Chapter 26 Feeding chickens.
SAMUEL BUTLER, 1878
Layers are chickens that produce eggs intended for use as food. The dominant layer is the Leghorn-type chicken. Leghorn-type chickens are relatively small in body size but are prolific layers.
The nutritional phases in the layer production cycle, illustrated in Figure 26-1, include:
Layer: starter, developer, grower, layer phase 1, layer phase 2, layer phase 3.
A sample growth rate is plotted in Figure 26-2.
At hatching, Leghorn-type pullets weigh about 40 g or 0.09 lb. Nutritional deficiencies and imbalances during the starter period may impair growth rate and future egg laying performance. The starter feed is generally fed for the first 6 weeks, at which time the birds are switched to a grower diet. Starter diets generally contain antibiotics to promote rapid, efficient growth and reduce mortality. Coccidiostats are also included to combat the protozoan parasites causing coccidiosis.
Birds are fed the grower diet from 6 weeks to sexual maturity. By the end of the grower phase--at approximately 21 weeks--the Leghorn-type pullet has reached 1,475 g or 3.25 lb. In some cases, birds may be placed on a developer diet after about 6 weeks on the grower diet.
[FIGURE 26-1 OMITTED]
A 28-day molt ration is sometimes fed after 8 to 12 months to extend the production period. Layer phase 1 ration is fed after the molt until rate of lay is no longer profitable.
[FIGURE 26-2 OMITTED]
A developer diet may be inserted between the grower and the layer diet in order to better address the changing nutritional needs of growing pullets. Although the feed for birds fed a developer diet will be less expensive than it would if they stayed on a grower diet, there will be an additional expense associated with managing these birds as a separate group. Developers are fed from 12 weeks of age until birds are up to 5 percent egg production (five eggs per 100 birds) at which time, birds are placed on the layer feed. This occurs at about the 21st week.
Feed for layers is provided free choice. Large amounts of calcium are required for egg-shell production by the laying hen. The calcium content of the egg shell determines its strength, and commercial eggs must be strong enough to withstand handling associated with automated collection, processing, and packaging. In addition, the calcium requirement may be affected by environmental temperature, the rate of lay, egg size, and the age of the bird. Diets for high-producing hens may contain ratios of calcium to available phosphorus as high as 12:1 (National Research Council, 1994).
Because methionine will be the first limiting amino acid in layer diets that are based on corn and soybean, methionine analog is often included in the layer ration formulation.
In a phase-feeding nutritional program for laying hens, nutrient requirements are adjusted according to expected requirements for maintenance and egg production. The adjustments amount to reductions in nutrient requirements as the laying period progresses. During the first phase, birds are growing and increasing in production, and at this time the feed formulation is at maximum nutrient density. During subsequent phases, the rate of lay declines as does nutrient density.
Layer phase 1: Phase 1 is the time from the onset of egg production until past the time of maximum egg mass output.
Layer phase 2: Phase 2 is a period following phase 1 of high (but declining) egg production and increasing egg weight. Egg production during phase 2 declines to about 65 percent of maximum.
Layer phase 3: Phase 3 is a period following phase 2 during which egg production continues to decline below 65 percent of maximum while egg weight increases only slightly.
The validity of the phase feeding system for laying hens has not been established (NRC, 1994). There is no evidence that the nutrient requirements of layers change during the period of lay. The companion application to this text does not use phase feeding to predict the nutrient requirements of laying hens. Rather, the companion application assumes that the amount of nutrient needed each day remains the same throughout a hen's production period.
Layers are sometimes molted to extend the production period. During a molt, feed and light are restricted. A molt can also be induced by feeding a diet containing a nutrient deficiency or excess. The molt may last 3 to 6 weeks. Following the molt, layers returned to a balanced diet will resume egg production, usually at a higher rate than that preceding the molt.
Growing Flock Replacements
Feed represents approximately 60 percent of the cost of raising replacement pullets in both the layer and broiler industries. Replacement pullets are chicks that are grown to produce viable eggs to hatch into chicks that will be sold as broilers or kept to produce hens that lay eggs for food. The nutrient requirement for birds producing hatching eggs is increased over that for hens producing eggs for human consumption for iodine, manganese, and zinc, and for the vitamins E, K, B12, folacin, pantothenic acid, pyridoxine, and riboflavin. From about the eighth week until replacement pullets are placed on the layer feed, feed is usually restricted to a level that supports less than maximal gain to delay the onset of sexual maturity. This is necessary to optimize production of viable eggs in the adult.
Originally, the broiler was a by-product of the egg industry: male pullets were grown and sold for their meat. Today, broilers are chickens that have been bred specifically for rapid and efficient gains, and a large body size. Although they may be marketed at a variety of ages and weights, broilers are usually marketed at 5 to 7 weeks of age when they weigh about 4.5 to 5.5 lb.
