The Effect of Varying Levels of Tryptophan on Growth Performance and Carcass Characteristics of Growing and Finishing Broilers

The purpose of this experiment was to study the effect of varying levels of tryptophan on the performance and carcass character of broiler. Trial 1: Ninety-six, five-week-old male Hubbard chickens, average weight 1.97 kg, were used in the trial. All birds were allocated into 3 treatments of 32 birds each. Each bird was kept in an individual cage. The trial period was 3 weeks. Treatment 1: Tryptophan content 0.198%. Treatment 2: Tryptophan content 0.228%. Treatment 3: Tryptophan content 0.258%. Trial 2: Ninety-six, three-week-old male Hubbard chickens, average weight 1.23 kg, were randomly distributed into the following two treatments. Each treatment had 48 birds. Treatment 1: Tryptophan content 0.167%. Treatment 2: Tryptophan content 0.198%. Trial 3: Ninety-six, twoweek-old Hubbard chickens, average body weight 0.72 kg, were used in this experiment. There were three treatments as follows. Treatment 1. Tryptophan content 0.136%. Treatment 2. Tryptophan content 0.167%. Treatment 3. Tryptophan content 0.198%. The result of Trial 1 showed that the feed intake, performance, and carcass characteristics were not influenced by tryptophan content in the diet which between 0.198% and 0.258% (p>0.05). There was no significant difference (p>0.05) in feed intake in either treatment in Trial 2. However, weight gain, feed conversion efficiency, and most carcass characteristics in the 0.198% treatment were significantly better (p<0.05) than in the 0.167% treatment. There was a trend that feed intake increased with increasing level of tryptophan, but there was no significant difference in Trial 3. The weight gain and feed conversion efficiency were significantly reduced for the broiler in the 0.136% treatment. This series of experiment showed that broilers need about 0.198% of tryptophan. (Asian-Aust. J. Anim. Sci. 2005. Vol 18, No. 2 : 230-234)


INTRODUCTION
Feed intake is one of the important factors which influences broiler live performance and carcass characteristics.The original ideal protein hypothesis is based on the balance between amino acids and their final products, meat, egg, milk, embryo, semen, etc.However, today's research findings indicate that the specific amino acid requirement may also need to consider some other special functions of amino acids.Two attractive examples for livestock and poultry production are: firstly, whether some amino acids control feed intake; and secondly, whether amino acid balance of feed depends on the specific requirements of antibodies.Tryptophan is the precursor of serotonin (Voet and Voet, 1995), which plays an important role in feed intake (Henry and Seve, 1993).NRC pointed out the requirement of tryptophan for 1-3 week broiler chickens has been reduced from 0.23 to 0.2%.Minimal research has been conducted on tryptophan requirement of the broiler at more than 3 weeks (NRC, 1994).The purpose of the present experiment was to study: (i) whether tryptophan can improve feed intake, live performance, and carcass characteristics of broiler, and (ii) what is the optimum tryptophan level for growing and finishing broiler.

MATERIALS AND METHODS
There were three trials involved in the experiment.Trial 1 was to study whether the feed intake of finishing chickens could be improved by adding more tryptophan when control diet reached the recommended level for young chickens (1-3 weeks) (NRC, 1994).Trial 2 was to study whether a reduced tryptophan level can cause detrimental effect on growing and finishing chickens.Trial 3 was to study whether reduced tryptophan level can cause differences between sex of growing and finishing chickens.

Trial 1
Ninety-six, five-week-old male Hubbard chickens, average weight 1.97 kg, were used in the trial.All birds were distributed into the following 3 treatments.Each treatment had 32 birds.Each bird was kept in an individual cage.The trial continued for 3 weeks.Feed formulation is shown in Table 1.Each cage had 3,400 cm 3 space, 24 cm×10 cm feed trough, and a nipple drinker.Feed and water were provided ad libitum.Everyday routine management included: (1) 17:00 clean excretion, (2) 17:30 collect and weigh residue feed, (3) 18:00 feed weighed feed.Except for the initial body weight, the body weight of chicken was weighed every three days.After the last weighing, twelve chickens of each treatment were fasted for 24 h and then slaughtered.The twelve chickens were selected from each treatment according to the rank of final body weight.The selected chickens in each treatment were on the ranks of final body weight of 1-4, 16-19 and 33-36.Blood and feather were also weighed.The following parts were also weighed: head, neck, wing, breastbone, breast, thigh, leg, back, leaf fat, organs, heart and liver.

