of freedom), there were clear differences in amino acid digestibilities due to soybean processing method. The strongest effect of phytase was on lysine digestibility, one of the most critical nutrients in feeds.

The second objective of the work was to determine the response of chicks to phytase supplementation when fed five different corn cultivars. Five hybrid 'non-transgenic' identity-preserved corn cultivar samples from the 2002 growing season with a variety of genetic backgrounds were obtained to test the hypothesis that variation in corn may be responsible for variation in the responses to phytase supplementation observed in broiler chickens. The cultivars were grown in three different locations, to provide a good geographical range.

Edwards and Pesti report that they could detect significant differences in the way chicks fed the different cultivars responded to phytase supplementation but the differences were very small. It was concluded that there were no large differences in the responses to phytase supplementation for the five corn samples tested. There were, however, clear differences in the incidence of phosphorus-deficiency rickets among the chicks fed the different corn samples.

The amino acid digestibility values that nutritionists use should be different for different sources of soybean meal. It is important that nutritionists not rely solely on published digestibility values. Diligence is required to have consistently accurate feed formulations.

It is concluded that the different results from phytase supplementation observed by researchers around the world may indeed be due to soybean processing methods. Corn cultivars definitely influenced the incidence to phosphorus-deficiency rickets and phytase effects, but the magnitude of the differences due to corn × phytase interactions appears to be very small.

December 2009