| چکیده انگلیسی مقاله |
Introduction: Stocking density is an important issue in the poultry industry, because it is highly related to the outcome of poultry productivity as well as animal welfare issues. Stocking density for broiler chickens is defined as the number of birds or the total live weight of birds in a fixed space (Estevez 2007). Therefore, high stocking density increases profitability due to a higher production of chicken meat per space (Puron et al. 1995). However, high stocking density was reported to decrease the absorptive capacity of broiler chickens by impairing villus structures of small intestine (Shakeri et al. 2014; Li et al. 2017). Regardless, physiological alterations in the gastrointestinal tract (GIT) of broiler chickens due to high stocking density in relation to productive performance have not been examined. Meanwhile, it is indicated that with increasing in stocking density, the productivity decreases because of increasing in health problems and decreasing in the performance of broiler chickens (Estevez, 2007). Birds cannot synthesize threonine (Thr); therefore, it is one of essential amino acids. Threonine participates in protein synthesis and its catabolism produces many products important in metabolism. Threonine is considered as the third limiting amino acid after methionine and lysine for broiler chicks (Kidd et al. 1999; Ayasan and Okan 2006; Baylan et al. 2006; Ayasan et al. 2009). Kidd (2000) reported that threonine deficiency resulted in decreasing utilization of methionine + cyctein and lysine. In addition, threonine incorporated in a high concentration in γ-globulin, it affects the immune function (Smith and Greene 1947; Crumpton and Wilkinson 1963; Tenenhouse and Deutsch 1966; Azzam et al. 2011a, b). Recently, Houshmand et al. (2012) observed significant interactions between protein level and stocking density for body weight gain and final body weight. Therefore, the aim of our study was to investigate the effects of dietary threonine levels on performance and meat quality of broilers chickens under different stocking density condition. Materials and methods: For this reason, 495 male Ross-308 broiler chickens from 1 to 42 days of age were used based on a completely randomized design with 4 treatments and 5 replicates. Two different stocking densities were achieved by raising a different number of broiler chickens per pen with identical floor size (1.2 m ×1.5 m). Two different stocking densities included 10 birds/m2 (18 birds/pen) or 15 birds/m2 (27 birds/pen). The dietary treatments consisted of: 1) basal diet based on corn-soybean meal as control group that containing 0.79, 0.69 and 0.6 percent standardized ileal digestible Thr in starter, grower and finisher periods, respectively; 2) high stocking density group fed basal diet; 3) high stocking density group fed basal diet supplemented with 10 percent L-Thr higher than control group (diets containing 0.88, 0.76, and 0.66 percent standardized ileal digestible Thr in starter, grower and finisher periods, respectively); and 4) high stocking density group fed basal diet supplemented with 20 percent L-Thr higher than control group (diets containing 0.96, 0.83, and 0.72 percent standardized ileal digestible Thr in starter, grower and finisher periods, respectively). The bell feeder (56 cm diameter and 172.6 cm circompherence) were used and feeder space for control and stocking density groups were 9.5 and 6.4 cm per bird, respectively. The diets were formulized to meet the requirements of broilers as established by the Ross 308 broiler nutrition specification in starter (1-10d), grower (11-24d) and finisher (25-42d). The birds were reared under controlled conditions for temperature, ventilation, and lighting based on Ross-308 management guide booklet recommendations. The broiler chickens' diets were formulated based on standardized ileal digestible amino acids recommendations (Hoehler et al. 2005) and other requirements were obtained from Ross-308 nutrition specification. Broiler chicken performance (feed intake, body weight gain, feed conversion efficiency, and European production efficiency factor), distribution and shape of distribution parameters, carcass parameters, and meat fat and protein percentage, and jejunum morphology were measured. Data were evaluated for variance homogeneity by Minitab 18 version (Minitab 2019 LLC) software and analyzed using GLM procedure (SAS 2004) based on a completely randomized design with 4 treatments and 5 replicates in each treatment. The mortality data were subjected to arc sine transformation prior to analysis, and then analyzed using GLM procedure (SAS 2004). Results and discussion: The results showed that high stocking density significantly decreased feed intake, body weight, feed conversion ratio, and carcass yield of broilers, while Thr supplementation could not ameliorate the negative effects of high stocking density which were in agreement with Azzam et al. 2011a, b. European production efficiency factor (EPEF), breast, thigh and abdominal fat percentage, intestinal morphology, and breast fat and thigh meat protein percentage were not affected by treatments. Maximum weight of the birds was decreased by increasing in stocking density, but the minimum weight was not affected by treatments. The skewness was negative for high stocking density groups, which means that body weight of the most of the broiler chickens in these groups were lower than mean of the group. Conclusion: In conclusion, high stocking density lowered broiler chickens' performance. Although final body weight per m2 in high stocking density treatments were higher than control group, but high stocking density led to decrease in the maximum weight of the birds and had a negative skewness in the distribution of the birds. On the other hand, threonine supplementation could not compensate the negative effects of stocking density, though 10 percent Thr supplementation only increased breast meat protein as compared with control group. It is suggested that Thr supplementation is not a good treatment for improving the bird's performance under high stocking density. |