Trial 2
Ninety-six, three-week-old male Hubbard chickens, average weight 1.23 kg, were randomly distributed into the following two treatments.Each treatment had 48 birds.
Feed formulation is shown in Table 2.Each bird was kept in an individual cage, which had the same size, a trough, and a drinker as used in Trial 1.The trial lasted for 4 weeks.Feed and water were provided for chickens ad libitum.The weighing frequency was every 7 days.The cleaning, feeding, and weighing of chickens were the same as practiced in Trial 1.The room temperature was kept constant at 25°C.
The slaughter methods and carcass measurements were the same as for Trial 1.The total number of selected chickens was 30 birds.The selected method for slaughtering chickens was the same as for Trial 1.

Trial 3
Ninety-six, two-week-old Hubbard chickens, average body weight 0.72 kg, were used in the present experiment.There were three treatments.
Feed formulation is shown in Table 3.
The housing facility and temperature control program used in the trial were the same as for Trial 2. Body weight was measured at 13:30 every day from the first week.Residual feed was cleaned about 17:30.Chickens were fed at 18:00.The slaughter methods, carcass measurements, and selection methods for slaughtering chickens were the same as for Trial 1.The slaughtered number of each treatment was 12 chickens.Half of them were male and half of them were female.

Trial 1
The results of this trial showed that there were no significant differences (p>0.05)among the three treatments on feed intake, weight gain, feed conversion rate (Table 4), or carcass characteristics (Table 5).NRC (1971,1977,1984,1994) pointed out that tryptophan requirement for broiler was about 0.18 and 0.22%.The present trial showed that the feed intake, performance and carcass were not influenced by tryptophan content in the diet between 0.198% and 0.258%.

Trial 2
There was no significant difference (p>0.05) on feed intake when tryptophan content in diet was either at 0.167% or 0.198%.However, weight gain, feed efficiency (Table 6), and most broiler carcass (Table 7) characteristics in the   0.198% treatment were significantly better (p<0.05)than those in the 0.167% treatment.This result could be explained by the fact that 0.167% tryptophan content may be enough for control of feed intake but not enough for amino acid balance.Consequently, the 0.167% level caused lower weight gain and carcass weight.This result was different from the results of Hewitt and Lewis (1972) and Smith and Waldroup (1987), who showed that the tryptophan requirements of broiler are less than 0.17% and 0.16%, respectively.

Trial 3
There was a trend that feed intake increased with an increase in the level of tryptophan, but without any significant difference (p>0.05).The weight gain and feed conversion rate were significantly worse for the broiler in the 0.136% treatment (p<0.05) (Table 8).The 0.136% tryptophan treatment showed significantly lower (p<0.05)weight on the following carcass characteristics, breast, thigh, back, leaf fat, feather and heart (Table 9).There was a very clear result that feed intake, weight gain, feed conversion rate (Table 10) and some parts of carcass (Table 11) were significantly better (p<0.05) for male broilers than for

Table 2 .
Feed formulation of Trial 2

Table 3 .
Feed formulation of Trial 3

Table 4 .
Effect of tryptophan level of feed on performance of

Table 5 .
Effect of tryptophan content of feeds on broiler carcass

Table 6 .
Effect of tryptophan level of feed on performance (

Table 7 .
Effect of tryptophan level of feed on carcass

Table 8 .
Effect of tryptophan level of feed on performance of

Table 9 .
Effect of tryptophan level of feed on broiler carcass Means within the same row without the same superscript letters are significantly different (p<0.05).* Means viscera without heart and liver.

Table 10 .
Effect of sex on broiler performance Means within the same row without the same superscript letters are significantly different (p<0.05).