Performance and Carcass Yield of Broilers Fed with Different Digestible Amino Acid Profiles Recommended by Nutrients Requirements Tables

An experiment was conducted to evaluate the effect of different digestible amino acid profiles on the performance of broilers in two phases: from 1 to 21 days and from 22 to 42 days of age. At the end of the experiment, carcass yield and cut percentages were evaluated. Nine hundred and sixty AgRoss birds were distributed in a randomized block design with four treatments (four digestible amino acid profiles): Rostagno et al. (2000), Baker & Han (1994b), Degussa (1997) and those recommended by AgRoss (2000), with six repetitions and 40 birds per repetition (20 males and 20 females). The results showed that the four diets were able to provide the requirements of the birds, since no significant differences were seen among the treatments on the performance in the two phases and on the carcass yield and cuts at 42 days of age. Males had better yields of feet, head and neck, and lower percentage of abdominal fat (p<0.05). The best cost/benefit ratio was seen for the profile established by Rostagno et al. (2000) when diets were evaluated in an ideal protein situation.


INTRODUCTION
Broiler diet formulation has been based on crude protein requirements for a long time, frequently resulting in diets containing amino acid levels above the real requirements of birds.Nevertheless, any amino acid in excess is inefficiently used by the birds, so that the excess is deaminated and excreted as uric acid.Advances in the knowledge about amino acid requirements, protein metabolism and the possibility of producing economically feasible synthetic amino acids increased the general efficiency of protein utilization.This, in turn, enabled diet formulation containing amino acid levels more appropriate to the animal needs, although protein levels were kept high.In recent years, many studies have directed animal nutritionists to formulate diets based on the ideal protein concept.This is defined as the ideal amino acid balance in the diet, without deficiencies or excesses, providing the requirements of all amino acids needed for maintenance and production of the birds (Baker & Chung, 1992).According to Penz (1996), amino acid should be added in levels that are as close as possible from the requirements of the birds in each production phase and thus, amino acid excess would be minimized in the diets.
Nutritional programs are commonly based on foreign requirement tables such as NRC (1994), ARC (1975), INRA (1984), Rhône Poulenc (1993) and Degussa (1997), and also on Brazilian tables such as Rostagno et al. (2000) or the recommendations from feeding or husbandry manuals of commercial strains.
Amino acid and protein requirements in broilers may vary according to the production phase and the expected goals.Thus, it is difficult to establish the requirements for the birds since they can be affected by a considerable number of factors, such as: metabolizable energy of the diet, age, gender, feed intake and environmental conditions.A diet based on the concept of ideal protein is established by selecting a reference amino acid.Lysine was standardized as the reference since it can be easily evaluated and is available in crystalline form.Besides, lysine has been previously described in studies and has great importance in the protein synthesis.
The aim of this study was to evaluate the applicability of the ideal protein concept according to the recommendations of some broiler nutrients requirements tables, based on the results of carcass yield and cost/benefit ratio.

MATERIAL AND METHODS
Nine hundred and sixty AgRoss chicks were distributed in 24 experimental units with 20 male and 20 females each.Although it is known that male and female requirements are different, sex-mixed birds were used, similar to what is seen in the majority of the commercial poultry farms.
Mean body weight of one-day-old chicks was 38.0g.They were housed in a randomized block design, with four treatments (four digestible amino acid profiles, according to Table 1) and six repetitions.The evaluations were performed in the initial phase (1 to 21 days), grower phase (22 to 42 days) and total period (1 to 42 days).
The basal diet and the amino acid lysine were established according to "Tabelas Brasileiras para Aves e Suínos" (Rostagno et al., 2000).Lysine levels were fixed and other amino acid profiles were established as a percentage of lysine.Birds were fed isocaloric and isoproteic diets in all treatments (Tables 2 and 3).
At the end of each phase, weight gain (WG), feed intake (FI), feed conversion (FC), mean daily weight gain (DWG), energy conversion (EC), protein conversion (PC) and mortality rate were evaluated.Productive efficiency index (PEI) was also evaluated for the total period (1 to 42 days).At the end of the trial, the birds were weighed, male and female mean body weight were determined for each experimental unit.Afterwards, in each experimental unit, one male and one female with body weight similar to the mean were killed in order to evaluate the yield of the carcass and commercial cuts.
For economical analysis, the cost of the diet per kg of live bird was considered for each digestible amino acid.Diet cost was calculated and expressed in US$/kg (Tables 2 and 3).All results were statistically analyzed using the software SAEG (UFV, 1997).

RESULTS AND DISCUSSION
No effect of treatment was seen on weight gain (WG), feed intake (FI), feed conversion (FC), mean daily weight gain (DWG), energy conversion (EC) and protein conversion (PC) from 1 to 21 days and from 22 to 42 days (Table 4).
Few data are available about the different profiles of digestible amino acids in broiler chickens based on lysine nutritional requirements, which is used as a reference for establishing the levels of the other amino acid.For instance, Conhalato et al.(1999) evaluated the effect of different levels of digestible lysine (0.84, 0.93, 1.02, 1.11 and 1.20%) for broiler chickens and the best weight gain was obtained with the predicted levels of approximately 1.02%, with 33% of methionine, 61% of threonine and 15% of tryptophan.These levels are similar to ideal levels established by Baker & Han (1994a), i.e., 36% of methionine, 67% of threonine and 16% of tryptophan.
The percentage of digestible lysine was 1.06% from 22 to 42 days of age.This is very similar to the level of 1.07% reported by Valério et al. (2000) showing better performance and carcass characteristics, with similar

Table 1 -
Profile of digestible amino acids expressed as a percentage of lysine suggested by requirement tables for broilers.

Franco SG, Flemming JS, Borges SA, Sillus PP Performance and Carcass Yield of Broilers Fed with
D i f f e r e n t D i g e s t i b l e A m i n o A c i d P r o f i l

e s Recommended by Nutrients Requirements Tables 32Table 3 -
Composition (%) and cost (US$) of the experimental diets (22 to 42 days).

Table 4 -
Performance of broilers fed with different profiles of digestible amino acid suggested by requirements tables.

Table 5 -
Performance of broilers fed different profiles of digestible amino acid from 1 to 42 days of age.

Table 6 -
Carcass and cut yield of sex-mixed broilers fed different profiles of digestible amino acids from 1 to 42 days of age.