A study published in 2003 comparing the nutrition and genetics of broilers from 1957 to a commercially available broiler strain estimated that within less than 50 years of development, broilers were reared to maturation in a third of the time (Havenstein et al., 2003). This shows clear evidence that improvements in genetics, nutrition and management of rearing meat producing chickens has continually improved the growth rate of birds. However, the effects of rapid growth rate on immune function and nutrient requirements of the immune system has not been well defined.
To date requirement studies define the amino acid requirement of the broiler, based on growth and feed efficiency curves in clean experimental conditions. A growing body of research suggests that this definition of requirement may be too narrow, as the requirement for specific amino acids in terms of optimum growth and immune function may be above those required for growth alone (Rama Rao et al., 2003; Abdukalykova et al., 2008; Jahanian, 2009; Ruiz-Feria and Abdukalykova, 2009; Jahanian and Khalifeh-Gholi, 2017; Sigolo et al., 2017). Therefore, under field conditions where environmental stress and pathogen exposure can challenge broilers during the growth cycle, the current requirements may underestimate the real needs for proper immune development and response to these challenges. As impaired health status due to disease or stress can reduce the ability of a broiler to grow at its genetic potential, it is important for us to know how to feed for optimum growth as well as immune function in order to prevent or quickly ameliorate performance losses when such challenges occur.
While management and hygiene practices can and should be utilized to prevent disease, it is not possible to prevent all encounters with pathogenic organisms. To best prevent detriments to growth performance associated with disease, it is advisable to ensure the animal has a strong, fast and adaptable immune system that can efficiently and swiftly manage threats. This will reduce the time in which nutrients may be diverted to the immune system and away from growth. However, in order to raise birds with a highly functioning immune system we need to understand the requirements of the immune system. It is essential to identify which nutrients and the levels of those nutrients that are required for the development of the immune system and to support response to vaccines. Equally important, is to identify if nutrient requirements change when an animal undergoes a disease challenge so we know as a last resort how we can assist the birds to best cope with minimum impact on growth. Finally, it is advisable that we understand the requirements for the arms of the immune system that deal with regulation and can ameliorate inflammation after recovery from an illness or prevent inflammation from being induced by harmless organisms or feedstuffs. Identifying the essential nutrients for all of these diverse functions of the immune system is no small task and to date there is no single solution or easy answer.