Introduction
Application of enzymes to improve nutrients digestibility of plant-based feed ingredients for swine and poultry has now been studies for decades. Initially, the main focus was phytase to break down the phytate molecule and release the attached phosphorus molecules. In the last two decades, enzymes to assist digesting NSP were developed, tested, and commercialized. In the meantime, enzymes to assist digesting starch, protein and fat have been tested as well. A large array of chemical characteristics exists among plant-based feed ingredients, and success of enzyme application will depend on these characteristics. The substrate must match the enzyme and be a limitation for nutrient digestibility or voluntary feed intake. Two diet formulation methods exists to apply enzyme treatments in practice: (1) formulate diets to a regular nutrient content and supplement with an enzyme, while hoping for an improvement in feed efficiency, or (2) formulate diets to a reduced nutrient content and count on an uplift by the enzyme to a regular nutrient content, while reducing feed costs. An overview of considerations and practical application of enzyme supplementation in swine will be presented.

Ingredients
Seeds of plants crops or fractions thereof each contain some of the three main energy categories: carbohydrates [divided into sugars, starch and non-starch polysaccharides (NSP)], protein, and oil (fat). Among the listed feed ingredients, a large array in content of these main energy categories exist, ranging from 10 to 37% NSP, 14 to 63% starch, 9 to 47% protein, and 1 to 5% fat (Table 1).

In least-cost diet formulation, the greatest cost-pressure exists against digestible or available energy (Zijlstra et al. 2001). Overall in swine nutrition, the inverse relationship between NSP content and energy digestibility has been well described for several feed ingredients, for example wheat (Zijlstra et al. 1999) and barley (Fairbairn et al. 1999). Logically, enzymes that degrade fiber and thereby improve energy digestibility or voluntary feed intake will thus have a high chance to be beneficial economically, whereas phytase to improve phosphorus digestibility may also reduce nutrient excretion and thereby improve sustainability of the swine industry.
Among ingredient, large differences in digestibility of the main macronutrients exist (Figure 1). Among the cereal grains, oats has the lowest digestibility of crude fiber, then barley, wheat, while corn has the highest digestibility of crude fiber. Both peas and soybean meal have a high digestibility of crude fiber. By-products from value-added processing, including wheat middlings from wheat flour milling, generally have a lower nutrient digestibility than the parent cereal. Digestibility of other carbohydrates, including starch, sugars, and the remainder of the fiber fractions was lower for wheat middlings, oats and barley compared to the other four feed ingredients. Protein digestibility followed and similar pattern as digestibility of other carbohydrates with the highest protein digestibility observed for soybean meal. According to the database (CVB 1994), fat digestibility showed a large variation among feed ingredients. Phosphorus digestibility was consistently below 40%, likely due to the phytate contained in plant-based feed ingredients.