Broiler breeders are birds that produce eggs expected to hatch into the pullets that are grown to become meat birds. The production cycle of the meat bird--the broiler--is illustrated in Figure 26-3. It includes the phases of egg incubation, starter, grower, and finisher. Feed conversion in broiler production is about 2 lb. of feed for each pound of bird. A sample growth rate is plotted in Figure 26-4.
[FIGURE 26-3 OMITTED]
[FIGURE 26-4 OMITTED]
The starter diet for broiler chicks corresponds to the starter diet for the Leghorn-type pullets. However, broiler pullets are capable of more rapid growth rates than Leghorn-type pullets, so a nutritionally balanced ration for broiler pullets will be of higher nutrient density than one for layer pullets. Antibiotics and coccidiostats are usually included in the broiler starter diet. The starter diet is usually fed for 2 to 3 weeks.
At about 3 weeks of age, broilers are switched to diets of lower nutrient density. The grower phase is inserted between the starter phase and the finisher phase to take advantage of the reduced nutrient densities required by these older birds compared to the starters. Grower diets are generally fed from 3 to 6 weeks of age.
By the end of the grower phase--at 7 weeks--the male broiler has reached 2,590 g or 5.7 lb. and the female has reached 2,134 g or 4.7 lb. The finisher diet is fed until birds reach market weight. The finisher diet is lower in nutrient content than the grower diet and is therefore less expensive. Finisher rations do not contain antibiotic and are sometimes referred to as withdrawal rations.
LAYER AND BROILER FEEDING AND NUTRITION ISSUES
Dry Matter/Feed Intake
Knowledge of the dry matter/feed intake is essential in order to be able to fortify the diet appropriately. The formulas that have been developed to predict dry matter/feed intake by poultry consider the variables of environmental temperature, energy concentration in the diet, and rate of egg production. Undoubtedly, there are other factors that affect dry matter/feed intake. In the poultry industry, flow meters are sometimes used to measure actual feed consumption, replacing predictive equations.
The Energy Requirement
Through the cloaca, urine and feces are excreted together. Add to this the fact that chickens produce negligible amounts of gaseous products during digestion, and it becomes apparent why nutritionists have chosen to express chicken energy requirements and feedstuff energy values in terms of metabolizable energy: metabolizable energy excludes the energy content of feces, urine, and gas. In poultry nutrition, the metabolizable energy is expressed as "nitrogen-corrected" or MEn. The nitrogen correction involves accounting for feed protein that is retained in the body rather than metabolized for energy.
Because appetite is driven by the need for energy, the energy concentration of the diet has an impact on dry matter/feed intake (Table 26-1). Rations of high energy density should be fortified with high concentrations of amino acids, minerals, and vitamins because the intake of such diets will be relatively low.
Conversely, rations of lower energy density should be fortified with lower concentrations of amino acids, minerals, and vitamins because the intake of such diets will be relatively high. The optimal energy density will generally depend on which formulation results in the lowest feed cost per unit of weight gain or egg production. However, the relationship between dietary energy concentration and feed intake is not precise, and for this reason, caution must be used in feeding untested or unusual formulations. Further discussion on the relationship between ration energy density and dry matter/feed intake is found in Chapter 5.
The poultry NRC (1994) gives energy concentrations based on what is typical in the poultry industry. The companion application to this text uses these typical energy concentrations as targets in ration formulation.
Male broiler chickens grow more efficiently than do females. This is because females tend to deposit more fat in the carcass and fat production requires more energy than does muscle production.
Amino Acids and Nitrogen Requirements
The amino acids that are most often first limiting in corn-soy based chicken diets include methionine, lysine, arginine, and tryptophan. Because the amino acids cysteine and tyrosine are made from the essential amino acids methionine and phenylalanine, respectively, they may become limiting with marginal levels of their precursors. Glycine and serine are amino acids that have been reported as essential for growing chickens. The two are interconvertible, so the requirement may be met in one of three ways:
1. Enough glycine to meet the need for glycine and support synthesis of serine
2. Enough serine to meet the need for serine and support synthesis of glycine
3. Adequate amounts of both glycine and serine (Akrabawi & Kratzer, 1968).
In addition to the need for essential amino acids, the chicken's diet must contain enough protein (nonspecific nitrogen sources) to allow metabolic synthesis of the nonessential amino acids.
The amino acid tryptophan can and will be converted into the B vitamin niacin when intake of niacin is inadequate. Likewise, the amino acid methionine will be converted into the B vitamin choline when intake of choline is inadequate. These conversions may be applied to diet formulation given a consideration of the cost of supplements.
While the mammalian dietary requirement for arginine is due to inadequate synthetic activity to meet metabolic needs, the poultry requirement for arginine is due to a complete lack of synthesis.
Requirements for individual amino acids, just as with all nutrients, should be viewed as targets. Missing the target through dietary excess may be just as bad as missing the target through dietary deficiency. An excess of lysine interferes with utilization of arginine and may result in a depressed appetite. A general excess of amino acid intake may create a deficiency of the first limiting amino acid.
