International Journal of Biotechnology and Recent Advances Growth and Immune Response of Broilers in Relation to Varying Dietary levels of Methionine and Threonine

Amino acids are structural and functional units of protein, nutritionally categorized into two groups; essential and non-essential amino acids. Amino acids play important physiological functions inside the body. Extensive research has been done on the use of synthetic amino acids in poultry feed. The uses of synthetic amino acids in diets make it easy to study dose dependent response in poultry birds. Methionine is considered as a first limiting amino acid in corn soya poultry diets. It is a sulphur containing amino acid (SAA) and plays important role for optimum growth performance of broilers. It is involved in muscle accretion, feather synthesis and important biochemical processes (as methyl-group donator). Methionine has been extensively supplemented in broilers diet to enhance the production performance and to decrease the cost of production in commercial broiler farming systems. Nutrient recommendations are available in order to achieve high broiler performance, but the levels of SAA can varies depending upon the production goal such as the optimization for breast meat yield or feed efficiency etc. Threonine is third limiting amino acid in poultry diet. A lot of research work has been done to find optimal level of threonine to get high broiler’s performance. Threonine has an impotent role in the gut health as an essential component of mucin. Threonine is the part of immuno-globulins and also works as proteinogenic amino acid. The health status of birds has a direct link with their immune system and broilers with strong immune system grow better. Research has showed that birds shows better performance at higher dietary methionine and threonine levels with better immune system.

The dietary protein level can be reduced by the supplementation of synthetic amino acids [18]. The use of amino acids reduces the loss of nitrogen during protein metabolism which ultimately results in low ammonia excretion in environment and improves growth performance of the birds [18]. Information about minimum requirements of amino acids is available [28]. These requirements were established under ideal conditions whereas under commercial broiler farming conditions these established values are lower than requirements [24].
Supplementation of amino acids in poultry diets improve the growth performance and strengthen the immune system [20,31]. Immune proteins isolated from mammalian plasma have higher concentration of Met and Thr than other amino acids [36]. The level of methionine +cystine (M+C) and Thr modulated the expression of immune genes in broilers [4]. Methionine enhances the production of antibodies in broilers [37]. Threonine is required for gut health [1], and stimulates the production of antibodies in lymphocytes [8]. It is an essential component of intestinal mucin [8,23] which protects internal linings of GIT. Threonine is required for synthesis of mucin, because Thr has hydroxyl group that is used to develop ester linkage with carboxyl groups [27]. So the use of Thr for synthesis of mucin may be compromised if diet is deficient in Thr which may affect the growth performance of broilers [26].
Dressing percentage can be significantly improved by extra supplementation of synthetic amino acids in diet than NRC recommendations [30]. Higher level of Met in diet decreasesthe fat % in meat [15,35]. Carcass and breast meat yield can be increased by using higher concentration of Thr in broiler diet without affecting the abdominal fat production [25,16].
This review explores existing information about effect of dietary Met and Thr levels on production performance of broilers and to deliver vision on its future commercial application in the poultry industry.

Effect of methionine supplementation on growth performance
Effect of different levels (starter phase 0.31-1.10%; finisher phase 0.29-0.87%) of Met on growth performance of broilers is presented in Table 1. Many studies have been conducted to evaluate the effect of different dietary Met levels on growth performance of broilers. Results of these studies showed that the increasing dietary Met level increase the body weight gain. [34] observed better FCR at Met level of 0.51% compared with 0.31% Met in diet. [11] also found better FCR at 0.54% Met in starter diet compared to 0.42% Met level.
Dietary Met level also affects the feed efficiency. [13] reported that the FCR decreased by increasing the Met level from 0.50 to 0.56%. [2] conducted two experiments on broilers during starter phase and got lowest FCR at dietary Met level of 1.10%. Similarly, research has been conducted on broilers during finisher phase to find optimal level of Met for production performance. Best FCR was found at 0.41% dietary Met [11] while the results of another experiment [35] showed better FCR at 0.51% Met in diet. Great variation in Met requirement to get optimum performance was observed in different studies [34,2]. Difference in Met requirements might be due to different strains of broilers and environmental variations.

