Exogenous Phytase Enhances Weight Gains and Increases Ileal Amino Acid Disappearance Rates in Broiler Chickens

The inclusion of exogenous phytases in diets for broiler chickens has developed into a routine practice because of their positive phosphoric and extra-phosphoric effects. On one hand, phytase liberates phytate-bound phosphorus (PP) which is found in all plant-sourced feedstuffs as the dominant P fraction. On the other, phytase enhances amino acid (AA) digestibilities, essentially because they prevent the de novo formation of binary protein-phytate complexes in the gut (Selle et al., 2012). Whilst several phytases are available in Australia, the exogenous phytase used in this study is a new entrant into the Australian market. Thus, the purpose of this study was to evaluate this exogenous phytase in respect of growth performance, bone mineralisation and AA disappearance rates from the ileum in broiler chickens offered broiler starter crumbles from 1 to 21 days post-hatch using typical Australian-sourced feed ingredients.

 

The experimental protocol was approved by the Animal Ethics Committee of The University of Sydney. A total of 960 day-old male Ross 308 birds were randomly assigned to 4 treatments with 6 replicates of 40 birds per pen. The design was a 2 × 2 factorial array of treatments with wheat-based diets, either a positive (PC) or a negative control (NC), without and with a bacterial (Escherichia coli) phytase expressed in Pichia pastoris (Microtech 10000 plus; Guandong VTR Biotech Co Ltd) at an inclusion of 1000 FTU/kg. The NC diets were formulated with reductions of 1.6 g/kg Ca, 1.3 g/kg P and 0.3 g/kg Na relative to the PC diets as shown in Table 1. The diets were steam-pelleted at a conditioning temperature of 80ºC and phytase activities analysed. The two non-supplemented diets contained an average of 354 FTU/kg whilst the two supplemented diets contained 1134 FTU/kg phytase activity, post steam-pelleting. Growth performance was monitored from 1 to 21 days post-hatch. At the conclusion of the starter phase, six birds per replicate pen were euthanased and digesta samples from the distal ileum and toes taken to determine apparent AA digestibility coefficients (ADC) and bone mineralisation by standard procedures. AA disappearance rates (g/bird/day) were calculated from the following equation: disappearance rate_(g/bird/day) = dietary AA concentration_(g/kg) x daily feed intake_(g/bird) x ADC

Experimental data was subjected to two-way analyses of variance using the IBM® SPSS® Version 24 statistical package.

III. RESULTS AND DISCUSSION

Overall growth performance of birds in the present study compared favourably with the 2019 Ross 308 performance objectives for 1 to 21 days post-hatch. The transition from PC to NC diets depressed weight gain by 2.11% (976 versus 997 g/bird; P < 0.05) and phytase increased weight gain by 1.33% (988 versus 975 g/bird; P < 0.025) as shown in Table 2. Phytase similarly increased weight gains in both PC and NC diets by 2.44% and 2.18%, respectively and increased feed intake by 1.82% (1175 versus 1154 g/bird; P < 0.05). The transition from PC to NC diets compromised FCR by 1.71% (1.192 versus 1.172; P < 0.005). The 1.46% mortality rate was not influenced by treatment. A treatment interaction (P < 0.05) was observed for toe ash but the likelihood is that the overall result of 12.84% was indicative of adequate bone mineralisation. Dietary treatments on apparent ileal disappearance rates of sixteen AA are shown in Table 3 where treatment interactions were observed for lysine (P < 0.05) and proline (P < 0.05). Given this caveat, phytase significantly increased all AA disappearance rates culminating in a 4.67% increase (10.31 versus 9.85 g/bird/day; P < 0.001) in total disappearance rates. The transition from PC to NC diets significantly increased disappearance rates of arginine, histidine, leucine, methionine and threonine but decreased that of tyrosine.

Phytase increased ileal AA disappearance rates or, effectively, their uptakes from the small intestine. The likelihood is the result of phytase enhancing both protein digestion by retarding the formation of binary protein-phytate complexes which are refractory to pepsin digestion and AA absorption. Phytase has been shown to have positive impacts on sodium pump activity along the small intestine in poultry, which would facilitate the co-absorption of AA and sodium via several Na⁺-dependent transport systems (Selle et al., 2012; Akter et al., 2019).