A Novel Phytase Enhances Growth Performance in Broiler Chickens Offered Reduced Crude Protein Diets

Phytate is ubiquitous in plant-based feed ingredients and phytase is routinely supplemented in poultry diets. Moreover, there is considerable interest within the chicken-meat industry in increasing dietary inclusion rates of crystalline or synthetic amino acids to develop reduced protein diets. An increasing array of both essential and nonessential non-bound amino acids is becoming commercially available at inclusion costs that are becoming increasingly feasible. Reduced CP diets reduces the industry’s dependency on imported soybean meals and was reported to have less nitrogen excretion (Nahm, 2007), enhance litter quality and reduce the incidence of footpad lesions. It is straightforward that non-bound amino acids partially replace soybean meal in reduced CP diets which leads to the lower concentration of phytate in the diets. Therefore, the intention of the present study is to evaluation the efficacy of a novel consensus bacterial 6-phytase variant in broiler diets containing conventional and reduced CP levels.

The feeding study complied with specific guidelines approved by the Animal Ethics Committee of The University of Sydney. A novel consensus bacterial 6-phytase variant (PhyG, Axtra® PHY GOLD, DuPont Animal Nutrition) was supplemented at 1500 FTU/kg to diets with three different CP concentrations (235, 215, 195 g/kg for starter; 215, 195, 175 g/kg for grower; 195, 175, 155 g/kg for finisher). For each phase, the six experimental diets were formulated to be iso-energetic and contained the same dietary levels of digestible lysine (12.8 g/kg for starter, 11.5 g/kg for grower and 10.2 g/kg for finisher), total sulphur amino acids (TSAA), threonine, and valine within each phase. The phytase supplemented diets were formulated with reduction of avP (1.95 g/kg), Ca (2.1 g/kg) and Na (0.45 g/kg). Starter, grower, and finisher diets were fed from 0 to 10 days, 11 to 28 days, and 29 to 41 days, respectively. The average analysed phytase activities in the control and supplemented diets were 200 and 1926 FTU/kg, respectively. All diets contained similar ideal protein ratios and were cold-pelleted. Starter diets were offered as crumble, whereas subsequent diets were in pellet form. Each of the six dietary treatments was offered to 8 replicate floor pens (15 birds per pen) or a total of 720 off-sex male Ross 308 chicks (parent line). Chickens had ad libitum access to feed and water. Initial and final body weights were determined, and feed intakes were recorded from which feed conversion ratios (FCR) were calculated. The incidence of dead or culled birds was recorded daily and their body-weights used to adjust FCR calculations. ANOVA and linear and quadratic correlation were performed using JMP® 13.0.0 and significance was determined at P < 0.05 by Student t-test.

Table 1 - Dietary compositions and calculated nutrient specifications in control diets

Table 2 - Growth performance in broiler chickens from 0-10, 0-28 and 0-41 days post-hatch

There was no interaction between phytase and dietary CP on growth performance (P > 0.05). Predictably, reducing dietary CP depressed FCR for all the three growing phases (P < 0.05) and moderate dietary CP reduction did not influence weight gain from 0-10 and 0-41 days posthatch (P > 0.05). Collectively, phytase supplementation improved weight gain and feed conversion regardless of the dietary CP levels. For instance, from 0-41 days, phytase supplementations improved weight corrected FCR by 0.56%, 3.68% and 2.14% in diets containing High, Medium and Low CP, respectively. It also increased weight gain by 0.98%, 3.07% and 2.47% in High, Medium and Low CP diets, respectively. It is encouraging phytase had more pronounced impact on growth performance in reduced CP diets compared to conventional diets, this may be attributed to increasing phytate to intact protein ratios (Table 3). Consistently, Liu and Selle (2017) reported increasing dietary phytate-P concentration from 2.6 g/kg to 3.0 g/kg in a diet containing higher concentration of crystalline amino acids reduced weight gain by 16% (1628 versus 1374 g/bird respectively), in comparison with a 5% reduction in weight gain when the same levels of phytate-P were added to a conventional diet; similar impact were observed on FCR where increasing phytate-P in high non-bound amino acid diet compromised FCR by 22% (1.130 versus 1.374 g/g respectively), but did not influence FCR in conventional diets. Phytate may influence protein digestion and utilization by forming binary or ternary phytate-protein complexes depending on the isoelectric points of the ingredients and environment (Selle et al., 2012). In the present study, the High, Medium and Low CP diets were estimated to contain 39.2, 42.8 and 47.7 g phytate relative to 1 kg of intact protein, respectively. Another possibility is phytate may have larger negative impact on amino acid and glucose absorption in reduced CP diets. Eighty per cent of dietary glucose is actively absorbed by Na+-dependent transport systems and phytate has been shown to decrease sodium-pump activity and glucose absorption in rats (Dilworth et al., 2005). The present study focused on growth performance only and further research is required to understand the impact of phytate on nutrient absorption and the benefit of phytase on nutrient utilizations in reduced CP diets.

Table 3 - Estimated phytate: intact protein ratios in grower diets

ACKNOWLEDGEMENTS: The authors would like to thank DuPont Animal Nutrition for funding this research project. We also acknowledge the technical assistance of Ms Joy Gill, Mr Duwei Chen, Ms Kylie Warr and Mr Peter Bird from Poultry Research Foundation for their assistance with feed manufacturing, bird management, sample collection and laboratory analyses.