Effect of threonine supplementation on growth performance
Threonine is third limiting amino acid in poultry diet [18,19]. Extensive research work has been done to find optimal level of Thr for growth performance of broilers for many years [30,9, dietary levels (0.64 to 1.55%) of Thr during different phases is presented in Table 2. [1] conducted experiment in three phases and used varying Thr levels, during first phase (1-12 days) best FCR with highest weight gain was observed at 0.98% Thr. During second phase (13-25 days) best FCR and weight gain was at Thr level of 0.95%, while in third phase (26-45 days) highest weight gain and better FCR was obtained at dietary Thr level of 0.78 and 0.85%, respectively. [12] got best FCR with highest weight gain at 0.87% Thr, during starter phase of broilers. [32] used Thr in diet and observed lowest FCR at 1.55% Thr (1 -21 days). [9] conducted experiment in two phase, during first phase (14-28 days) highest weight gain and lowest FCR was obtained at Thr level of 0.73 and 0.77%, respectively while four levels of Thr were used (0.69, 0.73, 0.77 and 0.81%). During second phase of experiment (28-42 days), Thr levels were used form 0.60-0.72% with difference of 0.4 between successive groups and result showed highest weight gain with lowest FCR at 0.72% Thr. The results of experiment conducted by [30] showed better Thr level for FCR and weight gain was 0.87% at age of 21-42 days of broiler. In above mentioned experiments feed intake during starter phase was increase by increasing dietary Thr levels up to certain level and then start to decrease. During finisher phase intake was positively correlated with graded levels of Thr used in diets.

Effect of methionine supplementation on carcass characteristics
Different Met supplementation levels affect the carcass characteristics in broilers. [11] found higher carcass% in broilers fed diet containing Met level of 0.24 and 0.29% during starter and finisher rations, respectively. [30,13] reported positive relationship between dressing% and level of Met used. [33] conducted research on male and female broilers separately and the results showed higher carcass% and breast yield at dietary Met levels of 0.69 and 0.93% for male and female broilers, respectively.The increase in the Met level in diet increased the thigh% [11] but abdominal fat% was decreased [11,29,17,32]. Relative weight of liver also improved by feeding higher dietary Met level to broilers [3]. Effect of threonine supplementation on carcass characteristics [1] used three dietary Thr levels in starter (0.89, 0.98 and 1.07%), grower (0.79, 0.87 and 0.95%) and finisher (0.71, 0.78 and 0.85%) phase and observed highercarcass and breast meat% at dietary Thr level of 0.98, 0.87 and 0.78% during starter, grower and finisher phase, respectively. The highest thigh% was found at Thr level of 1.07, 0.95 and 0.85%, respectively. (32) reported increase in breast meat (22.26 to 23.67%) and decrease in abdominal fat (1.70 to 1.62%) by increasing dietary Thr level from 0.08 to 0.16% during starter phase in broilers. [9] also observed higher carcass and breast meat yield by feeding diet containing Thr 0.77 and 0.68% during starter and grower phase, respectively. [5] found higher carcass and breast meat yield and lower abdominal fat at Thr levels of 0.65 and 0.86%, respectively. [7] conducted trials on male and female broilers separately and found highest carcass and lowest abdominal fat at Thr level of 0.63 and 0.52% for male and female broilers, respectively. [22] used different levels of Thr in broilers diet and observed higher breast meat yield at 0.67% and lowest abdominal fat at 0.85% Thr level in diet during finisher phase.

Effect of methionine and threonine supplementation on immune response
Growth performance of living organisms directly related to their health status. A strong immune system means that individual is less prone to attack of diseases. Immune response is reviewed by investigating antibody titer levels mainly against infectious bursal disease virus (IBDV) and Newcastle disease virus (NDV). Data on the effect of Met and Thr supplementation on immune response in broilers is presented in Table 5. [24] found higher antibody titer levels against NDV at Met level of 0.75 and 0.49% during starter and finisher phase, respectively. But the higher titer level against IBDV was found at Met levels of 1.27 and 0.88% during starter and finisher phase, respectively. [2] conducted two experiments to find Met level for maximum antibody titer levels during starter phase. In both experiments, highest titer level against NDV was found at 0.40% Met in diet. In another experiments [35] maximum antibody titer level against NDV and IBDV was found at Met level of 0.87 and 0.37%, respectively.
[1] used different dietary Thr levels in starter, grower and finisher diets of broilers and found highest antibody titter levels at dietary Thr level of 0.98, 0.87 and 0.78% in starter, grower and finisher phase, respectively. [32] found highest antibody titer level against NDV at dietary Thr level of 0.11 and 0.16% after 1 st and 2 nd vaccine, respectively during starter phase; but second antibody titer level was comparatively higher than that of first. [24] used different dietary Thr levels in starter and finisher diets and found higher antibody titer levels by feeding 1.62 and 1.51% Thr during two phases mentioned above, except 1 st antibody titer level against NDV which was highest at 0.88 and 0.76% Thr during starter and finisher phase, respectively. [38,39]

Conclusıon
Keeping in view the present review it may be concluded that; Broilers showed better performance at higher dietary level of Methionine and threonine, broiler's diet should be supplemented with these amino acids to enhance growth performance and to strengthen their immune system