Generations of selection have led to incredible changes in the
potential for growth of broiler chickens. Selection for traits such as
rapid growth and enhanced breast muscle mass has been accompanied by an
increase in voluntary feed intake
Generations of selection have led to incredible changes in the potential for growth of broiler chickens. Selection for traits such as rapid growth and enhanced breast muscle mass has been accompanied by an increase in voluntary feed intake.1 Broiler breeder producers are forced to employ some form of feed restriction in order to avoid an array of reproductive problems that arise in full fed birds. The positive effects of feed restriction include reduced body weight, delays of sexual maturity, increased egg production, reduction in number of unsettable eggs, reduced incidence of multiple hierarchies and increased livability during the laying period.2-5
|As genetic progress is made, nutritional, health and management inputs must keep pace if the true potential of the bird is to be realized.
Primary breeders endeavour to produce strains with a balance between reproductive potential in the parent stock and rapid growth and excellent feed efficiency in their offspring. As progress is made genetically, the nutritional, health and management inputs must keep pace if the true potential of the bird is to be extracted. In order to provide the best possible nutrition to a broiler breeder that has changed over the years, it is important to understand what has changed and how that impacts the nutritional requirements of the hen.
Aviagen recently conducted a trial comparing a modern (2005) Ross 308 broiler to a population that had been random bred since 1972. Three different “balanced protein” levels were fed to the birds from the two different eras to determine if the response to amino acids had changed over time. The term “balanced protein” refers to the complete ideal amino acid profile rather than to crude protein. The three levels tested were 80, 100 and 120 per cent of the recommendation for the Ross 308 (Ross 308 Broiler Nutrition Specification, 2007). The 100 per cent treatment had a digestible lysine level of 1.27, 1.10 and 0.97 per cent in the starter, grower and finisher diets respectively. Some of the results from the trial are depicted in figures 1 through 3.
Figure 1 shows the live weight of as-hatched broilers at 35 days. It is clear that genetic selection has resulted in a radical improvement in growth rate (approximately 35 grams per year). What is perhaps more interesting from a nutritional perspective is that the response to increasing amino acid density in the modern broilers appears to be more significant than that of the random bred broilers.
|Figure 1. The influence of balanced protein density on 35-day body weight of straight run broilers from either a modern (2005) population or a random bred population (1972)
|Figure 2. The influence of balanced protein density on mortality adjusted feed conversion ratio (corrected to two kilograms) of straight run broilers from either a modern (2005) population or a random bred population (1972)|
|Figure 3. The influence of balanced protein density breast yield (corrected to two kilograms), abdominal fat pad and total carcass fat of straight run broilers from either a modern (2005) population or a random bred population (1972)
The results presented in Figure 2 show that there has been a vast improvement in feed conversion ratio (FCR) since 1972. This improvement equates to roughly two points per year. Again, it is clear that the modern broiler is also more responsive to dietary amino acid density.
Figure 3 (page 35) shows the difference in carcass composition of the modern and random bred strains in the 100 per cent treatment. The random bred strain had almost 9 per cent less breast meat and double the abdominal fat pad percentage of the modern strain. The total carcass fat of the modern strain was 7.1 per cent lower than the random bred strain. Modern strains do not carry as much fat as older strains. This is an important point to consider when feeding broiler breeders, as there is a need for at least some fat reserve on the young hen.
The results of the trial clearly show the improvements that have been made thanks to genetics. However, what is also clear is that the modern strains are more responsive to dietary amino acid density and that they tend to carry a lot more breast muscle and a lot less fat. While broilers are fed to maximize growth and meat yield, the propensity for rapid growth and breast yield must be controlled in parent stock. Further, a certain amount of fat is desirable in broiler breeders.
The traits that make the modern broiler so successful are often the same ones that need to be suppressed to ensure good parent stock performance. For example, feeding unnecessarily high levels of amino acids to broiler breeders will lead to over-fleshing. This is particularly true of high yielding strains. This extra muscle requires a lot of energy to maintain. This in turn places a further demand on the energy requirements of a bird that is already less inclined to accumulate fat. The balance of amino acids to energy in the modern broiler breeder diet is very important. The diet must allow a producer to feed sufficient volume to meet the energy requirement of the bird without over feeding amino acids in the process.
