Chromium yeast (CY) supplementation has the potential to alleviate the negative effects of heat stress in dairy cows, but the mechanism remains elusive. We aimed to identify the metabolic mechanisms whereby CY supplementation alleviates the negative effects of heat stress in mid-lactation dairy cows. Twelve Holstein dairy cows with similar milk yield (24.6 ± 1.5 kg/d), parity (2 or 3) and days in milk (125 ± 8 d) were fed the same basal diet containing 0.09 mg of Cr/kg DM. They were allocated randomly to 2 groups: a control group (CON, without CY supplementation) and a CY group (CY, administered 0.36 mg Cr/kg DM). The experiment was performed over 8 weeks during a hot summer, in which the mean temperature-humidity index was 79.0 ± 3.13 (>72), indicating that the dairy cows were exposed to heat stress. Chromium yeast supplementation reduced rectal temperature (P = 0.032), and increased the lactation performance by increasing the yield of milk (+2.6 kg/d), protein, lactose and total solid, and protein and lactose percentages in the milk of the heat-stressed dairy cows (P < 0.05). Supplementation with CY increased the serum glucose and thyroxine concentrations, but reduced the urea nitrogen, insulin, and triiodothyronine concentrations on d 56 (P < 0.05). Furthermore, plasma metabolomic analysis was performed using liquid chromatography tandem-mass spectrometry, which identified 385 metabolites in the two groups. Subsequently, 16 significantly different metabolites in the plasma, were significantly higher in the CY group (variable importance for the projection >1.0, P < 0.05), and found to be involved in 6 Kyoto Encyclopedia of Genes and Genomes pathways, including those involved in nicotinate and nicotinamide metabolism. Specifically, plasma concentration of nicotinamide was higher after CY supplementation, which might also contribute to the reduction of rectal temperature, the regulation of glucose homeostasis, and an improvement in the lactation performance of heat-stressed dairy cows. In conclusion, CY supplementation reduces rectal temperature, influences metabolism by reducing serum insulin concentration and increasing serum glucose and plasma nicotinamide concentrations, and finally increases lactation performance of heat-stressed dairy cows.

铬酵母 (CY) 补充剂有可能减轻奶牛热应激的负面影响，但其机制仍不清楚。我们旨在确定补充 CY 减轻泌乳中期奶牛热应激负面影响的代谢机制。12 头产奶量 (24.6 ± 1.5 kg/d)、胎次 (2 或 3) 和产奶天数 (125 ± 8 d) 相似的荷斯坦奶牛饲喂含有 0.09 mg Cr/kg DM 的相同基础日粮。他们被随机分配到 2 组：对照组（CON，未补充 CY）和 CY 组（CY，给予 0.36 mg Cr/kg DM）。实验在炎热的夏季进行了 8 周，平均温湿度指数为 79.0 ± 3.13（>72），表明奶牛处于热应激状态。P  = 0.032)，通过提高产奶量（+2.6 kg/d）、蛋白质、乳糖和总固体，以及热应激奶牛乳汁中蛋白质和乳糖的百分比来提高泌乳性能（ P <  0.05 ). 补充 CY 增加了血清葡萄糖和甲状腺素浓度，但降低了第 56 天的尿素氮、胰岛素和三碘甲腺原氨酸浓度 ( P  < 0.05)。此外，使用液相色谱串联质谱法进行血浆代谢组学分析，鉴定了两组中的 385 种代谢物。随后，血浆中 16 种显着不同的代谢物在 CY 组中显着更高（投影的变量重要性 > 1.0，P < 0.05)，并被发现参与 6 个京都基因和基因组百科全书途径，包括那些与烟酸和烟酰胺代谢有关的途径。具体而言，补充 CY 后烟酰胺的血浆浓度更高，这也可能有助于降低直肠温度、调节葡萄糖稳态以及改善热应激奶牛的泌乳性能。总之，CY 补充降低直肠温度，通过降低血清胰岛素浓度和增加血清葡萄糖和血浆烟酰胺浓度影响新陈代谢，最终提高热应激奶牛的泌乳性能。