Poultry convert feed into food products quickly, efficiently, and with relatively low environmental impact compared with other livestock. The high rate of productivity of poultry results in relatively high nutrient needs. Poultry require the presence of at least 38 dietary nutrients in appropriate concentrations and balance. The nutrient requirement figures published in Nutrient Requirements of Poultry (National Research Council, 1994) are the most recent available and should be viewed as minimal nutrient needs for poultry. They are derived from experimentally determined levels after an extensive review of the published data. Criteria used to determine the requirement for a given nutrient include growth, feed efficiency, egg production, prevention of deficiency symptoms, and quality of poultry product. These requirements assume the nutrients are in a highly bioavailable form, and they do not include a margin of safety. Consequently, adjustments should be made based on bioavailability of nutrients in various feedstuffs. A margin of safety should be added based on the length of time the diet will be stored before feeding, changes in rates of feed intake due to environmental temperature or dietary energy content, genetic strain, husbandry conditions (especially the level of sanitation), and the presence of stressors (such as diseases or mycotoxins).
Water is an essential nutrient. Many factors influence water intake, including environmental temperature, relative humidity, salt and protein levels of the diet, birds’ productivity (rate of growth or egg production), and the individual bird’s ability to resorb water in the kidney. As a result, precise water requirements are highly variable. Water deprivation for ≥12 hr has an adverse effect on growth of young poultry and egg production of layers; water deprivation for ≥36 hr results in a marked increase in mortality of both young and mature poultry. Cool, clean water, uncontaminated by high levels of minerals or other potential toxic substances, must be available at all times.
Energy Requirements and Feed Intake:
The energy requirements of poultry and the energy content of feedstuffs are expressed in kilocalories (1 kcal equals 4.1868 kilojoules). Two different measures of the bioavailable energy in feedstuffs are in use, metabolizable energy (AMEn) and the true metabolizable energy (TMEn). AMEn is the gross energy of the feed minus the gross energy of the excreta after a correction for the nitrogen retained in the body. Calculations of TMEn make an additional correction to account for endogenous losses of energy that are not directly attributable to the feedstuff and are usually a more useful measure. AMEn and TMEn are similar for many ingredients. However, the two values differ substantially for some ingredients such as feather meal, rice, wheat middlings, and corn distiller’s grains with solubles.
Poultry can adjust their feed intake over a considerable range of feed energy levels to meet their daily energy needs. Energy needs and, consequently, feed intake also vary considerably with environmental temperature and amount of physical activity. A bird’s daily need for amino acids, vitamins, and minerals are mostly independent of these factors. The nutrient requirement values in the following tables are based on typical rates of intake of birds in a thermoneutral environment consuming a diet that contains a specific energy content (eg, 3,200 kcal/kg for broilers). If a bird consumes a diet that has a higher energy content, it will decrease its feed intake; consequently, that diet must contain a proportionally higher amount of amino acids, vitamins, and minerals. Thus, nutrient density in the ration should be adjusted to provide appropriate nutrient intake based on requirements and the actual feed intake.
Because of the ability of poultry to adjust their feed intake to accommodate a wide range of diets with differing energy content, the energy values listed in the nutrient requirement tables in this section ( Nutrient Requirements of Growing Pullets a Nutrient Requirements of Growing Pullets a through Linoleic Acid, Mineral, and Vitamin Requirements of Leghorn-type Chickens a Linoleic Acid, Mineral, and Vitamin Requirements of Leghorn-type Chickens a ) should be regarded as guidelines rather than absolute requirements.
Appropriate body weight and fat deposition are important factors in rearing pullets for maximal egg production. Most strains of White Leghorn chickens have relatively low body weights and do not tend, under normal feeding, to become obese. Feed is normally provided for ad lib intake to this strain of pullets. For brown-egg strains of chickens, some degree of restriction is often practiced (~90% of ad lib feeding) to prevent precocial onset of lay. Broiler strains tend to become obese if fed ab lib; feed restriction is necessary for broiler pullets and broiler breeders. When feed restriction is practiced, the feed levels of amino acids, vitamins, and minerals must be proportionally increased to prevent deficiencies. Most large commercial breeders provide feed restriction and dietary nutrient guidelines specific for their strains.
Amino Acid Requirements:
Poultry, like all animals, synthesize proteins that contain 20 L-amino acids. Birds are unable to synthesize 9 of these amino acids because of the lack of specific enzymes: arginine, isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine. Histidine, glycine, and proline can be synthesized by birds, but the rate is usually insufficient to meet metabolic needs and a dietary source is required. These 12 amino acids are referred to as the essential amino acids. Tyrosine and cysteine can be synthesized from phenylalanine and methionine, respectively, and are referred to as conditionally essential because they must be in the diet if phenylalanine or methionine levels are inadequate. The diet must also supply sufficient amounts of nitrogen to allow the synthesis of nonessential amino acids. Essential amino acids are often added to the diet in purified form (eg, DL-methionine and L-lysine) to minimize the total protein level as well as the cost of the diet. This has the added advantage of minimizing nitrogen excretion.
Requirements for vitamins A, D, and E are expressed in IU. For chickens, 1 IU of vitamin A activity is equivalent to 0.3 mcg of pure retinol, 0.344 mcg of retinyl acetate, or 0.6 mcg of β-carotene. However, young chicks use β-carotene less efficiently.
One IU of vitamin D is equal to 0.025 mcg of cholecalciferol (vitamin D3). Ergocalciferol (vitamin D2) is used with an efficiency of <10% of vitamin D3 in poultry.
One IU of vitamin E is equivalent to 1 mg of synthetic dl-α-tocopherol acetate. Vitamin E requirements vary with type and level of fat in the diet, the levels of selenium and trace minerals, and the presence or absence of other antioxidants. When diets high in long-chain highly polyunsaturated fatty acids are fed, vitamin E levels should be increased considerably.
Choline is required as an integral part of the body phospholipid, as a part of acetylcholine, and as a source of methyl groups. Growing chickens can also use betaine as a methylating agent. Betaine is widely distributed in practical feedstuffs and can spare the requirement for choline but cannot completely replace it in the diet.
All vitamins are subject to degradation over time, and this process is accelerated by moisture, oxygen, trace minerals, heat, and light. Stabilized vitamin preparations and generous margins of safety are often applied to account for these losses. This is especially true if diets are pelleted, extruded, or stored for long periods.
Much of the phosphorus in feedstuffs of plant origin is complexed by phytate and is not absorbed efficiently by poultry. Consequently, it is critical that only the available phosphorus and not the total phosphorus levels be considered. Appropriate calcium nutrition depends on both the level of calcium and its ratio to that of available phosphorus. For growing poultry, this ratio should not deviate substantially from 2:1. The calcium requirement of laying hens is very high and increases with the rate of egg production and age of the hen.
Other Nutrients and Additives:
The chick has requirements for 38 nutrients, together with an adequate level of metabolizable energy and water. Some additional nutrients may be necessary for growth and development under certain conditions. These include vitamin C, pyrroloquinoline quinone, and several heavy metals.
Non-nutrient antioxidants, such as ethoxyquin, are usually added to poultry diets to protect vitamins and unsaturated fatty acids from oxidation. Antibiotics at low levels (5–25 mg/kg of feed, depending on the antibiotic) and surfeit copper (150 ppm) are sometimes included to improve growth rate and feed efficiency. Enzymes that increase the bioavailability of dietary phosphorus, energy, and protein are often used in poultry diets when their costs are not prohibitive. In some cases, phytase enzymes are used to decrease the amount of phosphorus in the excreta to meet environmental regulations.