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The impact of phytate as an anti-nutrient in poultry diets

The role of phytates in reducing the availability of phosphorus (P) within plant-based feed ingredients is widely recognised, with the use of phytase feed enzymes to release phytate-P and reduce feed costs almost ubiquitous. Yet the significant negative impact of phytate is much less understood.

Phytate itself, also sometimes referred to as phytin or phytic acid, is the major storage form of P in seeds, and is part of a complex that also contains potassium, magnesium and calcium. 
Originally recognised as a key source of P during seed germination, the presence of phytate is now also known to play an important role in reducing oxidative stress during the germination process, preventing plant embryo death.

Phytate anti-nutrient effects

In animal nutrition, however, phytate has long posed a challenge. Initially viewed as an extremely valuable potential source of P that could be made available through targeted use of phytase feed enzymes, the focus in recent years has shifted to the anti-nutrient role phytate plays within the digestive tract. This anti-nutrient effect is substantial, and clearly demonstrated by the significant reductions in performance seen in Figure 1, where normal (high phytate) and genetically modified (low phytate) cereals were compared using 0 to 21-day-old broilers. The higher phytate content in the unmodified grains increased feed conversion ratio (FCR) by 4.6% in the corn-based diets, and reduced bodyweight gain (BWG) in the barley-based diets by 5%. There are several modes of action by which phytate has this negative effect, though all act to reduce digestibility and utilisation of important nutrients supplied in the diet. Key amongst these is the ability of the phytate molecule to bind with both proteins and other minerals present in the gastro-intestinal tract, resulting not only in reduced availability, but also detrimental endogenous responses by the bird.

Figure 1 - Bodyweight gain (BWG) and mortality-corrected feed conversion ratio (FCR) in 0 to 21-day-old broilers fed cereals with normal or low phytate contents. (see larger image)

Lower mineral availability

Even though phytate has a higher affinity for cations such as copper and zinc, it is the molecule’s affinity for calcium that is of greatest concern in poultry nutrition. For example, it has been shown that the dietary calcium requirement for broilers increases from 0.60% to 0.95% when the phytate-P level in the diet increases from 0 to 0.25%, the latter being approximately equivalent to the phytate concentration found in a standard corn-soybean diet. The impact on other minerals can be seen when assessing the impact of progressive phytate reduction through application of increasing doses of phytase feed enzyme. The results in Figure 2, for example, show how yolk selenium content rises when a diet containing no phytase (the positive control) diet is supplemented with a commercial phytase (Quantum Blue) at 700, 1400 and 2100 FTU/kg feed. Note that these dose rates are well above the standard dose of 300 FTU/kt studies

Figure 2 - Impace of phytate elimination on selenium nutrition in layers. (see larger image)

showing reduced pepsin activation between pH 0.8 and 2.8 in the presence of phytate. Since the pH of the stomach contents is typically between pH 2.0 to 3.0, it is likely that the subsequent reduction in pepsin activity results in less protein being initially broken down during the acid phase of the digestive process, leading to a lower overall protein digestibility. Whilst this reduced pepsin activation may be overcome by higher production of its precursor, pepsinogen, in the stomach, phytate presence also directly reduces protein solubility and subsequent digestibility. The presence of phytate can reduce the activity of the sodium / potassium ‘pump’, which is crucial to amino acid uptake across the gut wall.

Phytate elimination leads to better nutrient use
of animal feed and savings in costs.

The reduction in protein digestibility also has a further negative effect on the bird. The resulting rise in the level of undigested protein reaching the duodenum can increase secretion of the hormones gastrin and cholecystokinin, stimulating additional production of hydrochloric acid (HCl) and pepsinogen in the stomach whilst reducing gastric emptying. The result is a substantial increase in endogenous losses. A greater amount of sodium bicarbonate has to be secreted into the duodenum to neutralise gut content pH, whilst the additional irritant effect on the gut mucosa leads to extra mucus production to maintain protection levels.

Phytate elimination benefits

Perhaps the clearest indication of the impact on performance from the anti-nutrient effects of phytate comes from trials where phytate is effectively eliminated through phytase ‘superdosing’. The results in Figure 3, for example, show that increasing incremental levels of phytase by 500 FTU/kg improves bodyweight corrected FCR linearly, with the 1500 FTU/kg dose showing a 4 point improvement when compared to a positive control (PC) diet formulated to meet the nutrient needs of the bird. This response to superdose levels of phytase can be attributed to phytate breakdown and not P release since there was no further improvement over the PC when inorganic P was added to the diet.

Critical phytase differences

However, not all phytases are sufficiently active at the pH found in the stomach – or able to continue degrading phytate towards elimination as concentrations fall – to be effective for such superdosing. However, the results in Figure 3 clearly show the efficacy of Quantum Blue phytase in producing performance benefits beyond simple mineral supply (the standard dose equals the positive control performance) through phytate elimination. The net effect is a four point improvement in FCR beyond that achieved by the positive control or the standard phytase dose, equivalent to a cost saving of €4‑6/tonne of feed. It is a gain that has far reaching implications, with estimates suggesting the anti-nutritional effects of phytate could be costing the global monogastric feed industry over €2 billion/year.

Figure 3 - Analysis of 0-35/42d bodyweight-corrected feed efficiency with increasing doses (composite analysis of 6 trials). (see larger image)

As such, there is little doubt that as awareness of the negative impact of phytate on bird performance and profitability increases, so too will the use of high doses of phytase to achieve phytate elimination. The arrival of commercial phytases like Quantum Blue specifically developed to maximise phytate destruction will also help to ensure end-users are able to achieve consistent results, a factor likely to be critical to the widespread uptake of superdosing and to reclaiming the revenue currently lost to the anti-nutrient phytate.

This article was featured in World Poultry magazine no. 5 - 2015 - To read more published articles see World Poultry Digital

Dr Gilson Gomes, AB Vista Global Technical Manager

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