Many poultry producers have commented that feeding last year’s corn
crop has resulted in increased feed intakes. In poultry, dietary energy
has a significant effect upon feed consumption. As the energy density
of the diet decreases, there is a tendency for the bird to increase
feed intake in a bid to ensure that energy intake meets the bird’s
Many poultry producers have commented that feeding last year’s corn crop has resulted in increased feed intakes. In poultry, dietary energy has a significant effect upon feed consumption. As the energy density of the diet decreases, there is a tendency for the bird to increase feed intake in a bid to ensure that energy intake meets the bird’s energy requirements. Since feed accounts for the biggest single cost of poultry production, focusing on feed efficiency is crucial to maximizing profitability.
A starchy problem?
Starch accounts for around two-thirds of the energy content of corn. For a poultry diet formulated to contain 50 per cent corn, around one-third of the metabolizable energy in the diet will be supplied by the cornstarch.
At Danisco, over the past four years, we have been particularly focusing on corn quality and its effect on bird performance. We have established that both the level of starch and the corn starch digestibility can vary significantly from year to year and between different batches of corn.
|Some broiler and layer producers have observed increased feed intakes when feeding the latest new harvest corn. Dr. Janet Remus discusses the possible reasons for these trends and solutions to a potentially expensive problem.
Having analyzed 55 samples of corn taken from this year’s Canadian harvest we have measured that the starch content of corn averaged 65% (coefficient of variation, c.v. 2%), and the in vitro starch digestibility averaged 37.9% (c.v. 11%, range 22-44%). This contrasts markedly with our 2007 corn survey for Canada (43 samples) where starch content was found to be similar (average 64%, c.v. 2%), but the average in vitro starch digestibility was higher (41.4%) and much less variable (c.v. 7%, range 35-49%).
So what are the main factors that affect starch digestibility? There are several that Danisco has identified that can contribute to variations in starch digestibility including the presence of resistant starch, starch granule size, starch composition and starch encapsulation.
Resistant starch (RS) is not digested in the ileum, so does not contribute to ileal digestible energy and cannot be utilized by the bird. It can, however, be fermented by microbes in the hindgut. There are three types of resistant starch: RS1, which is not digested as it is physically trapped by cell walls; RS2, which is an intrinsically resistant starch granule; and RS3, which is retrograded starch. Retrograded starch forms during severe heat processing, in the presence of moisture. The key components of starch (amylose and amylopectin) can undergo retrogradation where the starch crystals re-associate after heating and form resistant particles, which are very difficult for the bird to digest in the small intestine.
Starch granule size will also affect starch digestibility. The digestibility of the corn starch increases as the granule size decreases. The smaller the granule size, the larger the relative surface area for digestion by the starch-degrading enzyme amylase.
|Figure 1: Cornstarch content and digestibility can be highly variable. For larger image
|Figure 2: Protein Solubility (%) in Canadian corns, harvested 2008. for larger image
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The composition of the starch will also affect starch digestibility. Amylose and amylopectin in starch link together to form complex structures. The relative amounts of amylose and amylopectin vary between samples of corn. Amylose more easily forms retrograde starch, is harder to gelatinize and is more resistant to breakdown by amylase. For these reasons, amylose is less digestible than amylopectin. Therefore, the higher the amylose:amylopectin ratio, the lower the digestibility of the starch.
The fourth factor impacting starch digestibility is starch encapsulation by protein. Protein encapsulation reduces starch digestibility by limiting its exposure to the bird’s starch-digesting amylase.
Ontario had a late spring in 2008 and a very wet growing season. In many cases mechanical drying would have been used during the harvest season. Harsh drying of corn after a wet harvest can reduce energy digestibility due to both starch retrogradation and an increase in starch/protein binding in the corn. Also differences in drying processes between farms (e.g., drying temperature used) may have contributed to increased variation in the corn quality seen, as demonstrated by the wider range of in vitro starch digestibility versus 2007.
Mechanical drying of grains is also known to increase the binding of certain proteins to the starch in the corn, making that protein less soluble, and the starch less available for digestion. Indications of this have been found in Danisco’s standard in vitro measurements of the protein solubility (%) in the Canadian corn harvest samples. For the 2008 harvest this protein solubility averaged 18.1% (c.v. 40%) versus much higher, and less variable values, of 28.4% (c.v. 23%) for the 2007 harvest.
Some of these differences between the 2007 and 2008 Canadian corn harvests, highlighted by these in vitro tests, are likely to underlie the rise in feed intake seen commercially when feeding 2008 corn. The birds are basically having more of a problem with the “available” energy in this year’s corn – and are increasing their feed intakes to compensate.
The key to success
Feed enzymes can be an extremely cost-effective solution to these issues. The secret to success is selecting the right enzymes, those that will target and break down the anti-nutritional problems present in corn. These include:
- Xylans – present in cell walls – restrict enzyme access to cell contents, e.g., starch granules.
- Storage proteins in corn – encapsulate starch and restrict starch digestibility. Heating of corn by mechanical drying will also increase the binding of certain proteins to starch.
- Starch digestibility – often incomplete at the terminal ileum.
The ideal enzyme solution is one that cost effectively targets each of these anti-nutritional factors, and contains the right balance between xylanase, amylase and protease. The xylanase acts to break down cell walls to expose the starch for digestion. Protease helps to release the starch encapsulated by, and bound to, storage proteins and helps break down certain residual anti-nutritional factors in vegetable proteins (e.g. trypsin inhibitors, lectins in soybean meal). Amylase, a starch-digesting enzyme, enhances starch digestion and is complementary to the animal’s own amylase production. Ultimately, a correctly formulated enzyme combination increases energy and protein digestibility in the diet.
To help to quantify how much the enzyme combination can increase the energy value of a corn-based diet, Danisco has developed a unique service – Avicheck™ Corn. The service measures in the laboratory the key parameters in corn that influence the effect of its enzyme blend (Avizyme 1502)
on corn energy digestibility. A corn energy improvement value (“EIV”) is then assigned to the corn sample. The EIV is the amount of additional energy that can be assigned to the corn when the diet is supplemented with the enzyme combination.
For the 55 Canadian corn samples the average EIV from using the enzyme combination was 170 kcal/kg. For a diet containing 50% corn the enzyme combination currently provides opportunities to reduce feed costs by around CAD$4.20/tonne. Just as important commercially is the fact that the enzyme combination will reduce variability between different batches of corn, resulting in more consistent feed and bird performance.
Dee Britney, a nutritionist at Wallenstein Feed & Supply Ltd, Ontario, observed changes in the feed intake of laying hens with the use of new harvest corn last fall. After thorough testing of the new corn to determine a complete nutrient profile and evaluation of flock data, results seemed to indicate poorer energy availability compared to previous years.
“We use enzymes to help reduce cereal variability and to improve nutrient availability. To determine if we needed to use an enzyme with this new corn, we started by sending samples to Danisco and other suppliers for evaluation.” This year, tests on Ontario corn show poorer energy availability compared to previous years. In addition, the tests show what energy release they estimate can be achieved with the use of carbohydrase enzymes.
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