Current Trends in Nutrient Density
Commercial feeds are often still formulated to minimum crude protein levels, which results in feeds that, with the exception of TSAA, have amino acid levels significantly higher than required. For example, a corn/soy diet has excess lysine and isoleucine. When formulating to minimum crude protein levels, the lysine levels are often up to 40 per cent above requirement. This excess lysine can drive muscle deposition. The negative effect of excess crude protein on fertility has been clearly illustrated, and researchers have identified lysine and isoleucine as two amino acids directly affecting the fertility of breeders.6,7
The amino acid requirement of a growing bird includes two components, a requirement for maintenance and a requirement for tissue protein accretion. In the case of breeder hens there is also a requirement for egg production. Several experiments were recently conducted to determine the amino acid requirements of broiler breeders at peak production.7 These requirements were determined using test diets in which a specific amino acid was added at increasing levels while all other amino acids were kept at constant levels. Separate trials were conducted to determine maintenance and production requirements. Maintenance was considered to be the point of zero accretion of the specific amino acid being tested.
Linear regressions were used to determine the maintenance requirements, and polynomial regressions were used for the production requirements. The summarized results of the trials are presented in Table 1 (page 44). Table 1 also shows the requirements as suggested by Fisher (1998) and those published by the NRC (1994).
Very little data exists to suggest that there are specific amino acid requirements for fertility. However, hens from the above trials were artificially inseminated with 5×107 sperm weekly and every egg was set the Wednesday after it was laid in order to determine if there was a fertility response to increasing levels of each amino acid.
|Figure 4. Average fertility of broiler breeder hens fed increasing levels of digestible isoleucine|
|Figure 5. Average fertility of broiler breeder hens fed increasing levels of digestible lysine
A reduction in fertility with increasing levels of lysine and isoleucine was found (figures 4 and 5). In repeats of the trials, the trends were again evident. The specific mechanisms whereby these reductions in fertility occur are not clear.
In attempting to elucidate these mechanisms it was of interest to determine the effect of storage in the hens’ sperm host tubules on fertility. It is possible that a change in the microenvironment of the tubules may be responsible for the eventual changes in fertility.
Figures 6 and 7 (page 39) show that a peak in fertility is reached around four days after insemination for both isoleucine and lysine. It is clear from the figure that in both cases the drop off in fertility after the peak is far greater in the hens on high isoleucine and lysine diets than in the hens on the low diets. This suggests that the differences in fertility may be partly explained by differences in the quality of the sperm cells after a period of storage in the hen’s sperm storage tubules. It is of great interest to determine the possible differences in storage conditions created in the sperm host tubules as a result of different isoleucine and lysine levels in the diet.
Figure 8 (page 42) shows the difference in pH of hens on low and standard isoleucine diets. The extremely low inclusion of isoleucine causes the diet to be unbalanced. The hen is forced to catabolize any excess amino acids in order to counteract the imbalance. Excess nitrogen can be very toxic and must be expelled from the body. This process of breakdown and excretion most likely leads to the observed differences in urine pH. The sperm host tubules are found near the utero-vaginal junction. There is a possibility that the tubules experience a different pH and perhaps some different ionic concentrations and ratios as a result of the low isoleucine diets, which may lead to the observed differences in fertility.
Energy and Amino Acid Levels in Current Pullet and Breeder Diets
A summary of energy and crude protein levels in pullet and breeder diets in the United States over the last four years is presented in figures 9 (pullets) and 10 (breeders) (page 42).
The trend in the U.S. industry in pullet developer diets shows that integrators are feeding both lower energy and crude protein levels to broiler breeder pullets. Although the exact reason for these trends is not clear, several possibilities can be speculated on. For example, as appetites continue to become more voracious, feeding a lower density diet has benefits in terms of feed volume and clean-up time. Lower density diets help to promote uniformity. Part of this could also be related to cost as ingredient prices have soared over the last 18 months to two years. Lower crude protein levels also make it easier to control the fleshing of the pullet.
|Figure 6. Average fertility by day for low and high levels of isoleucine intake (hens were inseminated on Wednesday of each week)|
|Figure 7. Average fertility by day for low and high levels of lysine intake (hens were inseminated on Wednesday of each week)
Trends in breeder I diets indicate that energy levels are being increased and that crude protein levels are being decreased on average across the U.S. industry. The available research indicates that the amino acid requirements of the birds are easily being met and the tendency toward lower crude protein levels in the U.S. should be seen as a positive step. This is particularly true in light of the tendency for various modern breeds to develop a very large breast. In fact, based on the information in figure 10 it would appear that meeting the energy requirement, without over feeding crude protein, has become a focus of many U.S. integrators.
