Oil for animal feed or for fuel? That's a part of the debate sparked worldwide with the explosive growth of biofuel development. The cost of energy in poultry and swine feed formulations is expected to undergo a significant increase in the coming years. According to a report from Rabobank International, the world consumption of vegetable oil derived from soybeans and other plants will have a record 27 percent growth until 2010 as a result of the demands from the food and biofuel industries. According to projections, the use of this oil type will reach 121 million tons in 2010. The demand for biofuels is stimulated by the record oil prices and by the increasingly strict environmental legislation that limits the use of fossil fuels.
This will be the largest increase ever in demand for vegetable oil in such a short time period. But it is also a great opportunity for the use of enzymes, allowing for better utilization of the diet's energy.
Enzymes involved in starch digestion
The use of enzymes that improve the utilization of starch has recently been the subject of many studies with corn- and/or sorghum-based diets for poultry and swine. Unlike mammals, the ability of poultry to use starch through microbial degradation in the distal portion of the digestive tract is very low (Carré, 2004). According to the same author and Tester et al. (2004), three enzymes are involved in starch digestion: alpha-amylase, maltase and isomaltase, while pancreatic alpha-amylase is responsible for most of the hydrolysis in the duodenum.
In its natural state (unprocessed), starch is stored as granules (in the endosperm of grains), with variable shapes and sizes. These granules contain two different glucose polymers: amylose and amylopectin. Amylose is an almost linear polymer formed by four to 100 glucose units, with nine to 20 branches, linked by alpha-1,4 bonds (Oates, 1997, cited by Weurding, 2002).
Amylopectin, on the other hand, is a highly branched polymer and the glucose molecules are linked by alpha-1,6 bonds, formed by more than 20 glucose units (Weurding, 2002). According to the same author, starch is formed exclusively by alpha-1,4 and alpha-1,6 glucose bonds, and is therefore easily digested by enzymes in animals. Glucose molecules, on the other hand, also make up cellulose, but they are linked by beta bonds and cannot be accessed by enzymatic digestion in the small intestine of monogastric animals.
Weurding (2002) classified the conventional starch sources into three categories based on the amylose amount (high, intermediate and low) (Table 1).
According to the author, there are several reasons for starches with high amylose levels to have a low digestibility:
- Small amylose molecule (as compared to amylopectin, which is quite larger) and, as a consequence, a smaller surface for the enzymes’ action;
- Hydrogen bridge bonds of amylose make it less susceptible to the action of amylase enzyme; and
- Amylose forms complexes with other compounds very easily, especially with lipids (fatty acids), making the enzymes’ activity difficult. The formation of these complexes with amylopectin, however, has not been reported.
The amount of amylose in conventional starch sources is between 17 and 33 percent, but GMO (Genetically Modified Organism) grains as corn, rice, sorghum and barley can have different levels. Waxy corn, for example, that has specific industrial uses, has a maximum of 1 percent amylose.
When starch is submitted to thermal treatment, there is a direct relationship between the amylose amount and the formation of resistant starches, a process known as retrogradation. Using in vitro digestion, Sievert and Pomeranz (1989) found a positive correlation between amylose level and resistant starch formation (Table 2).
According to Newcombe (1999), the formation of retrograde starch as well as the presence of large starch granules leads to an incomplete digestion of this starch in the small intestine. As a consequence, a significant fraction of starch is fermented in the terminal ileum and in the cecum, with low energy utilization by poultry.
Main energy source
In corn and soybean meal-based diets given to broilers, corn provides approximately 70 percent of the final energy. According to Yu & Chung (2004), differences in starch digestibility are the main cause for Metabolizable Energy (ME) variation in corn used for poultry and swine
Through the years, the National Research Center for Swine and Poultry (CNPSA) of EMBRAPA (Brazilian Agricultural Research Company) in Concordia has carried out several experiments to determine the energy amount in corn produced in different areas of Brazil and used to feed poultry and swine. The results that were obtained show a large variability not only due to the different existing corn types, but also the distinct composition of these corns (Table 3).
Newcombe (1999) reported that the variability in corn quality could be due to genetic factors as well as the type of starch, climate factors during growth and harvest, and the drying process of grains. According to Cowieson (2005), the energy content of corn used to feed poultry is affected by variations in amylase inhibitors as phytic acid, resistant starch and other anti-nutritional factors. Englyst et al. (1992) suggested the following starch classification based on its digestibility:
- Rapid digesting starch
- Slow digesting starch
- Resistant starch
Working with different inclusion levels of slow-digesting starch, Weurding (2002) found a significant effect on production parameters of broilers. In this study, the author used cassava and regular corn to feed the group with low inclusion level of slow digesting starch, and waxy corn; peas and sorghum to feed the group with high inclusion level of slow digesting starch. The author concludes that high inclusion levels of slow digesting starch are beneficial for broilers (Table 4).
In a review on the different resistant starch (RS) types, Cowieson (2005) states that they can be divided into three subcategories:
- RS1 starch that is not digested due to association or encapsulation in the food matrix with other compounds, as carbohydrates or proteins.
- RS2 starch that is not digested due to the granules structure and conformation.
- RS3 is associated with the effects of starch processing, as gelatinization resulting from thermal action and formation of hydrogen bridges.
Enzymatic action in the small intestine
Tester et al. (2004) suggest a fourth classification (RS4) including the formation of a new chemical binding, cross-linking, sterification and etherification, different from the already known alpha-1,4 and alpha-1,6. After the feed leaves the gizzard, the digesta is subjected to enzymatic action in the small intestine. Diets with rapid-digesting starch are digested in the upper part of the small intestine, while diets with slow digesting starch are digested in the lower portion of the small intestine. The resistant starch will undergo fermentation in the lower part. Weurding (2002) states that broilers do not utilize most resistant starches.
According to Zanella (1999), the ileal digestibility of starch in 37-day old birds is 87 percent when corn and soybean-based diets are used. Similar figures were found by Noy & Sklan (1995) and these authors conclude that digestibility of corn starch in the lower part of terminal ileum can be less than 85 percent. With two levels of an enzyme compound based on alpha-amylase, Bertechini et al. (2006) observed a significant effect on the performance of broilers using a feed (negative control) with a 70 kcal reduction (Table 5).
Feed processing with appropriate moisture and temperature conditions (pelleting, expansion and extrusion) improves digestibility of starch in the ingredients, improving the action of the animal's endogenous enzymes. Due to variations that can occur in the processes, however, such as excessive temperature, low moisture level or low steam addition, for example, retrograde starches can be formed and will have a negative effect on the animals. Besides, as enzymes are temperature-sensitive proteins, only products with proven thermostability to thermal processes should be used.
Since energy is the most expensive nutrient in feed formulations, the use of energy-releasing enzymes is a technically effective way of reducing the formulation cost without interfering with the animal's performance. Starches from different species have different digestion rates.
The nutritionist must have a very good knowledge about the substrate used in the diets and take it into consideration when choosing the enzymes to be used. There are several enzymes and enzyme blends with similar characteristics, but different enzymatic activity and efficacy. Thus, each company should evaluate the products that are available to be used under its particular conditions.