Energy is the most expensive “nutrient” in every animal diet. In fact, the major source of energy, starch, makes up about 50 percent of most diets for monogastrics (pigs and poultry). But, energy is also derived from lipids and non-starch carbohydrates, such as non-starch polysaccharides (after suitable enzyme supplementation).
Increasing the utilization of energy from cereals and protein sources, such as soybean meal, sunflower meal and peas, which also contain substantial amounts of energy-yielding carbohydrates, can only result in improved feed efficiency, reduced feed cost, less environmental impact and of course, higher profitability.
Carbohydrates is the collective name for starch, sugars and “fiber” polysaccharides. Carbohydrates can be classified into two categories: storage carbohydrates and structural carbohydrates. Storage carbohydrates include starch and simple sugars, such as fructose and saccharose. These carbohydrates, along with lipids in the embryonic part of seeds, are the main energy sources for the new plants that will be emerging from cereal seeds (if they were to be planted).
Structural carbohydrates, on the other hand, including the well-known non-starch polysaccharides, are responsible for cellular form and structure, and are located mostly in the outer cellular membrane. Structural carbohydrates, commonly referred to as “fiber,” are hardly digested by monogastric animals due to lack of suitable endogenous enzymes. Thus, the majority of structural carbohydrates are fermented in the hind gut, where they may release limited useful energy levels (covering less than 5 percent of daily needs for the animal), in the form of volatile fatty acids.
Carbohydrases are specific commercial enzyme preparations that attack carbohydrates releasing energy that would be otherwise lost for the animal. They work mainly by opening the cell wall structure of intact plant cells, release thus not only energy (starch), but also other nutrients, such as protein, minerals and lipids. In addition, plant cell wall fractions increase intestinal viscosity that leads to reduced nutrient absorption, accelerated proliferation of pathogens, such as Escherichia coli, and other problems, such as sticky droppings and dirty eggs. Today, the majority of commercial carbohydrases are for use against structural carbohydrates, although carbohydrases for starch (amylases) are becoming increasingly popular.
In cereals, up to 80 percent of the dry matter is starch. For pigs and poultry, starch digestibility is rather high, ranging from 92 percent to 95 percent, and a fraction lower for very young animals. Nevertheless, given the extremely high concentration of starch in cereals, even the slightest improvement in digestibility will have a significant impact on energy utilization. Thus, the use of carbohydrases in animal feed has a clear financial interest, especially when cereal prices are high.
Starch is made up of glucose molecules linked together to form either a linear (amylose) or branched (amylopectin) polymer. Although monogastric animals produce enough endogenous amylase, supplementation with an exogenous amylase – that degrades amylase and amylopectin – has been shown to marginally improve starch digestibility, especially in young animals. However, results remain ambiguous because the use of amylase is affected not only by animal age and cereal type, but also by the degree of cereal grinding and other feed processing treatments.
The cell wall of cereals, made of structural carbohydrates, is composed mostly of arabinoxylans and beta-glucans. The exact chemical structure of arabinoxylans and beta-glucans varies among the different cereal types and their varieties, and it is also influenced by the local growing conditions (soil, weather and irrigation). As such, the chemical structure and concentration of non-starch polysaccharides is highly variable from season to season and even from batch to batch. In addition, many complex factors influence the number of intact cells remaining in the feed after processing. Feed grinding, conditioning, pelleting and extrusion all enhance cell wall rupture, while chewing (pigs) or gizzard grinding (poultry) further increase cell wall rupture. Still, many cells frequently remain intact upon reaching the small intestine, and thus, fail to release important nutrients like protein, starch, minerals and lipids.
To enhance the “opening” of cells within the gastrointestinal tract, exogenous enzymes are required to be added in the feed. For cereals, these enzymes include xylanases and beta-glucanases, which have been shown to improve energy utilization and also reduce digesta viscosity, especially when the levels of non-starch polysaccharides are particularly elevated (Figure 1). This last aspect is frequently the source of considerable confusion in the interpretation of many scientific trials, as in most cases the exact levels (or quality) of non-starch polysaccharides in cereals is not measured.
A diet based solely on maize contains very few anti-nutritional factors, as maize contains only 2.5 percent cellulose and 5 percent arabinoxylans. That is why enzymes are not very popular in all-maize diets (except in certain cases for very young animals). In contrast, a diet based entirely or heavily on barley is extremely rich in non-starch polysaccharides, as it contains 5 percent cellulose, 7 percent arabinoxylans and 5 percent beta-glucans. Wheat, being somewhere in between maize and barley, with 2.5 percent cellulose, 6 percent arabinoxylans and only 1 percent beta-glucans, can also benefit from enzyme supplementation. Of course, triticale and rye, with the highest levels of non-starch polysaccharides among cereals, almost always benefit from enzyme supplementation.
Although the major nutrient in most protein sources is, of course, the protein, they also contain substantial amounts of carbohydrates, including starch, but mostly structural carbohydrates. Most protein sources have their origin in legume plants. In legume seeds, the cell wall structure is quite different compared to the cell wall structure of cereals. Here, xyloglucans and pectins make the major cell wall fiber network – and, this structure is more complex and complicated to rupture. Today, there is a limited range of suitable commercial enzymes capable of degrading xyloglucans and pectins. Results are promising, again depending on the level of non-starch polysaccharides, whereas combination with other energy-releasing enzymes appears to offer the most benefit. Clearly, there is a dearth of evidence and knowledge on how to release more energy from carbohydrates in legumes, and this topic deserves more attention from the scientific community.