REZA ABDOLLAHI* provides a brief overview of wheat, sorghum and barley, as potential alternatives to corn in broiler diets.
Introduction
Several factors including the global population growth, shift towards high protein diets, finite land availability for crop production, climate uncertainties, increase in fossil fuel prices and diversion of feed ingredients, especially of cereals, to biofuel manufacture presents challenges to the global feed supply. This is a major threat for sustainable poultry production, in both the developing nations, as they move towards animal protein food regimen due to rising incomes, and developed countries that rely heavily upon a limited number of corn-producing nations (USA, Argentina and Brazil). Therefore, sourcing of quality and sustainable feed ingredients at acceptable prices has become the major challenge for the industry.
Wheat
Wheat is a major energy source in poultry feeds in many parts of the world, including the European Union, Canada, Australia and New Zealand. The use of wheat as a poultry feed ingredient has increased significantly over the years, due to favorable price conditions. Compared to corn, wheat generally delivers energy into a diet at low cost. Wheat can contribute up to 60-65% of the metabolizable energy and 35% of the protein in broiler diets.
Wheat is highly diverse with different types of grain. The major factors used to distinguish between types of wheat are hardness and softness of the grain, whether they are spring or winter wheats, what colour bran they produce (red or white) and protein content. The composition and feeding value of wheat is usually more variable than that of other cereals. Protein levels in wheat, for example, can vary from 10-18%. Wheat provides 10% less metabolisable energy than corn (3050 vs 3350 kcal/kg), mainly due to the presence of soluble non-starch polysaccharides. Non-starch polysaccharides (NSP) in wheat are mainly arabinoxylans and (1→3) (1→4)-β-D-glucans. These NSP can encapsulate nutrients such as starch and protein, and/or cause increased digesta viscosity in the gut, leading to reduced nutrient digestibility and metabolisable energy values, especially in young birds.
Apparent metabolizable energy (AME) is the most important measurement used in characterizing the feeding value of wheat for poultry. Among cereal grains, wheat is known to be the most variable in the available energy content for poultry. The AME of wheat for broilers varies considerably (Figure 1). Such variation is a nutritionists’ nightmare and can cause serious management problems and economic losses to poultry producers. Unfortunately, no rapid test is available to separate the wheats of different AME, but exogenous enzymes, containing xylanase activity, can be effectively used to mitigate the adverse effects of non-starch polysaccharides and to minimize the variation in AME and the performance of broilers fed wheat-based diets. With the inclusion of exogenous enzymes, wheat is an excellent energy source for poultry and can completely replace corn in broiler diets. Moreover, as wheat contains 35-40% more protein than corn, less soybean meal is needed in wheat-based diets. Another favorable aspect is that wheat inclusion, even as low as 10%, can improve the physical pellet quality during feed manufacture. Feeding of whole wheat grains to poultry has also generated interest in recent years as a mean to reduce processing and production costs and improving gut health.
Figure 1: Variation in AME of wheat.
Barley
Barley is extensively cultivated, ranking fourth in world cereal production with an annual production of 160 million tonnes. Characteristics such as resistance to drought and saline soils and ability to mature in climates with a short growing season have encouraged the cultivation of barley over corn and wheat. The proportion of barley used in poultry diets remains very limited (less than 1% of total barley utilized as animal feed) due to its lower energy, relatively high fibre content (220 g/kg), and high content of β-glucans, the dominant NSP. Due to its higher level of soluble NSP, barley is categorized as a viscous cereal together with rye, wheat, triticale, and oats. Though both wheat and barley have higher levels of NSP compared to corn, barley NSP consists mainly of the soluble fraction compared to wheat.
Ways to improve the feeding value of barley in poultry diets has been studied over the years, however, the published data have been contradictory, resulting in variable range of inclusion levels being recommended in broiler diets. The ratio between amylose and amylopectin has been given special attention as an important factor determining the nutritive value of barley for monogastric animals. Even relatively small variations in total dietary starch supply and changes in starch amylose: amylopectin ratio can affect the growth performance of poultry. In comparison to amylopectin rich starch, high amylose starch is less susceptible to enzymatic degradation by α-amylase in small intestine, highlighting that waxy starch may be more digestible than the normal starch type.
