Heat stress and a high metabolic rate of broilers increase the production of reactive oxygen species, which must be removed by antioxidants to prevent oxidative stress. Selenium (Se) is a potent antioxidant as it is a structural part of glutathione peroxidase (GPx). Dietary supplementation of Se can help reduce the oxidative stress caused by heat-stress conditions. The aim was to evaluate the effect of Se supplementation on the performance and antioxidant status of broilers under heat stress, by using a meta-analysis approach, and to quantify the contribution to heterogeneity of the explanatory variables through a meta-regression procedure. A database of 74 trials was created from 56 published studies that met the following inclusion criteria: papers that reported a measure of effect size, sample size, measure of variability and random procedure. The response variables were GPx and malondialdehyde concentrations, DM intake, average daily gain, and feed conversion rate. Broiler line, Se source, and dosage, heat-stress level, days of experimentation, sample source (blood, liver or meat) and basal Se concentration were considered to be explanatory variables. All statistical analyses were performed in R software, by using the ‘meta’ and ‘Metafor’ packages for meta-analysis and meta-regression respectively. High concentrations of GPx were found in Se-supplemented broilers (>1.76; P = 0.001), in comparison to control groups. However, when the random model was applied to GPx studies, it showed high heterogeneity (I2 = 95.4%), which was reduced (I2 = 61.5) when heat-stress temperature, Se source and its dosage and tissue sample were included as covariates in the meta-regression analysis. The highest standard mean difference of GPx was founded in studies that reported supplementation with inorganic Se sources (2.92), in comparison to supplementation with organic and nano-Se sources (1.66 and 1.44 respectively). The standard mean differences of malondialdehyde and feed conversion rate were significantly lower (<0.66 and <0.11 respectively) in supplemented broilers. Heterogeneity of all variables decreased when the explanatory variables were included in the mixed-regression model. Our findings confirmed that Se supplementation improves the broiler’s antioxidant status and productive performance. However, the response level was affected by dosage and source of Se and the level of heat stress.

中文翻译：

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补硒对热应激下肉鸡生产性能和抗氧化状态的影响：荟萃分析和荟萃回归

热应激和肉鸡的高代谢率会增加活性氧的产生，必须通过抗氧化剂去除这些活性氧以防止氧化应激。硒 (Se) 是一种有效的抗氧化剂，因为它是谷胱甘肽过氧化物酶 (GPx) 的结构部分。膳食补充硒有助于减少由热应激条件引起的氧化应激。目的是通过使用荟萃分析方法评估硒补充剂对热应激下肉鸡性能和抗氧化状态的影响，并通过荟萃回归程序量化对解释变量异质性的贡献。从 56 项符合以下纳入标准的已发表研究中创建了一个包含 74 项试验的数据库：报告了效应量、样本量、变异性和随机程序量度的论文。响应变量是 GPx 和丙二醛浓度、干物质摄入量、平均日增重和饲料转化率。肉鸡品系、硒源和剂量、热应激水平、实验天数、样品来源（血液、肝脏或肉）和基础硒浓度被认为是解释变量。所有统计分析均在 R 软件中进行，分别使用“meta”和“Metafor”包进行元分析和元回归。与对照组相比，在补充硒的肉鸡中发现了高浓度的 GPx（>1.76；P = 0.001）。然而，当将随机模型应用于 GPx 研究时，它表现出高异质性（I2 = 95.4%），当热应激温度、Se 源及其剂量和组织样本作为协变量加入时，异质性降低（I2 = 61.5）元回归分析。GPx 的最高标准平均差异建立在报告补充无机硒源 (2.92) 与补充有机硒源和纳米硒源 (分别为 1.66 和 1.44) 的研究中。补充肉鸡的丙二醛和饲料转化率的标准平均差异显着较低（分别<0.66 和<0.11）。当解释变量包含在混合回归模型中时，所有变量的异质性降低。我们的研究结果证实，硒补充剂可改善肉鸡的抗氧化状态和生产性能。然而，反应水平受硒的剂量和来源以及热应激水平的影响。与补充有机硒和纳米硒（分别为 1.66 和 1.44）相比。补充肉鸡的丙二醛和饲料转化率的标准平均差异显着较低（分别<0.66 和<0.11）。当解释变量包含在混合回归模型中时，所有变量的异质性降低。我们的研究结果证实，硒补充剂可改善肉鸡的抗氧化状态和生产性能。然而，反应水平受硒的剂量和来源以及热应激水平的影响。与补充有机硒和纳米硒（分别为 1.66 和 1.44）相比。补充肉鸡的丙二醛和饲料转化率的标准平均差异显着较低（分别<0.66 和<0.11）。当解释变量包含在混合回归模型中时，所有变量的异质性降低。我们的研究结果证实，硒补充剂可改善肉鸡的抗氧化状态和生产性能。然而，反应水平受硒的剂量和来源以及热应激水平的影响。当解释变量包含在混合回归模型中时，所有变量的异质性降低。我们的研究结果证实，硒补充剂可改善肉鸡的抗氧化状态和生产性能。然而，反应水平受硒的剂量和来源以及热应激水平的影响。当解释变量包含在混合回归模型中时，所有变量的异质性降低。我们的研究结果证实，硒补充剂可改善肉鸡的抗氧化状态和生产性能。然而，反应水平受硒的剂量和来源以及热应激水平的影响。