The application of exogenous enzymes to counteract anti-nutritive properties of targeted dietary components and improve broiler growth performance is routinely practiced. The performance of broilers offered sorghum-based diets may be inconsistent and is not usually enhanced by non-starch polysaccharide-degrading enzymes. However, amongst cereal grains, sorghum contains relatively high phytate concentrations (Selle et al., 2003) and it follows that broilers on sorghum-based diets should be advantaged by phytate-degrading enzymes. Nevertheless, there are indications that, in poultry, responses to phytase with sorghum are less than with other cereals (Wu et al., 2004; Pourreza and Ebadi, 2006). To investigate this aspect, the present study determines the effects of phytase in sorghum-based broiler diets with substantially lower phosphorus (P) concentrations generated by reduced dietary inclusion levels of meat-and-bone meal.
II. MATERIALS and METHODS
Positive control (PC) diets formulated to commercial standards were offered to an initial total of 840 male Cobb chicks on deep litter, from 1-14, 15-28 and 29-42 days post-hatch. The diets contained averages (g/kg) of 571 sorghum, 197 soybean meal, 100 canola meal, 58 meat-and-bone meal (MBM) and 25 rice bran. The PC diets contained 76, 54 and 44 g/kg MBM which was reduced to 20 g/kg in the starter diets and eliminated in the grower and finisher negative control (NC) diets. Compensation was not made for the analysed 36 g/kg P in MBM; consequently nonphytate-P levels were reduced from 4.40 to 2.51, 3.62 to 1.79 and 3.21 to 1.72g/kg in the three dietary phases. The NC diets were supplemented with 500 and 750 FTU/kg Escherichia coli-derived phytase (Phyzyme® XP) and each of the four dietary treatments were offered to six pens of 35 birds from 1-42 days post-hatch to determine their effects on growth performance. From 15-28 days post-hatch, five birds from each pen were relocated in cages in an adjacent facility for total collection of excreta to determine apparent metabolisable energy (AME) and nitrogen (N) retention. At day 28, the birds were euthanised and ileal digesta samples collected to determine apparent ileal digestibility (AID) of amino acids. More details for the bioassay methodologies adopted in the resent study are outlined by Selle et al. (2006).
III. RESULTS and DISCUSSION
Treatment effects on growth performance and nutrient utilisation are shown in Tables 1 and 2. From 1-42 days, reduced dietary specifications in the control diets significantly depressed weight gain by 20.3% and feed intake by 17.9% and tended to depress feed efficiency. On average, phytase inclusions in NC diets significantly improved weight gains by 20.7% and feed intakes by 21.9% but not feed efficiency. However, on the basis of gain-corrected feed conversion ratios (FCR), phytase improved feed efficiency by 8.5%. Phytase significantly increased AME by 0.29 MJ (2.0%); however, dietary treatments did not influence N retention. Also, phytase reduced mortality rates of birds offered NC diets from 6.7 to an average of 1.9%, which may have been related to the liberation of phytate-bound P.
Table 1. Effects of phytase supplementation (500, 750 FTU/kg) on growth performance of broilers offered modified (NC) sorghum-based diets
Table 2. Effects of phytase supplementation on AME, N retention, gain-corrected FCR and mortality rates of broilers on modified (NC) sorghum-based diets
The effects of phytase on AID coefficients of 15 amino acids where acid insoluble ash was used as the dietary marker are shown in Table 3. It is evident that amino acid digestibility was higher in the NC than PC diets which may reflect the lower amino acid digestibilities of MBM in the NC diets. Phytase linearly increased (P < 0.05) the digestibility of isoleucine, methionine, valine and tyrosine and tended to enhance (P < 0.10) the digestibility of arginine, histidine, leucine, phenylalanine, threonine, alanine and glutamic acid.
Table 3. Effects of phytase supplementation AID coefficients of amino acids in broilers offered modified (NC) sorghum-based diets
As a result of lower MBM inclusions in NC diets the average reduction in non-phytate P levels across the three diets was 1.74 g/kg, which is substantial. This is reflected in the marked depressions in weight gain and feed intake between PC and NC diets to 42 days; however, phytase supplementation of NC diets essentially reversed these reductions. NC diets contained an average of 3.15 g/kg phytate-P (or 11.2 g/kg phytate) of which 41% was derived from sorghum. Assuming phytase degraded 45% of dietary phytate this would release 1.42 g/kg P to compensate for non-phytate P reductions. This confirms the capacity of phytase to degrade phytate in sorghum-based diets and, under in vitro conditions, Chivandi et al. (2010) reported that phytase released fractionally more inorganic P from sorghum than maize.
While the capacity of microbial phytase to degrade phytate and liberate phytate-bound P is fundamental, the negative influences of phytate on nutrient availability are not limited to P. Indeed, they extend to amino acids, energy and other minerals and this forms the basis for the so-called "extra-phosphoric" effects of phytase (Ravindran, 1995). While AME was enhanced in the present study, phytase did not significantly improve the AID of the majority of amino acids or N retention. This raises the possibility that the "protein effect" of phytase was muted. The negative impact of phytate on protein/amino acid digestibility and its attenuation by phytase has received considerable attention in poultry (Selle and Ravindran, 2007) and is probably pivotal to the extra-phosphoric effects of phytase. In sorghum, the dominant protein fraction is kafirin and it may represent up to 70% of total protein (Hamaker et al., 1995). However, kafirin is hydrophobic, deficient in basic amino acids, with extensive disulphide linkages in the β-and γ-fractions located in the periphery of protein bodies in sorghum endosperm (Selle, 2011).
Logically, the extent to which phytate interacts with kafirin protein is crucial to the magnitude of any protein-related, extra-phosphoric responses to phytase in sorghum-based diets. While an alternative has been proposed (Cowieson and Cowieson, 2011), the accepted mechanism for direct protein-phytate interactions was initially outlined by Cosgrove (1966). Proteins carry a net positive charge under acidic conditions when pH is less than the isoelectric point of the protein. The negatively-charged phytate molecule may bind protein in binary protein-phytate complexes via basic amino acid residues (arginine, histidine, lysine). Moreover, proteins bound in insoluble protein-phytate complexes are refractory to pepsin digestion at very low pH levels (Vaintraub and Bulmaga, 1991). However, the paucity of basic amino acids in kafirin suggests that phytate may not readily bind kafirin in binary complexes. Thus, if phytate interactions with kafirin are limited, the magnitude of extra-phosphoric responses to phytase in sorghum-based diets would be less pronounced as a consequence. This would not, however, impact on the phytase-induced liberation of phytate-P in sorghum-based diets. It is possible that phytase supplementation of sorghum-based diets would be advantaged by the simultaneous inclusions of proteases and/or reducing agents to target the kafirin fraction of sorghum protein.
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