Effect of Breeder Nutrition on Progeny
There is an abundance of evidence that confirms that breeder nutrition directly impacts the progeny. Fifteen years ago peak feed levels were higher and egg production was lower than it is today. Measuring breeder nutrition impact on progeny is difficult but under conditions of poor breeder flock uniformity, low vitamin and trace mineral levels in breeder diets or some form of stress in the broiler house, the impact of breeder nutrition on progeny performance can be significant. Small or no differences may be seen under trial or ideal commercial conditions, but under stress (disease, low broiler nutrient levels, chilling) the responses will be more apparent.
All nutritionists are aware of the critical importance of macro minerals such as calcium and available phosphorus on bone and eggshell integrity. However, there is very good evidence that maternal levels of trace minerals especially zinc, manganese, copper and selenium impact levels in the egg and influence progeny. Hen feeds deficient in zinc were shown to cause slow growth of chicks, and to cause weak chicks, poor feathering and high mortality; while supplementing with inorganic and/ or organic zinc increased levels in the bones and increased bone weight.8-10
In addition to zinc’s role in DNA and enzymes, it is particularly important in the young bird in the synthesis of two key proteins: collagen and keratin. Keratin is a structural protein in skin and feathers while collagen is the major structural protein of internal tissues, including cartilage and bone. The cost of ensuring a sufficient and an available zinc source in breeder and early broiler feeds is minimal considering the impact of poor skeletal development and compromised immunity on the profitability of broiler production.
As with zinc, manganese, copper and selenium levels can be affected in the egg by maternal supplementation. Manganese is vital in embryonic and post-natal bone development. Copper is essential for reproduction and development. Selenium has a sparing effect on vitamin E as an antioxidant.
Although severe vitamin deficiencies will cause a wide variety of deformities and severely affect hatch, grossly underfeeding vitamins is not commonly seen in practice. It is more common to see marginal deficiencies caused either by low supplementation, sources of questionable quality/availability and less dominant breeders consuming less than calculated feed quantities. The progeny will not exhibit classical deficiency syndromes, but they will not perform to their potential. Aviagen conducted a study to assess the impact on progeny where vitamins E, K and B vitamins were supplemented at 20 per cent above Aviagen breeder recommendation. Although broiler body weights were only 20 grams heavier at term, mortality of the supplemented group was 2.2 per cent lower than the control birds, with a yield advantage of 0.2 per cent at two-kilogram body weight.
|Figure 8. Urine pH in the morning and evening over three days, of colostomized breeder hens fed either low or standard levels of isoleucine in the diet
|Figure 9. Trends in metabolizable energy and crude protein levels in pullet grower diets in the United States (Source: Industry reporting service)
|Figure 10. Trends in metabolizable energy and crude protein levels in breeder I diets in the United States (Source: Industry reporting service)
Egg yolk vitamin E levels were measured and a 50 per cent increase in alpha-tocopherol was seen in the supplemented birds. In addition, candling clears at 60 and 64 weeks of age were 12.2 versus 17.3 and 17.9 versus 26.9 per cent for the higher supplementation group.
Adequate vitamin and trace mineral supplementation with quality vitamins and available minerals is an inexpensive way to ensure that the young chick is prepared for optimal skeletal growth and a healthy immune system, to help deal with challenges during brooding.
Broiler breeder producers utilize feed restriction programs to control the growth of their pullets. During rearing the daily intake is severely restricted and may be reduced to one-third of the intake of ad libitum fed birds of the same age or half of the intake of ad libitum fed birds of the same weight.11,12 Compared to full-fed broiler breeder hens, restricted hens produce more eggs, persist in lay longer and have longer sequences, lay fewer abnormal eggs, and have fewer multiple ovulations.13-15 Despite these successes, feed restriction has been implicated in numerous welfare issues such as increased stereotyped spot-pecking, over-consumption of water and even changes in plasma heterophil to lymphocyte ratios.16 Apart from changes in indices of stress, feed restriction has also been shown to alter other metabolic characteristics of the hen such as its capacity for lipogenesis.17
During restricted feeding, competition for feed is intense and smaller birds are often at a distinct disadvantage. In many cases a skip-a-day or 4-3 program is preferred to an everyday restriction program because of the improvement in body weight uniformity. By feeding a larger amount of feed every second day, feed clean-up time is increased. This gives smaller, less competitive birds the chance to consume more feed once the larger birds have eaten.