Figure 2: AME response to enzyme supplementation in barley.
Currently, all barley-based broiler diets worldwide are supplemented with glycanases (xylanases and β-glucanases). When supplementing barley-based diets with exogenous enzyme, the rule of thumb adopted by the poultry industry is “barley + β-glucanase = wheat”. The improvements in growth performance and nutrient utilisation in response to the supplementation of carbohydrase in barley-based diets have been attributed to the viscosity reduction caused by the partial degradation of soluble NSP. In addition, supplemental carbohydrases minimise the variability in nutritional value of barley grains. To minimise the negative impact caused by the inherent variability of barley in poultry diets, grain specific determination of inclusion levels and processing conditions should be encouraged. The combination of enzyme supplementation with an appropriate feed processing practice may enable achieving maximum efficacy of supplemental enzymes in barley-based diets.
Sorghum
Sorghum is the world’s fifth cereal crop with an annual production of 62 million tonnes. Except in Australia and Mexico, sorghum is not extensively used as a feed grain in broiler diets. However, due to challenges to crop production imposed by climate change and the drought tolerance of sorghum, there is a huge potential for the use of sorghum for broiler chickens. Whilst the prime reason for sorghum inclusion is for the provision of energy, which is largely derived from starch, sorghum contributes some protein in broiler diets. Even though sorghum, is a ‘non-viscous’ grain, the starch digestibility and energy utilisation of sorghum in broiler chickens are inconsistent and in some cases inferior to corn. The kafirin, phenolic compounds and phytate are suggested as three ‘starch extrinsic’ factors, compromising energy utilisation in poultry.
According to a recent study at Massey University, corn and sorghum had the highest AME, barley the lowest, and wheat being intermediate (Figure 3).
Figure 3: AME of cereal grains.
Kafirin is the major protein fraction in sorghum, representing 54% of grain crude protein content, followed by glutelin (33%), globulin (7.0%) and albumin (6%). Kafirin has been shown to impede starch digestibility and energy utilisation in broilers fed sorghum-based diets, due to biochemical starch protein interactions. Kafirin protein bodies and starch granules are embedded in the glutelin protein matrix of sorghum endosperm that facilitates starch-protein interactions. Sorghum contains substantially higher concentrations of phenolic compounds than other feed grains. A negative linear relationship has been reported between total phenolic compound concentrations and energy metabolisability of sorghum in broiler chickens. Condensed tannin is a polyphenolic compound with potent anti-nutritive properties; however, condensed tannins are not present in contemporary sorghum crops such as those grown in the United States and Australia. Sorghum contains low concentrations of soluble NSP, and therefore, the magnitude of responses to NSP-degrading enzymes in sorghum-based diets appears relatively muted. However, supplementation of sorghum-based diets with appropriate proteases with the capacity to degrade kafirin, may enhance both protein and starch digestibility. An increased protein digestibility by 4.5% and AME by 175 kcal/kg, due to protease application in a sorghum-based diet has been reported. Application of sulphite reducing agents, such as sodium metabisulphite, in sorghum-based broiler diets can cleave disulphide cross-linkages, increase starch digestibility and AME. Further selection of grain sorghums with improved kafirin properties and lower polyphenolic compounds may advantage nutrient utilisation and growth performance in broiler chickens fed sorghum-based diets.
Table 1: Chemical composition of wheat (g/kg).
Conclusion
High variability exists even within the same grain type, and feed formulation should consider cultivar variability and differences in nutritional values. Using correct nutritional, especially AME and AA digestibility, values, wheat, and sorghum, can completely, and barley partially replace corn in broiler diets. Exogenous NSP-degrading enzymes have been effective in mitigating the adverse effects of NSP, minimizing the variation in AME and improving the performance of poultry fed diets based on viscous grains. Inherent grain quality can cause variation in response of grains to exogenous enzymes.
*Dr Reza Abdollahi ([email protected]) is with the Monogastric Research Centre, Institute of Veterinary, Animal and Biomedical Sciences, Massey University, New Zealand. References are available on request to the author.
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