Phytase inclusion in monogastric animal diets is commonplace in the industry today, primarily driven by its ability to release phytate-bound phosphorus (P) from cereal-based feed ingredients. The benefits of phytase in monogastric systems have been well documented, with standard rates typically releasing sufficient levels of phytate-P to meet the animal’s requirements while simultaneously reducing inorganic P inputs at formulation. More recently, the practice of supplementing phytase in the diet at three to five times the recommended rate, commonly known as superdosing, has been used as a way to promote an extra-phosphoric effect in animals by alleviating the antinutritional effects of phytate in feed.
Why more phytase?
While the jury may still be out on the actual commercial benefits of superdosing, some published literature does suggest corresponding improvements in feed conversion ratio (FCR) and/or apparent metabolizable energy when phytase is supplemented at higher than recommended rates. Phytase activity does not respond in a linear fashion; doubling the phytase dose will not result in a doubling of enzyme efficacy. In fact, the numbers indicate that doubling a standard 500 FTU/kg dose will only result in a 30 percent increase in phytase efficacy, and doubling that dose again will halve that efficacy rate. So, if enzyme performance doesn’t necessarily go hand in hand with the amount of enzyme added, are there other limiting factors in the feed that are hindering phytase efficacy?
Not all phytases are created equal
There is a diverse range of phytase enzymes currently on the market, each typically derived from a different biological source. Whether these phytases come from natural fungal fermentations or heterologous expression systems defines their mode of action and dictates how and to what extent phytate can theoretically be hydrolyzed. The effects associated with superdosing may be achieved through extensive phytate hydrolysis, whereby the products of phytase-treated phytate (known as IP esters) are further hydrolyzed. Similar to phytate, IP esters still retain antinutritional properties, perhaps the most detrimental of which, from a dietary perspective, is the impairment of mineral absorption. Subsequently, phytase efficacy becomes a very relevant factor when trying to maximise the nutritional value of feed.
To achieve the effects of superdosing, optimal phytase efficacy is required. While enzyme source, feed grain, and calcium and phosphorus levels can be controlled during diet formulation, other essential feed components have been shown to interact with enzymes, influencing and typically hindering activity. Trace minerals are a prime example of this. In order to alleviate potential deficiencies and to maximize animal health and performance, minerals such as copper (Cu) and iron (Fe) are routinely supplemented to monogastric feeds, typically at levels exceeding National Research Council (NRC) recommendations. While the NRC updated the nutritional requirements for swine as recently as 2012, many of the recommendations made were based on research data from two decades ago. In that intervening period, the feed industry has moved forward considerably, particularly in the area of trace minerals.
Not all minerals are created equal
Feed minerals come in a wide variety of shapes and forms, commonly found as either inorganic trace minerals (ITMs) or organic trace minerals (OTMs). The underlying chemistry behind OTMs has caused confusion in the industry over the last few of years, fuelled largely by a multitude of differing terminologies and vague definitions. However, the generic term “OTM” typically refers to a mineral bound to organic molecules, such as amino acids or peptides. Enhanced stability is a key feature of OTMs, imparted by the manner in which the mineral is bound by the organic bonding group.
Mineral form impacts quality
Studies demonstrating improved mineral stability, deposition and bioavailability have resulted in a far better appreciation of the role and benefits of OTMs over their ITM counterparts. These benefits translate to reduced mineral inputs and better mineral efficacy. Interestingly, more recent research has demonstrated that dietary minerals can have antagonistic effects on phytase activity, with mineral form being a key determinant of enzyme efficacy. In vitro assessments on the influence of copper for five different commercial phytases indicated a considerable difference in enzyme function when exposed to either organic or inorganic copper analogues. In this study, the effect of OTMs on relative phytase activity equated to a 20 to 50 percent retention of enzyme function compared to equivalent recommended levels of ITMs. A similar response was observed with iron, whereby OTMs exhibited a considerably higher retention of phytase activity, in this instance anywhere from 10 to 50 percent activity depending on the phytase source. While these studies were carried out in vitro, the findings provide exciting insights for future in vivo feed trials assessing the impact of mineral-enzyme interactions on an animal performance level.
Understanding the importance of feed interactions
Depending on the mineral source utilized in the diet, the acidic conditions of the gastric environment can result in the release of mineral ions in their charged cationic form. While ITMs are readily dissociated, OTMs are affected to a lesser extent, depending on the bonding group used. In this dissociated form, mineral ions such as copper (Cu2+) and iron (Fe2+) are free to interact with a range of components, the most recognized of which is with phytic acid. This association forms an insoluble mineral-phytate complex, which significantly reduces mineral absorption and prevents phytase from binding to the substrate. However, the potential to interact with other feed ingredients is often overlooked, with charged mineral ions reacting with amino acids in phytase-active sites, which are responsible for substrate binding and degradation.
Before adding to excess, there needs to be clearer insights into how ingredients interact.
In reality, readily dissociated minerals may be having a dual effect on phytase efficacy, both directly on the enzyme’s active sites and also indirectly, by preventing phytase-substrate binding through the formation of mineral-phytate complexes. While this gives an interesting insight into future phytase research, the findings presented here demonstrate that mineral source has a considerable effect on phytase efficacy.
What does this mean for feed formulation?
As commercial profitability is being driven more and more by system efficiencies as opposed to individual unit sales, the traditional approach of adding any one feed ingredient to excess needs to be balanced with current insights into feed ingredient interactions. Trace mineral form has been shown here to considerably influence the efficacy of a number of commercial phytases, which in turn has the potential to reduce phosphorus release and impede mineral uptake. While phytase superdosing continues to be a debated topic within the industry, maximizing enzyme function by mitigating antagonist interactions between feed ingredients needs to be the first consideration for getting the most out of feed. The importance of mineral form reaches beyond enzyme function and can extend to other high-value feed ingredients, such as vitamins and antioxidants. These often antagonistic interactions suggest that there are steps that can be taken to better stabilize feed composition before resorting to supplementing any one ingredient to excess.
Advancements in organic trace mineral and exogenous enzyme technologies have helped the industry identify areas in the feeding process, as early as the formulation stage, that can be tailored to potentially improve not only feed efficiency, but also feed value.