The effects of skip-a-day versus everyday feeding have been compared in various trials.18,19 The results were very consistent among the various trials. Skip-a-day feeding improved uniformity of breeder pullets. Everyday fed birds grow with improved efficiency over skip-a-day fed birds. They also tend to achieve sexual maturity in a shorter period of time after photostimulation. This occurs even when body weights are equal between the two groups. The earlier onset of production translates to more eggs in everyday fed birds. The eggs of skip-a-day fed birds were relatively larger than those of everyday fed birds. The studies also demonstrated that skip-a-day fed breeder pullets have elevated levels of hepatic lipogenesis after feeding, compared to everyday fed birds. Body composition was not dramatically affected by feeding regimens. Most of the lipids made after a meal were utilized by the skip-a-day birds in the fasting period before the next meal. The livers of skip-a-day fed birds were larger than everyday birds during the rearing period and contained more total lipid. Skip-a-day feeding increased heterophil to lymphocyte ratios over everyday feeding. This was considered an indication of the stress associated with hunger during the longer fasting periods in skip-a-day fed birds. However, in general, the heterophil to lymphocyte ratio returned to levels similar to everyday birds during the latter parts of the rearing period. This suggests that there is some level of adaptation to these forms of feed restriction. Mortality, fertility and hatchability were not affected by feeding regimens.
|Table 1. Digestible amino acid requirements for both
maintenance and production from empirical data with broiler breeder hens * Egg mass; ** Body weight change
Further, expression of lipogenic genes and levels of plasma hormones and metabolites were determined at various times over the course of a single feeding cycle for everyday and skip-a-day birds at 16 weeks of age. It was found that skip-a-day regimens dramatically elevated the expression of several key lipogenic genes such as acetyl-CoA carboxylase, fatty acid synthase and malic enzyme after feeding. Malic enzyme is responsible for providing reducing equivalents for biosynthesis of fatty acids. It was also found that the expression of aspartate amino transferase and isocitrate dehydrogenase was depressed after feeding in skip-a-day birds. These genes were more highly expressed during the fasting periods, reflecting their roles in providing glucose from gluconeogenesis.
Uniform flocks are essential for maximal breeder performance. Such flocks make it easier for nutritionists to accurately meet the nutritional needs of each individual. The effects of skip-a-day feeding have now been well documented and present the possibility that such systems may in fact be harmful to the ultimate performance of the bird. It is difficult to quantify the potential benefits in performance against the potential negative effects of reduced uniformity. As feed distribution methods are modified, the uniformity of everyday fed birds may be improved, which will make it possible to exploit the benefits of reduced feed costs, improved growth efficiency, reproductive performance and welfare associated with such programs.
Selection practices have resulted in vast improvements in broiler growth rate, feed conversion and yield. Understanding how these changes affect the nutritional requirements of the parent stock is one of the keys to maximizing parent stock performance. Given the modern birds’ tendency to develop a large breast mass, it is important to ensure amino acids are not being over fed. In the pullet phase, feeding low-energy diets is quite common in order to increase feed volume. However, it is important to ensure that amino acid levels are reduced in accordance with energy levels when employing such a strategy. This will prevent excessive crude protein intakes. In the breeder phase, trends show that energy levels are increasing while amino acid levels are decreasing. Vitamin and trace mineral nutrition is important not only for breeder performance but also for the performance of the offspring. While acute vitamin and trace mineral deficiencies are rare, marginal deficiencies can have a significant impact on broiler performance. Feed restriction programs are useful management tools but it should be kept in mind that the schedule of feeding (everyday, skip-a-day, etc.) can have a significant impact on performance, welfare and other physiological parameters. It is the responsibility of the nutritionist to provide a high quality, consistent diet that meets all the requirements of the bird without over feeding any nutrients. With a consistent feed, a production manager can “learn” to allocate feed accurately and maximum performance can be achieved.
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below recommended levels. Poultry Science 70:26-36.
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National Research Council (1994) Nutrient Requirements of Poultry. 9th rev. ed. National Academy Press, Washington, D.C.
Presented at the European Symposium of Poultry Nutrition
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