Phytase addition to poultry diets has been shown to improve the utilization of dietary phosphorus (Simons, Versteegh, Jongbloed, Kemme, Slump, Bos, Wolters, Beudeker, & Verschoor, 1990). This is important for two reasons. First, there is an economic benefit: the use of phytase will enable poultry to acquire the phosphorus from corn meal and soybean meal, thereby removing the need to purchase supplemental sources of phosphorus. Second, there is an environmental benefit: the improved utilization of dietary phosphorus will reduce the excretion of phosphorus.
The addition of enzymes that act on carbohydrates has also had positive effects in poultry diets. Such enzymes have been shown to improve weight gain, feed intake, and feed efficiency in broilers (Mathlouthi, Lalles, Lepercq, Juste, & Larbier, 2002). The use of these enzymes in commercial diets is limited due to their high cost.
Xanthophylls and Carotenoids
Feeds that are rich in xanthophylls and carotenoids will produce a deep yellow color in the beak, feet, skin, shanks, fat tissue, and egg yolks of chickens. For some consumers, this coloration is desirable, and these substances are included in diet formulations in the form of additives and natural feedstuffs. Chapter 3 presents additional information on xanthophylls and carotenoids.
The optimal electrolyte balance for poultry has been suggested to be 250 mEq/kg of excess cations, calculated using the sodium and potassium cations and the chloride anion (Mongin, 1981). The 250 mEq/kg is an as fed value, which, assuming a ration of 90 percent dry matter, would be 278 mEq/kg on a dry matter basis. Sodium, potassium, and chloride are the primary dietary ions that influence acid-base status in animals. Dietary electrolyte balance will influence growth, bone development, egg-shell quality, and amino acid utilization in chickens.
Feeding Antibiotics at Subtherapeutic Levels
Antibiotics are often fed at subtherapeutic levels to chickens to improve feed efficiency, rate of gain, and egg production. A discussion of issues related to use of antibiotics in livestock feed is found in Chapter 3.
Poultry are sometimes fed grit to improve the grinding efficiency in the gizzard. Grit is usually composed of the ground shells of oyster, clam, or coquina, or may be made from limestone or other types of stone. Feeding grit is usually not necessary at commercial chicken farms because nutrients are efficiently released from highly processed feeds without extensive grinding in the gizzard.
1. What is the relationship between feed energy density and the level of feed intake?
2. Give four economically important factors that are influenced by the electrolyte balance in chicken diets. What has been suggested as the optimum electrolyte balance for poultry, expressed as mEq/kg, as fed basis?
3. Describe a phase-feeding program for layers.
4. How does the required nutrient density for a broiler change with increasing age? Give the names that describe the rations fed during the three phases of broiler production.
5. What does the "n" in the energy value used in chicken nutrition, MEn, represent? Define MEn.
6. List two metabolic routes for meeting the chicken's requirement for each of the following: niacin, serine, and choline.
7. How are chickens different from most mammals in arginine metabolism?
8. What is grit? Describe its importance in commercial broiler and layer production.
9. What role is played by xanthophylls and carotenoids in feeding poultry?
10. Characterize the availability to chickens of phosphorus in corn meal and soybean meal. In this regard, what role can be played by phytase? Give two potential benefits of using phytase in chicken diets.
Akrabawi, S. S., & Kratzer, F. H. (1968). Journal of Nutrition. 95, 41-48.
Butler, Samuel 1878. Life and Habit.
Leeson, S., Caston, L., & Summers, J. D. (1996). Broiler response to energy or energy and protein dilution in the finisher diet. Poultry Science. 75, 522-529.
Mathlouthi, N., Lalles, J. P., Lepercq, P., Juste, C., & Larbier, M. (2002). Xylanase and [beta]-glucanase supplementation improve conjugated bile acid fraction in intestinal contents and increase villus size of small intestine wall in broiler chickens fed a rye-based diet. Journal of Animal Science. 80, 2773-2779.
Mongin, P. (1981). Recent advances in dietary cation-anion balance: Applications in poultry. Proceedings of the Nutrition Society. 40, 285-294.
National Research Council. (1994). Nutrient requirements of poultry (9th revised edition). Washington, DC: National Academy Press.
Simons, P. C. M., Versteegh, H. A. J., Jongbloed, A. W., Kemme, P. A., Slump, P., Bos, K. D., Wolters, M. G. E., Beudeker, R. F., & Verschoor, G. J. (1990). Improvement of phosphorus availability by microbial phytase in broilers and pigs. British Journal of Nutrition. 64, 525-540.
Table 26-1 Effect of ration energy concentration on DMI in broilers Diet Metabolizable Energy (kcal/lb.,as fed basis) Dry Matter Intake (lb.) (1) 1,452 36.6 1,315 39.2 1,179 41.1 1,043 46.4 862 52.0 726 60.9 (1) per day average over day 35(49 for 100 birds Adapted from Leeson et al., 1996.
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|Author:||Tisch, David A.|
|Publication:||Animal Feeds, Feeding and Nutrition, and Ration Evaluation|
|Date:||Jan 1, 2006|
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