Glycine and Threonine interactive effects in low crude protein diets on performance and carcass traits of broiler chickens

Introduction: Todays, low crude protein (LP) diets are interested for poultry nutritionists and the poultry feed industry as well. One important concern using LP diets is occurring poor performance in broilers which has been reported by some authors. Dean et al. (2006) studied different dietary manipulation tools for improving performance in LP diets and found that glycine (Gly) has a basic role; as the supplementing 2.44 % Gly+Ser in the LP diet led to the equal performance with the diet containing standard CP level. Threonine can be used as procures of glycine. Therefore, Gly supplementation increases the amount of Thr available for the physiological processes. On the other hand, Thr at the levels more than required level could serve as a source of Gly+Ser in LP diets (Ospina-Rojas et al. 2013b). Based on this information, the objective of the present study was to evaluate Gly requirements in LP diets with different levels of digestible (dig) Thr, and their effects on supplementation in comparison with recommended levels of crude protein (RCP) diet on performance and carcass traits in broiler chickens from 8 to 28d of age.
Material and methods: A four hundred and sixty-two male Ross-308 broilers distributed in a completely randomized design in a 3× 2 +1 factorial arrangement (3 levels of dig Gly‌+Ser, 2 levels of dig Thr in LP diets and 1 recommend CP (RCP) diet ) with 6 replicates with 11 birds per replicate. Factorial arrangements of The LP diets (18.3% CP) were 3 levels of digestible Gly‌+Ser (1.41, 1.51, and 1.61%) and two levels digestible Thr (0.76, 0.86%, corresponding to 100 and 110 % of Thr requirements, respectively). A RCP diet was formulated to be adequate in essential AA except for Gly, as recommended by Rostagno et al. (2011). L- Glu acid was used as nonspecific nitrogen for the need to the synthesis of Non-essential amino acids. Other six experimental diets with different levels of Gly and Thr were obtained by supplementing the basal diet with Gly and Thr as replacement of L-Glu acid. Body weight and feed intake were recorded at the beginning and the end of the experimental period 8 to 28 d of age. Mortality was recorded daily and corrected to its effect on feed intake. Data were analyzed using a completely randomized design as a factorial arrangement with R software (3.6.2) software (Team. 2020). Also, the fixed effects were expressed as Least Square Means (LSMEANS±SE), and to compare the mean of treatments, the Tukey Kramer test was used.
Results and discussion: In the present study, dig Gly supplementation at levels higher than 1.41% in the LP diets showed a significant improvement (P≤0.01) in the body weight gain index of broilers. This is in agreement with the study of Namroud et al. (2008) who reported that in LP diets below 19% CP supplemented with large amounts of essential amino acids, the concentration of ammonia in blood increases and leads to negative effects on tissue metabolism and reduced appetite and growth of poultry, so the addition of Gly and Glu increases the production of uric acid from ammonia and improves performance. In this study, the main effect of supplementation of different levels of dig Thr in LP diets did not significantly effect on performance of broilers (P≥0.05). The interaction effects between levels of dig Gly and Thr were significant (P≤0.05) on BWG and were the highest in the diets with a high levels of dig Gly (1.51, 1.61 %) and Thr (0.86 %) in comparison to the diets containing 1.41 and 0.76 % dig Gly and Thr (i.e., BWG 1146 vs.919g, respectively). These results are in agreement with the reports of Ospina-Rojas et al. (2013b) and Hillar et al. (2019), they suggested that in LP diets supplemented with Gly+Ser and Thr, the catabolic enzymes (Thr aldolase and Thr dehydrogenase) convert the excess Thr to Gly, so the energy used in the synthesis of Gly endogenous (9ATP/mol) is used to improve performance. In our study, there were not found significant (P≥0.05) differences in BWG, FI and FCR between RCP diet and LP diets. The results show that the growth performance of the birds fed LP diets was similar with the birds fed the RCP diet, this finding may be due that the LP diets only contained 2 % CP lower but the content of digestible essential amino acids similar in comparison with RCP diet These results are consistent with the report of Van Harn et al. (2018). In our study, the main effect of Gly supplementation in high levels (1.51, 1.61 %) significantly (P≤0.001) increased in the percentage of breast meat, these results are consistent with studies Kriseldi et al. (2017) that showed Gly amino acid has an important role in the synthesis of creatine. Energy metabolism in muscle tissue is affected by creatine. The interaction effect between levels of dig Gly (1.51, 1.61 %) and Thr (0.86 %) in comparison to the diet containing 1.41 and 0.76 % dig Gly and Thr were significant increased (P≤0.05) on breast and thigh meat percent. By increasing the dig Gly concentration in LP diets, the amount of Thr available for protein synthesis such as breast tissue increases.
Conclusions: It can be concluded that dig Gly supplementation in LP diets significantly improves BWG of broiler chickens. Also it seems interactions between dig Gly and Thr supplementation in diets allow reduction of dietary CP from 20.39% to 18.32% for broilers from 8 to 28 days of age, while maintaining growth performance similar to the RCP diet. However, to support maximum growth, it is recommended that the requirement for dig Thr in the LP diets be higher than 0.86%.