Several studies have been conducted to assess the effects of supplemental dietary chromium (Cr) on broiler chickens under heat stress (HS) conditions, but the shape and strength of the associations between Cr supplementation and broiler chickens' responses to HS remain unclear. Therefore, the current systematic review and meta-analysis investigated the effectiveness and dose-response relationship of Cr. The results indicated non-linear dose-response associations between Cr supplementation and body mass gain (BMG), feed intake (FI), feed conversion ratio (FCR), carcass, breast, leg, and abdominal fat relative weight (Pnon-linearity < 0.05). The maximum BMG, FI, and the relative weight of carcass, breast, and leg would be achieved with 1200, 1100, 900, 800, and 800 ppb of Cr, respectively, while the lowest FCR and abdominal fat relative weight might be obtained with the supplementation of 1100 and 1000 ppb of Cr, compared with no Cr supplementation. Referring to BMG, supplementation with 1200-1700 ppb inorganic Cr or 2700 ppb or less organic Cr had a significant beneficial effect on the BMG, while NanoCr supplementation did not influence this outcome variable. A non-linear association was observed for blood total cholesterol concentration (TC, Pnon-linearity < 0.05), with the maximum reduction of TC concentration observed at approximately 900 ppb of Cr. The cholesterol-lowering effect of Cr (≤ 2400 ppb) was only found in severe HS conditions. Moreover, supplemental Cr caused a significant linear reduction in the blood triglycerides and glucose concentrations (P < 0.05). The blood concentrations of triiodothyronine, thyroxine, and insulin increased linearly, and the corticosterone concentration reduced, with increasing supplemental Cr (P < 0.05). There was a non-linear inverse association between Cr supplementation and cortisol level (Pnon-linearity < 0.05), and the lowest concentration of cortisol was observed with the supplementation of 1000 ppb of Cr. Meanwhile, significant positive linear associations between Cr supplementation and bursa percentage, thymus percentage, infectious bronchitis vaccine titer, avian influenza vaccine titer, Newcastle disease vaccine titer, cutaneous basophil hypersensitivity response, and serum immunoglobulin G level were found (P < 0.05). However, Cr supplementation caused a linear reduction in the heterophil/lymphocyte ratio (P < 0.05). Based on the obtained results, the recommended optimum amount of supplemental Cr is 1100 ppb.

中文翻译：

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铬补充剂对热应激下肉鸡生长性能、胴体性状、血液成分和免疫能力的影响：系统评价和剂量反应荟萃分析。

已经进行了几项研究来评估在热应激 (HS) 条件下补充日粮铬 (Cr) 对肉鸡的影响，但补充铬与肉鸡对 HS 反应之间的关联的形状和强度仍不清楚。因此，目前的系统评价和荟萃分析调查了 Cr 的有效性和剂量反应关系。结果表明，Cr 补充与体重增加 (BMG)、采食量 (FI)、饲料转化率 (FCR)、胴体、胸部、腿部和腹部脂肪相对重量之间存在非线性剂量反应关联（P 非线性 < 0.05)。分别使用 1200、1100、900、800 和 800 ppb 的 Cr 可达到最大 BMG、FI 和胴体、胸部和腿部的相对重量，而与不添加铬相比，添加 1100 和 1000 ppb 的铬可能会获得最低的 FCR 和腹部脂肪相对重量。关于 BMG，补充 1200-1700 ppb 无机铬或 2700 ppb 或更少的有机铬对 BMG 有显着的有益影响，而 NanoCr 补充不影响这一结果变量。观察到血液总胆固醇浓度呈非线性关联（TC，P 非线性 < 0.05），在大约 900 ppb Cr 时观察到 TC 浓度的最大降低。Cr (≤ 2400 ppb) 的降胆固醇作用仅在严重的 HS 条件下被发现。此外，补充 Cr 导致血液甘油三酯和葡萄糖浓度显着线性降低（P < 0.05）。三碘甲状腺原氨酸、甲状腺素、随着补充Cr的增加，胰岛素线性增加，皮质酮浓度降低（P < 0.05）。Cr 的补充与皮质醇水平之间存在非线性负相关（P 非线性 < 0.05），并且在补充 1000 ppb 的 Cr 时观察到最低的皮质醇浓度。同时，Cr的补充与法氏囊百分比、胸腺百分比、传染性支气管炎疫苗滴度、禽流感疫苗滴度、新城疫疫苗滴度、皮肤嗜碱性粒细胞超敏反应和血清免疫球蛋白G水平呈显着正相关（P < 0.05）。然而，Cr 补充导致异嗜细胞/淋巴细胞比率线性降低（P < 0.05）。根据得到的结果，