New insights into phytase inclusion in broiler diets
Phytase added to the feed increases digestibility of amino acids, protein and energy.
Phytate, the mixed salt of phytic acid (myo-inositol hexaphosphate), is found in all feedstuffs of plant origin, predominantly as magnesium-phytate complexes. Phytate is invariably present in practical broiler diets at concentrations of approximately 10.0g/kg or 2.8g/kg phytate-phosphorus (phytate-P). In poultry, phytate is poorly digested so the phosphorus (P) component of phytate is only partially available, and phytate also possesses anti-nutritive properties. Phytate is a poly-anionic molecule with a tremendous capacity to bind positively charged nutrients, which is the probable basis of its anti-nutritive properties. Increased utilisation of phytate-P in poultry will both reduce P pollution of the environment and facilitate the preservation of finite P reserves. On a global basis, poultry consume approximately 250 million tonnes of feed annually. This corresponds to 700,000 tonnes of phytate-P, which is the equivalent of 4 million tonnes of dicalcium phosphate.
The solution: add phytase to feed
A phytase of fungal origin was introduced in The Netherlands in 1991. After a slow start, the acceptance of phytase feed enzymes rapidly expanded and it is now estimated that around half of the diets offered to pigs and poultry globally contain an exogenous phytase added to the feed. Contributing factors include increasing legislation designed to curb P pollution, lower feed enzyme inclusion costs relative to inorganic P supplements and the removal of meat-and-bone meal from monogastric diets in certain countries.
In practical feed formulation, poultry nutritionists usually assign matrix values for P and calcium (Ca) under the assumption that typical phytase inclusion rates, e.g. 500 phytase units (FTU) per kg in broiler diets, contribute approximately 1.15g/kg P and 1.00g/kg Ca to the final feed. This adjustment to dietary P levels with phytase facilitates reductions in both feed costs and in the amount of P excreted, which is beneficial to the environment. Phytase feed enzymes may also have the capacity to improve protein and energy utilisation, which offers further potential to reduce feed costs.
Exploiting the full potential of phytase
The modern broiler chicken is a highly efficient converter of cereal grains and protein meals into meat. Presently, 2-kg broilers achieve a feed conversion ratio of 1.45, which correspond to a conversion of 2.014kg of feed into 1.0kg chicken meat (given a 72% carcass yield). However, rising feed ingredient prices are a mounting challenge to viable chicken meat production. This is being driven by an escalating demand for global food production for man and animals, coupled with the diversion of grain from the food chain to ethanol production. Indeed, global grain outputs have consistently failed to meet demand as reserves have fallen from 115 days in 1999-2000 to a projected 53 days in 2007-08. This represents an average annual depletion of global grain reserves of 9.2% since the turn of this century.
Given this adverse price and supply situation for feedstuffs, the approach of assigning matrix values for amino acids and energy to phytase deserves closer attention. Studies have shown no differences in growth performance and carcas traits between broilers offered control diets and phytase-supplemented diets with reductions in P, Ca, amino acids and energy density. It is equally instructive to compare amino acid matrix values usually assigned to phytase with the quantities of essential amino acids generated by phytase in ileal digestibility studies. The quantities of ileal digestible essential amino acids generated by phytase are substantially above the assigned matrix values, emphasising the economic potential of applying soundly developed nutrient matrix values for phytase to ration formulations.
Some fifteen years ago in pigs, Ted Batterham provided the first tangible indication that phytase enhances amino acid digestibility. Nevertheless, the capacity of phytase to enhance protein digestibility remains contentious and consequently, nutritionists are sometimes reluctant to incorporate amino acid matrix values into their dietary formulations. There are two reasons for this:
Under acidic conditions, phytate has the capacity to complex protein and importantly, complexed protein is resistant to pepsin digestion. On the basis of in-vitro data, phytate may complex up to half the protein present in a diet. However, the extent of in-vivo protein-phytate complex formation is probably influenced by several factors including gut pH and the source and solubility of proteins and phytates inherent in the diet.
It follows that protein-phytate complexes can impede protein digestion in the proventriculus/gizzard, with negative implications for protein/amino acid digestibility in the small intestine. It is also possible that the refractory nature of phytate-bound protein stimulates gastric secretions of pepsin and hydrochloric acid (HCl) as a compensatory mechanism. It has been demonstrated that phytate increases mucin secretion in broilers, which is consistent with additional gastric outputs of pepsin and HCl.
