Chickens are living creatures who are supposed to be productive. To a large extent this is related to biological functions in their body. Different nutritional factors have an effect on growth, fertility, egg production, immunity, and on adaptation to various environmental stresses. Such relationships should provide useful tools to adapt nutritional regimens.
By Dr. Salah H. Esmail, Cairo, Egypt
The level of metabolic energy (ME) in the diet of poultry has an effect on growth. Generally, each increment of 110 kcal/kg would result in an increase of 45g in daily gain. This relationship could be used for achieving the target weights of chicks in short periods of time. It should be noted, however, that with such increases in energy level and growth performance there is a reduction of 0.1 point in the feed efficiency. The increased growth in this case must be balanced against the economic losses associated with the increased energy density of the diet in order to achieve optimal performance without adversely affecting economic feasibility.
With most commercial flocks, it was found though that the target weights matching consumer demand could be achieved with feeds having an ME density of 3,300 kcal/kg without any adverse effects on the cost of production. If the ME level of the diet is increased, so too should be the supply of the dietary protein in order to avoid excessive fat formation in the carcass (Table 1). A proportion of the body fat formed with high energy-low protein diets is deposited around the viscera in the abdominal area, resulting in a reduction of at least 3-4% in the dressing-out percentage.
Nutrition and fertility
Fertility can be influenced indirectly by nutrition, and even a fertile egg may fail to yield a viable chick if this egg is missing a certain nutrient. Beyond nutrition, of course, it is assumed that conditions during egg collection, storage and setting are kept optimal.
The control of body weight through manipulation of feed composition and its daily allowance after the peak of production is the first management strategy to be considered when attempting to reduce the rate of decline in fertility due to age. In one study, female broiler breeders that were about 500 grams lighter than recommended weight were more fertile (by 2-4%) than females kept under a standard body weight condition. Hatchability can also be affected by the concentration of vitamins in the broiler breeder feed. For example, a complete omission of riboflavin results in zero hatchability after seven weeks. The vitamin deficiency problem can occur if the vitamin premix is omitted inadvertently during feed preparation, or if some vitamins are omitted by the manufacturers. The deficiency problem may also result from destruction of vitamins during manufacturing or storage of feed under unfavourable conditions. In all these cases, it is important to closely monitor the problem and ensure adequate supply of vitamins which are most needed for improved fertility.
Nutrition and egg production
Protein level in the diet is an important factor determining egg production and egg weight (Table 2). Fats also have similar effects on laying performance, probably due to their effects on the metabolism of oestrogen, which controls the formation of albumen in the egg yolk and hence contributes to increased egg size and weight. Dietary fat source affects the degree to which egg size and weight increase. In a study with laying hens, egg size and weight were greater with vegetable oil included in the diet at 4% than with fish oil fed at the same level. This difference was attributed to the greater ability of the vegetable oil to stimulate oestrogen metabolism.
Other nutrients, such as fibres, may affect pigmentation of eggs. Among the different fibre sources, corn DDGS is known to have an effect on yolk colour, imparting reddish appearance thereon, which is preferred by most consumers. This is because corn contains relatively high levels of xanthophylls which are primary contributors of yolk pigmentation.
Nutrition is also an important factor in eggshell formation. The calcium content of the feed is probably the most limiting factor in this regard, being a major constituent of the shell. Increasing the dietary calcium level from 1% to 3% led to an increased thickness of the shell from 328 μm to 388 μm, with a subsequent reduction in the percentage of cracked eggs from 26.2% to only 10.2%.
Nutrition and immunity
Poultry flocks are exposed to a variety of disease challenges caused by microbial agents. For each of these cases, the nutritional programme can be adjusted to meet the increased or the decreased requirements for the specific nutrient under the adverse conditions. It has been found that the supply of dietary protein should be decreased when coccidiosis is prevalent. If the level of protein is maintained high, so too is the activity of the enzyme trypsin in the small intestine of the bird. This speeds up the release of coccidia from their oocysts to such an extent that they become less responsive to vaccination. Similarly, the supply of calcium should be reduced because this mineral also increases trypsin activity.
On the other hand, it may be necessary to increase the supply of some nutrients for better control of other disease problems. Birds should receive more vitamin E if infected with E. coli. In a study with day-old broiler chickens, there was a linear increase in the production of the antibodies specific to the E. coli infection at 2 weeks of age when the level of vitamin E in the diet was increased from 150 IU/kg to 300 IU/kg. Similar responses were noted with parent stocks fed either 300 IU or 450 IU vitamin E in the diet. At the higher level of the vitamin, there was an increased level of immunity against bacterial infection such as B. abortus in the offspring at 7 days of age.
There is a linear relationship between dietary vitamin A and diseases associated with M. tuberculosis bacteria. Increasing the level of this vitamin from 2,200 to 4,400 IU/kg resulted in increased serum immunoglobulin specific to this infection and reduced mortality.
