Introduction: The knowledge of the requirement of essential amino acids helps to optimize the different nutritional inputs at various stages of broiler production. Before production of synthetic amino acids, protein was included in diets at rates far above animal requirements as a safety margin against amino acid deficiencies. Today, most necessary amino acids can be obtained in the crystalline form. Addition of these amino acids firstly allows for reduced dietary crude protein inclusion rates and secondly supplies the animal with an amino acid balance that is closer to that requirement of the animal. Recent studies showed Valine (Val) to be the 4^th limiting amino acid in vegetable based of broiler diets (Han et al 1992). It seems that the amount of the 4^th limiting amino acid depends on the diet composition and animal species (Fernandez et al 1994). The NRC (1994) recommendation for total Val in the grower periods is 0.82 % of diet. Corzo et al. (2008) estimated dig Val requirement during 14 to 28 d (0.9% of diet) which were higher than NRC (1994) total Val recommendation of 0.82%. Little information is available on the response of Val usage on improving the tissue protein synthesis in broiler chickens. However, the appropriate programs of eugenics in recent decades will continue to increase poultry productions and therefore, it is necessary to constantly improve and update responses of animals to amino acids usage. The aim of this experiment will be determining the valine requirements and the effect of digestible Val on performance characteristics in commercial varieties of broiler chick (Ross 308).
Material and methods: To determine Val requirement for growth performance of male broiler chicks, a total of 450 chicks, was assigned to 6 treatments with 5 replicates (with 15 birds per each replicate) from 8 to 21 days of age using a completely randomized design. Corn-soybean meal based diet was formulated to provide all nutrients except for Val. Basal diet was formulated with 0.74% Val and 1.056% lysine (Lys) content. In other diets, Val content was increased by adding L-Val with simultaneous decreasing in L-Glutamic acid from the basal level to provide isonitrogenous feeds.  Dietary Val was supplied in 6 levels from 0.74 to 0.99% during the experimental period. Body weight gain and feed intake were measured during the period (d 8 -21). Val intake, feed conversion ratio (FCR), and feed efficiency ratio (FER) were calculated from feed intake data and there was no dead bird in experimental period. All data were analyzed based on a completely randomized design by GLM procedure of SAS (9.1). Duncan’s multiple range tests was used to study the differences between treatment means. Estimations of Val requirements were done at 95% of the plateaus of nonlinear regressions. Broken line Regression analysis was used to estimate Val requirements, when nonlinear regressions responses were statistically significant (p < 0.01).
Results and Discussion: Result of this study showed that digestible Val requirement for Ross 308 male broiler chicks is the well fitted based on weight gain, feed intake, and feed conversion ratio. Evaluated digestible Val content for these performance parameters were 0.85, 0.95 and 0.84 % of diet, respectively. The result of this experiment showed that all performance parameters were significantly affected by Val supplementation in the diet (p < 0.01). The broiler chickens fed with 0.89 % of digestible Val level had the highest feed efficiency ratio, but the lowest feed conversion ratio (p < 0.01). In this experiment, the digestible Lys content of the basal diets was 1.056% and the best digestible level for weight gain and feed conversion ratio were 0.85 and 0.84%, respectively. According to this information, the level of Val as a digestible ratio to Lys for weight gain and feed conversion ratio was 80.49 and 79.54 for linear broken linear model, respectively. Several studies have addressed the valine needs for broilers in starter phase (Thornton et al 2006). Baker et al. (2002) and Corzo et al. (2008) conducted two studies about of evaluation of the Val requirements for broiler chicks, but, there are many remarkable differences between their study and the present experiment. Although several researches with many similarities have been performed on Val requirement, their results are remarkably different. It could be originated from diet types (purified or practical diet), Leucine levels of diets, crude protein content of basal diet, immunological stress, amino acids interactions, metabolizable energy content of basal diet, broiler strains, age, extended experimental period, sex of broiler and amount of feed intake (D'Mello 1974).   
Conclusions: The results of the current study suggested that the level of 0.93% digestible Val is the best level for an optimum performance of body weight gain and the level of 0.91% digestible Val is the best level for an optimum feed conversion ratio in growing broilers from 8 to 21 days of age. Based on statistical model and evaluated performance parameters for requirements, clearly differences were observed between result of this experiment for Val requirement in grower period of Ross 308 broiler chicks and other data sources.