Improving soybean digestibility with mono-component protease

Exogenous feed enzymes were first introduced as commercially-relevant feed additives in the 1980s with an initial focus on reduction of the antinutritional effects of high molecular weight soluble pentosans in wheat- and barley-based diets for young broiler chickens (Bedford & Partridge, 2010). Exogenous phytase was launched in the early 1990s to increase the digestibility of organic phosphorus and to reduce the antinutritional effects of phytic acid (Selle & Ravindran, 2007). Subsequently there have been a plethora of additional feed enzyme launches including amylase, pectinase, alpha-galactosidase, beta-mannanase, protease and others (Bedford & Partridge, 2010). In general, these enzyme classes do not compete with one another for substrate but the cumulative response to their addition for a given diet tends to be sub-additive due to overlap in effect on released nutrients e.g. energy, amino acids or phosphorus (Cowieson, 2010). Importantly, the magnitude and consistency of the response to enzyme addition is linked to the inherent digestibility of the diet to which it is added with elevated effect in ingredients or nutrients with a lower digestibility and vice versa (Cowieson, 2010).

Microbial protease is a relative new-comer to the global feed enzyme market, especially when considered as a specific mono-component activity (Cowieson & Roos, 2016). In common with its predecessors, the effect of protease on the digestibility of various dietary ingredients is higher when inherent digestibility is low. For example, Douglas et al. (2000) noted a direct correlation between the ileal digestible energy content of various batches of soybean meal and the beneficial effect of an admixture of xylanase, amylase and protease. Otherwise, the effect of exogenous protease in animal nutrition shares little with alternative exogenous enzymes and there is considerable opportunity for substantial value creation through its use in concert with alternative enzyme classes. The major mechanism of action of exogenous proteases include reduction in the severity of effect of proteinaceous antinutrients e.g. trypsin inhibitors or antigenic proteins, improvement in the solubility and digestibility of dietary protein (and adjacent nutrients such as fat), reduction in the flow of endogenous protein in the intestine e.g. mucin and various beneficial effects on gut health e.g. increase tight junction integrity and intestinal tensile strength (Cowieson & Roos, 2016). A major source of dietary proteinaceous antinutrients (and, certainly, dietary protein per se) is soybean meal and this has been shown to vary considerably in chemical and mechanical characteristics (Dozier & Hess, 2011; Cowieson et al., 2017). The interaction between exogenous protease and soybean meal has not been systematically explored but a reasonably large body of evidence exists to suggest that benefits vary in line with the inherent quality of the meal, processing methods (and severity thereof) and the concentration of various antinutrients therein. It was therefore the purpose of the present paper to briefly outline the usefulness of exogenous protease to enhance the nutritional value of soybean meal for poultry and to suggest factors that may increase or decrease mean effect.

Following the pioneering work of Lewis and Baker in the 1950s (Lewis et al. 1955; Baker et al. 1956), who were able to show potential of exogenous protease sources in pigs, specific benefits of protease for soybean meal were not conclusively demonstrated until the early 1990s (Cowieson & Roos, 2016). Initially, these beneficial effects were focused on reducing the antinutritional effects of trypsin inhibitors in soybean meal and on examination of various classes of proteases e.g. alkaline vs. acidic. More recently, the effects of exogenous proteases on general animal performance, nutrient digestibility, gut health and intestinal physiology have been explored and reported. For example, Cowieson & Roos (2014) conducted a metaanalysis of the effect of exogenous protease on the apparent ileal digestibility of amino acids from 25 independently conducted studies and noted that the effect magnitude was directly linked to the inherent digestibility of amino acids in the control groups (Fig. 1). This relationship underlines the importance of quality control in raw material selection and use and suggests that microbial protease will reduce variance in the digestibility of protein in animal nutrition. This general relationship between ileal amino acid digestibility and the corresponding value of exogenous protease has recently been confirmed in soybean meal and full-fat soy meal (Cowieson et al. 2017; Table 1) where the inherent digestibility of various amino acids and also AMEn was well correlated for leveraging the scale of response to exogenous protease. It is therefore recommended that some estimate of soybean meal quality is available in order to optimize the value creation of exogenous protease in animal diets.

Soybean meal, as with many other feed ingredients, contains several antinutritional compounds that vary in nutritional significance. Trypsin inhibitors, lectins, antigenic and recalcitrant proteins are implicated in various intestinal perturbances and contribute to variance in the nutritional value of soy products, especially for neonates (Cowieson & Roos, 2016). In addition to direct hydrolysis of proteinaceous antinutrients (Rooke et al., 1998), exogenous proteases are able to enhance the resilience of the intestinal tract, putatively through improved tight junction and mucin integrity (Cowieson & Roos, 2016). Exogenous proteases have also been shown to increase the digestibility of energy and fat in poultry diets which suggest a generic enhancement of nutrient solubility and digestion following their addition (Cowieson & Roos, 2016).

Exogenous proteases, at least as an explicit mono-component activity, are a relative newcomer to the global feed enzyme market. However, their use has expanded substantially since their introduction and their effect extends far beyond ‘simple’ improvement in the digestibility of dietary protein to benefits in gut integrity, environmental sustainability and the digestibility of non-proteinaceous macro-nutrients such as lipid. The link between the inherent quality of the raw material and the degree of protease effect should be further explored and exploited via articulation between nutrient release recommendations and rapid raw material quality surveillance. In conclusion, the usefulness of exogenous protease to enhance the digestibility of soybean meal and full-fat soy (as well as many alternative feed ingredients) is clear. Adequate characterization of raw material quality and inherent nutritive value is an important consideration when assigning nutrient matrix values to a protease; however, such products are able to reduce variability in the digestible nutrient value of raw materials of unquantified quality. Further to this concept, integration of exogenous protease with adjacent enzyme technology such as phytase and carbohydrase is recommended for maximum consistency of response and value creation.