Response of Cobb 500 broiler chickens to different ratios of digestible valine to lysine during starter period

Introduction: Commercial availability of crystalline amino acid (AA) has reduced dietary coasts by reducing dietary crude protein (CP) without negatively influencing in broiler performance. With the availability of lysine, methionine, and threonine as feed grade AAs ready for supplementation on diet, the focus is now set on satisfying the needs of the fourth limiting AA in broiler diet. A review of the literature suggests that, valine (Val) is the 4th limiting AA in corn and soybean meal-based diets for growing chicks (Baker et al., 2002; Corzo et al., 2007, 2008, 2009; Rostagno et al., 2011). Inadequate dietary Val for chicks impair body weight gain and feed conversion ratio (Anderson and Warnick, 1967; Farran; Thomas, 1992a; Farran; Thomas, 1992b). The majority of the research estimated Val requirement for commercial male broilers (Tavernari et al., 2013; Corzo et al., 2008, 2009; Baker et al., 2002), the results however, were recommended to both sexes. NRC (1994) recommendations for Val to Lysine (Lys) ratio in the starter phase is 82%. Tavernari et al. (2013) indicated that the optimal digestible Val to Lys ratio for male Cobb ×Cobb 500 broilers is 77%. Available report on the Val need in the female broiler diet is scarce. However, some studies on AA requirement have reported lower AA needs for females to support maximum growth performance (Chamruspollert et al. 2002; Wang et al. 2016; Dozier et al. 2008); so using male Val recommendation to females, may be resulted in extra cost because of Val overfeeding. So, the purpose of the experiment was to determine the digestible Val to Lys need for female Cobb 500 broilers in starter phase.
Material and methods: To determine the requirement of digestible Val to Lys ratio, a dose-response trial was conducted, and 540 female broiler chickens were allocated to 1 of 6 treatments with 6 replicate pens of 15 birds in each. Six different ratios of digestible Val (to Lys) of 0.70, 0.75, 0.80, 0.85, 0.90, and 0.95 were formulated. Val deficient diet was formulated to meet the broiler nutritional requirements suggested in Rostagno et al. (2011), with the exception of Val and Lys levels. Lys was sub-limiting and provided at 90% digestible Lys (1.05%). To creating isonitrogenous experimental diets, crystalline L-glutamic acid in basal diet was replaced by crystalline L-Val. Body weight and feed intake were recorded at the beginning and the end of the experimental period (8 to 21 d) after a 6-h fasting to remove any residual feed from the gastrointestinal tract. Mortality was recorded daily and corrected to its effect on feed intake. The feed conversion ratio was calculated as the unit of eaten feed per unit of weight gain. Data for performance parameters were subjected to ANOVA procedures appropriate for completely randomized designs by using the GLM procedure of SAS software. The difference of treatments were separated using Tukey’s test at P ≤ 0.05 level. Linear and quadratic broken-line models was used to determine digestible Val to Lys ratio need based on weight gain and feed conversion ratio.
Results and Discussion: The result of current experiment showed that weight gain and feed conversion ratio were significantly affected by dietary treatment (P< 0.05), but, feed intake was not affected. As respected, Val intake increased linearly with increasing the digestible Val to Lys ratio in the feeds. Weight gain and feed conversion ratio showed quadratic response (P < 0.01) to increase digestible Val to Lys ratio, and the birds fed the 0.80 digestible Val to Lys ratio diet had the numerically greatest weight gain and was not different from the 0.85 and 0.90 digestible Val to Lys ratio diets. The birds fed Val deficient diet (0.70 digestible Val to Lys ratio) had the highest feed conversion ratio and the lowest weight gain. As feed intake did not differ among experimental diets, it seems that the improving in feed conversion ratio may result on weight gain. To determine the requirement of digestible Val to Lys ratio, performance traits (weight gain and feed conversion ratio) were fitted to statistical models like e.g. broken line model, described by Robbins et al. (2006) and the breakpoint was considered as the requirement. For all response criteria the quadratic broken-line model estimated a greater digestible Val to Lys ratio requirement than the linear broken line model. Using the linear broken-line analyses the requirement of digestible Val to Lys ratio for weight gain was estimated at 0.76 and feed conversion ratio at 0.757. The quadratic broken-line model determined digestible Val to Lys ratio need for weight gain and feed conversion ratio at 0.784 and 0.777, respectively. Based on the current study, it may be concluded that female Cobb 500 broilers need same ratio of Val to optimize weight gain and feed conversion ratio. Since, quadratic broken line model adequate than the linear broken line model to estimate the AA requirement for growing chicks. So, the requirement of digestible Val to Lys ratio to support maximum weight gain and minimum feed conversion ratio was 0.78 in the current study. The estimated requirement of digestible Val to Lys ratio in this study was in line with the findings of Baker et al. (2002), Rostagno et al. (2011), and Tavernari et al (2013), who suggested requirement of Val to Lys ratio of 0.775, 0.77 and 0.77, respectively, for male starter broilers. This finding is not supporting the recommendation of NRC (1994), who indicated that 0.82 Val in ratio to Lys needed to maximum performance in broiler chickens.
Conclusions: the obtained data of the experiment suggest that, the ratio of 0.78 digestible Val to Lys in female Cobb broiler, is the best ratio to optimize performance parameters, and the Val ratio need in female broilers not differ from males.