Amino acid requirements of broilers: Relationships with growth and meat quality - By R A Coleman and D R Korver, University of Alberta - Over the last sixty years there has been a rapid reduction in market age for broilers. This has been a direct result from genetic selection (increase in muscle yield), improved diets, and better management of the resources available to the producers.

Summary

A rapidly growing broiler needs to be supplied with sufficient nutrients to meet its requirements for maintenance and for the growth of all components of the bird, including feathers. The crude protein and amino acid status of a diet can influence the carcass composition of broilers, with increased carcass protein and reduced carcass fat accompanying increases in dietary protein or essential amino acid content.

Rapid increases in growth rate have been associated with improved feed conversion efficiency, while amino acid requirements as a percentage of the diet have not changed substantially. Although nutritional requirements expressed as proportions of the diet may not change linearly with improvements in performance, a greater understanding of the requirements for specific factors such as requirements for optimum immune function or breast muscle deposition requires further research.

Introduction

Each year the world's human population is predicted to increase by 70 - 80 million. The majority of this increase is from third world countries. With the economy of many third world countries improving, the demand for animal products and especially meat as a source of protein will increase. Chicken continues to be the least expensive meat in most of these countries. The broiler industry is ideally suited to meet this expected increase demand for animal protein with improved efficiency of production. The broiler industry in Australia and many parts of the world is relatively new compared with other livestock industries.

The broiler industry started on the east coast of the USA in 1920. By 1960, broiler meat made up 10% of the total meat consumption and increased by 5% annually to exceed the consumption of beef and pork in 1995 (Figure 1). By 2000, the total consumption of broiler and other poultry meats was 46.2% of total meat consumption (Kaku, 2001). Factors contributing to an increase in broiler production by 5% each year include genetic selection, dietary, environmental and managerial improvements, and economic factors.

The broiler industry has developed into an robust business with several advantages over it competitors, these include the ease of establishing integrated operations with the adaptability for further processing and economically competitive price of poultry vs. red meats. In the USA in 2000, the price of broiler meat was cheaper than beef (32%) and pork (47%). The actual price of beef and pork has increased by 3.5 times compared to broiler meat which has increased by only 2.5 times since 1960 (Kaku, 2001). Consumers consider broiler meat healthy and inexpensive, which will only lead to further increases in demand (Ishibashi and Yonemoch, 2002).

Increase muscle growth

To meet the market demands poultry selection has concentrated on growth rate and muscle mass in broiler. In the last 30 years, the production time needed to raise a 1.3 kg chicken has been halved (Dransfield and Sosnick, 1999). To meet requirements and maintain protein deposition in broilers, nutritionist must look at the demands of the rapidly growing broiler and adapt the formulated diets to meet those demands. To do this, the nutritionists has to look at two major issues -- what is the potential growth rate of the selected broiler and what are the broilers nutrient requirements to meet those needs. A rapidly growing broiler needs to be supplied with nutrients in order to meet its requirements for maintenance and for the growth of all other components of the broiler, including feathers (Gous, 1998). A good definition of potential growth is given by Gous, (1998), as the maximum possible growth rate that the genotype can achieve when given perfect nutritional and husbandry conditions.

Below the requirement, breast muscle development is sensitive to dietary lysine content in the diet. Many studies citing a positive correlation between dietary lysine level and breast muscle accretion have compared diets deficient in protein and/or lysine to diets adequate (Corzo et al., 2002; Tesseraud et al., 2003). The authors are unaware of any studies in which an increase in breast muscle or growth rate is associated with increases in dietary amino acids or protein at levels well beyond the NRC requirements.

Han and Baker (1991) determined requirements of 8 to 21 day old broilers to be not greater than 1.17% lysine for maximal weight gain. This was slightly higher than the National Research Council recommendations of 1.10% (NRC, 1994). Labadan et al. (2001) also determined that lysine requirements for broilers up to 2 weeks of age to be 1.32% compared to 1.10% recommended by the National Research Council (NRC, 1994). Muscle protein is high in lysine and the portion of breast muscle yield has increased (through genetic selection) compared to the total carcass meat in broiler. Breast muscle contributes about 30% of total carcass meat and accounts for 50% of total edible carcass protein (Summers et al., 1988); some researchers suggest that the lysine requirement has increased over time (Si et al., 2001). However, the lysine requirement for maximum breast muscle accretion was similar to the requirement for growth, but greater than for feed efficiency (Labadan et al., 2001).

Care must be taken when discussing changes in lysine requirements as a function of growth rate. Although broiler growth rate has increased dramatically in the past 60 years (NRC 1944; 1954; 1977; 1984; 1994), the actual lysine requirement, expressed in percentage of the diet, has changed very little over the years. The lysine recommendation given by the National Research Council was 0.902, 0.9, 0.85 to 1.2 and 0.85 to 1.1% of the diet for the years 1944, 1954, 1984 and 1994, respectively. The ranges given for 1984 and 1994 reflect the effect of age on lysine requirement as the bird ages; lysine requirements decrease with age.

Earlier versions of the NRC recommendations for poultry did not take into account the changing requirements of birds as they approached a target body weight. During the last 60 years, the growth rate of birds has increased dramatically. In 1944, a typical Rhode Island Red cockerel or pullet would reach 1.6 kg at 14 to 17 weeks, or 16 to 19 weeks, respectively. In 1944, cockerels would be expected to reach 2.27 kg in 17 to 22 weeks of age; females would reach this weight in 25 to 30 weeks (NRC, 1944). In 1954, "heavy breed" male chickens required 11, and females 14, weeks to reach 1.6 kg (ages to heavier weights were not given in the 1954 NRC). A contemporary publication suggests that New Hampshire males and females would reach 2.3 kg at 21 and 31 weeks, respectively (Titus, 1955).

Although different strains of chicken were reported in each of these publications, the standard for meat-type chicken production was moving towards an increased growth rate. In 1977, broiler chickens would be expected to reach 2 kg in 8 weeks (NRC, 1977). In 1984, that body weight was reached prior to 7 weeks of age (NRC, 1984), and by 1994 a 2 kg body weight would be expected in male broilers at approximately 6 weeks. Growth rates have continued to increase since that time. Concurrent with the improvement in growth rate has been a dramatic improvement in feed efficiency. In 1944, meat-type chickens required approximately 4.1 kg of feed for each kg of gain (NRC, 1944), whereas feed conversion efficiency had decreased to 1.8 kg of feed per kg of gain by 1994 (NRC, 1994).

Much of the improvement in efficiency can be attributed to the decrease in maintenance requirements due to a shorter time to a particular weight. As mentioned previously, the requirements for lysine as a percent of the diet, and presumably other amino acids, have changed very little since 1944. Expressed in terms of total lysine required to reach a particular body weight therefore, have decreased.

Further Information

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Source: University of Sydney - Added to the PoultrySite and published on March 2005