Because of their chemical composition and the presence of anti-nutritional factors, there are certain limits imposed on the use of some conventional feed ingredients in poultry feeding, such as maize, wheat, barley, soybean, sunflower, and fishmeal. The following is a review of these nutritional limitations, which hinder the use of such ingredients in excessive amounts in the diet.
Maize is a major feed ingredient having an energy value of about 3,370 kcal/kg. It is usually included in broiler diets at varying levels depending on the energy requirements at various growth stages. From 1-30 days of age, maize is usually included at a level ranging from 45-50% to provide an energy value of 3,100 kcal/kg in diets balanced to have 23% crude protein. Thereafter, the level of maize may be increased to 50-55% to provide an energy value of 3,200 kcal in diets balanced to have 20% crude protein.
In either case, the levels of maize in the diets should not exceed the recommended values, because of its ‘heating effect’, particularly in warm seasons, which may force the birds to go off feed if supplied in a larger amount in the total diet. The same is true with laying hens, although in many cases the level of maize is increased to 70% without any detrimental effects on feed intake, laying performance, or egg quality.
The low lysine content is also a problem causing the maize protein to have a poor biological value. Most of this deficiency in lysine is because zein, the protein from the endosperm, is lysine-deficient. Although high-lysine or lysine-rich varieties are now available, it is often more economical to supplement diets containing the low-lysine maize with synthetic lysine than it is to substitute the maize with the high-lysine varieties.
The imbalance of calcium: phosphorus ratio in maize (1:5) could also be a problem, particularly with laying hens, unless corrected through appropriate mineral supplementation.
Barley is generally an unacceptable component in chicken feed because of its low palatability. It also has a low energy value due to low starch content, which leads to limited uptake of nutrients and slow initial growth. It is, however important to incorporate barley into the diet at up to 150 g/kg because of its high fibre content, because it undergoes fewer nutrient losses during processing, and also because it is rich in antioxidants, minerals, and vitamins.
Increasing the level of barely in the diet above the recommended rate is not advised, especially when fed to young chickens. This is because of its high content of beta-glucans which bind with water in the intestine, resulting in the formation of gels and increasing the viscosity of the intestinal contents and sticky droppings, thereby creating bad litter, which can cause hock problems and damage the breast of broilers. Chickens fed high-barley diets have also been shown to be more susceptible to necrotic enteritis than those on corn-based diets.
For young birds, the use of feed enzymes in barley-based diets breaks down the beta-glucans in the diet, reduces intestinal viscosity, improves feed and litter quality, modulated the microbial profile in the GI tract, and improves bird performance. As the chicken grows older, the digestive tract undergoes functional changes and becomes more efficient at utilizing barley and coping with viscosity and pathogen problems even without enzyme supplementation.
Wheat is preferred to all other grains by poultry and is often included in their feed because of its palatability and to offer variety as well as its high nutritive value, which is equal to or slightly superior to maize. The level of wheat should not exceed 20% of the total grain mixture during the first part of the growing period of broilers or 30-35% in the later part. Higher inclusion of wheat may result in increased viscosity of the digesta. This is caused by the highly viscous pentosans and the other non-starch polysaccharides in wheat, which reduce the passage time of the digesta and impair the diffusion of digestive enzymes, with a resulting decrease in body weight and feed efficiency. Although various enzyme treatments have proven to be effective in alleviating tire viscosity problems, economic considerations may restrict the potential of such an approach under practical farm conditions.
In using soybean as the main protein supplement, it is important to correct deficiencies of calcium, riboflavin, and possibly phosphorus. With proper supplementation, soybean meal can replace up to half of the animal protein in the feed for poultry of all ages. When there is a shortage of animal proteins, soybean meal can form a larger portion of the feed although, for the rapid growth of broiler chicks and high hatchability of eggs, a minimum amount of animal protein supplements should be included in the feed.
Excessive amounts of soybean meal or use thereof over a long time may, however, cause trouble from “pasting up”, in which excreta sticks to the vent of the bird. This can lead to renal dysfunction arising from the retention of urates and interfere with the passing of droppings.
The presence of anti-nutritional components, especially fibrous complexes such as pectins and arabinoxylans are primary factors contributing to the lower protein bioavailability and digestibility in sunflower meal (SFM) compared to other feed ingredients. The high concentration of these compounds in SFM also causes alterations of gut microbial communities, and hence leads to autoimmune disorders and brings about health and quality problems such as wet droppings and dirty eggs.
Moreover, SFM is low in lysine and other essential amino acids. Thus, the use of SFM in poultry diets at levels of more than 5% requires amino acid supplementation to prevent imbalances and deficiencies, and alleviate any adverse effects from the high chlorogenic acid content of SFM (1.2% DM) which inhibits the activity of hydrolytic enzymes.
The nutritive value of SFM can be improved by enzyme treatment. Commercial enzyme products containing arabinase, pectinase, and xylanase are now available in dry, micro-granulated forms and liquid formulations for feed application. These allow a greater proportion of the protein component to be sourced from SFM rather than soybean, without detrimental effects on performance. Studies with broiler chickens, for example, indicate that enzyme-treated SFM can replace up to 43% of the soybean meal without affecting weight gain or feed efficiency.
Fishmeal has a high protein content – about 60% on average. Because of the bones, fishmeal is also high in calcium (5.5%) and phosphorus (3.2%), with a total mineral content of about 18%. It is also a rich source of vitamin B12 and other essential vitamins. The amount of fishmeal to be incorporated into the diet for poultry should not, however, exceed 10% or there will be a chance of fishy flavors in eggs and meat.
Fishmeal derived from sardine, menhaden, and white fish tend to have a higher protein content and better quality than those from redfish, tuna, or salmon. In either case, overheating during processing results in the formation of fatty acid breakdown products such as aldehydes, which react with some free amino acids in the fish proteins, and hence reduce their bioavailability
In some cases, fishmeal may be adulterated with other ingredients such as fish bones, sand, sawdust, etc., which adversely affect its nutritive value. It may also be treated with insecticides for the preservation of fishmeal, which may cause toxicity in poultry. Careful choice of fishmeal sources should, therefore, be duly considered.
References available upon request.