It has recently been established that phytate increases the flows of endogenous amino acids in broiler chickens. Elevated dietary phytate (8.5 to 14.5g/kg) increased the endogenous flow of seventeen amino acids by 27.2% and conversely, the addition of 500FTU/kg phytase reduced these flows by 20.0%. Also, it may be deduced that variations in endogenous amino acid flows induced by phytate and phytase are correlated to the amino acid profiles of pepsin and mucin. Because of its lack of basic amino acids, pepsin has a unique amino acid profile. Therefore, these significant correlations support the concept that phytate increases gastric secretions of pepsin, HCl and in turn, mucin. If so, increased secretions of mucin would, of itself, contribute to increased endogenous amino acid losses.
It has also been established in broilers that dietary phytate has the capacity to drag sodium (Na) into the small intestinal gut lumen and that this movement is counteracted by phytase addition to the feed. It is conceivable that phytate drags Na into the gut as sodium bicarbonate (NaHCO3) to maintain gut pH. It follows that this phytate-induced transfer of Na into the gut may compromise intestinal uptakes of amino acids via Na-dependent transport systems and has a negative impact on the ‘sodium pump'.
In a recent study, phytase increased the digestibility of thirteen amino acids by an average of 5.0% in broiler diets containing 1.5 and 1.8g/kg Na. In contrast, at a higher Na level of 5.2g/kg, phytase had no effect on amino acid digestibility. This discrepancy suggests that the sparing effect of phytase facilitated intestinal uptakes of amino acids at low Na levels, which was muted by high dietary Na levels. This supports the proposal that phytate and phytase, via their effects on Na secretion patterns, influence the absorption of dietary amino acids and re-absorption of endogenous amino acids.
Amino acid digestibility assays
In a recent summary of thirteen amino acid digestibility studies involving phytase in broilers, the results were inconsistent. It was evident that phytase responses are more pronounced when either titanium oxide or acid insoluble ash are used as dietary markers in contrast to the use of chromic oxide as a marker. In five studies using titanium oxide or acid insoluble ash, phytase increased ileal digestibility co-efficient by a weighted mean of 5.00% (range 3.46-5.92%). In contrast, phytase increased ileal digestibility coefficients by a weighted mean of 1.52% (range 0.47-3.09%) in eight chromic oxide studies.
Dietary marker selection has been an important cause of the ambiguity of phytase amino acid digestibility assays. Moreover, two research groups have considered that titanium oxide is a superior marker to chromic oxide. Consequently, chromic oxide studies should be treated with caution, leaving the balance of the research indicating that phytase is capable of enhancing ileal digestibility of amino acids in broilers.
In seventeen comparisons, phytase at an average inclusion rate of 662FTU/kg enhanced energy utilisation across a range of broiler diets by 0.36MJ from 13.27 to 13.64MJ/kg on a dry matter basis (or by 86kcal/kg from 3172 to 3260kcal/kg DM).
While similar energy uplifts are quite consistently recorded, the basis for these responses remains unclear. However, phytase addition increased the ileal digestibility coefficients of fat (3.5%), protein (2.6%) and starch (1.4%) in maize-soy broiler diets, suggesting that phytase has positive, additive effects on energy utilisation. As phytase enhances protein digestibility, a corresponding uplift in energy could be expected. Ca-phytate complexes are probably involved in the formation of metallic soaps in the small intestine, which would limit utilisation of energy derived from fats. By hydrolysing phytate in the upper gut, phytase decreases metallic soap formation and increases energy utilisation, particularly from saturated fats.
The situation with starch is less clear; while it has been suggested that phytate may complex starch, there is little evidence to support this argument. Phytate has the capacity to inhibit alpha-amylase activity but it is unclear if this is significant in broilers. Phytate has also been shown to reduce glucose absorption in humans offered glucose test meals, which raises the possibility that phytate directly interferes with small intestinal glucose uptakes rather than altering starch digestion.
There is a clear need to define the protein and energy effects of phytase accurately so that appropriate matrix values can be applied in feed formulation, thereby maximising feed cost savings. More effective phytase sources, with increased resistance to pepsin hydrolysis, is an exciting new development in this respect.