Vitamin C was found to have a strong action against S. gallinarum when fed at a high level (1,000 ppm) daily for 3 weeks. Similar responses were noted with iron when increased in the diet from 250 ppm to 450 ppm and mortality was reduced by 9%. The inclusion of acetic acid compounds such as ethylene-di-amino-tetra-acetic acid can also be beneficial as it increases the bioavailability of iron.
The physical form of the diet also interacts with the immunity issue. In one study, it was found that the feeding of whole wheat to broiler chickens reduced the number of Salmonella typhimurium and C. perfringens in the intestinal tract of birds, and hence reduced mortality associated therewith. Here, the use of wheat finely ground with a hammer mill has in some cases increased mortality to 28.9% compared to a mortality of 18.1% when using wheat coarsely ground with a roller mill. In other studies, however, the feeding of whole wheat or coarsely ground wheat to broiler chickens experimentally infected with coccidiosis has enhanced development of Eimeria tenella in the cecum. This resulted in significantly lower weight gain in the whole wheat group compared with ground wheat-fed broilers. From these studies, it can be concluded that when the GI tract is healthy, inclusion of whole wheat into the diet may help improve digestive tract function and immunity, but when integrity of the GI tract is already impaired through microbial invasion, then inclusion of whole grains may decrease performance.
Adaptation to heat stress
The reduced feed intake under heat stress conditions can be partly compensated by adding enzymes to the diet because they improve the utilisation of the smaller amounts of feed. A study was conducted using a mixture of enzymes (amylase, protease and xylanase). The enzyme-fed broilers had better metabolic rates and reached a live weight of 2,013 kg at 49 days of age compared to 1,886 kg for the control. Feed efficiency was improved by 0.07 points with the enzyme treatment.
The inclusion of more dietary fats should be considered during hot periods, particularly for broiler chickens, so that daily energy intake can match the requirements for growth. It is common practice on tropical farms to exclude fat from the diet during summer and include it during winter because it is thought that the energy requirement of broilers is less in summer than in winter. Recent studies, however, have shown that inclusion of fat in diets of heat-stressed broilers actually improves feed intake and performance. This is because heat increment of fat is lower than that of other energy sources such as carbohydrates or proteins. When the dietary energy density is increased by adding fat, it is important that the levels of other nutrients, particularly proteins, are proportionally adjusted to maintain a balanced feed and hence optimise utilisation.
Morning and evening
The timing of feeding is also an important factor in alleviating the effects of heat stress on feed intake and utilisation. The major part of the diet should thus be made accessible to the birds when it is cool (early morning and late evening). Broiler chickens are more responsive to the timing of feeding than laying hens. Adjusting feeding time can by itself alleviate many of the problems associated with heat stress in broilers but it is not as successful for layers. It is, therefore, suggested that other management factors need to be considered to achieve better laying performance during the hottest months.
Adaptation to high altitudes
For high-altitude chickens, diets should be duly supplemented with anti-oxidants such as vitamin E. This helps improve performance by alleviating much of the oxidative stress problems caused by reduced oxygen partial pressure, increased ultra-violet light, and increased metabolic rates at high altitudes.
A study was conducted in a Himalayan area north of India, where altitude varies from 3,050 to 3,600 meters above sea level, with the atmospheric oxygen pressure being 30% short relative to the sea level. In this study, broiler chickens were fed on 200 mg of vitamin E per kg of feed for six weeks, and had better performance compared to the control chickens receiving diets devoid of vitamin E (Table 3).
The improved performance with vitamin E supplement could be attributed to the improved feed intake and utilisation of the supplied nutrients, particularly protein which is essential for health, body weight gain and survivability. The vitamin also improves blood characteristics, mainly due to its effects on hematopoietic organs and erythropoiesis, thereby increasing RBC and Hb levels and helps adaptation to high altitudes.
The supply of B vitamins such as thiamine, riboflavin and niacin should also be considered at high altitudes. These vitamins help improve performance by releasing energy from the feed and hence compensate for the low energy intake observed under high altitude stresses.
It is also important to supply adequate amounts of iron at high altitudes where atmospheric pressure is lower and there is less oxygen in each breath. In this case, iron would increase the oxygen-carrying capacity of blood and facilitates its utilisation by the cells, and would hence improve performance and adaptation to high altitude. It should be noted here that most conventional cereals and legumes used in poultry feeding contain non-heam iron which is poorly absorbed and utilised. Therefore, the diet of high-altitude birds should be duly manipulated so as to contain heam-iron sources such as those from animal origin, supplying at least 80 mg of iron per kg of feed, with the protein and energy components to be kept at optimal ratios for each production stage.
References are available from the author upon request.