The objective of this study was to evaluate the effects of heat stress in late gestation independent of maternal reduced feed intake on performance, blood hormones and metabolites, and immune responses of dairy calves from birth through weaning. A total of 30 multiparous Holstein cows at 45 d before expected calving were randomly assigned to one of 3 groups: (1) thermal neutral (CL, n = 10) conditions with ad libitum feed intake (10% of refusals on an as-fed basis); (2) pair-fed thermal neutral (CLPF, n = 10) conditions to reduce feed intake to levels similar to the heat stress (HS) group while reared under thermoneutral conditions (80% of the CL group); or (3) heat stress (HS, n = 10) conditions with ad libitum feed intake. Pair-feeding was conducted to quantify the confounding effects of dissimilar feed intake. Calves (10/group) born to cows that were exposed to cooling (IU-CL), pair-feeding (IU-CLPF), or heat stress (IU-HS) were used from birth through weaning. After birth, all the calves were managed under identical conditions. IU-HS calves had lower birth weight, and hip height at birth and 14 d of age. Compared with IU-CL and IU-CLPF calves, IU-HS calves had lower serum concentration of IgG and apparent efficiency of IgG absorption but higher serum insulin concentrations. Cortisol concentration in serum was higher in IU-HS and IU-CLPF calves compared to IU-CL calves. The neutrophil percentage was lower in IU-CL calves than in IU-HS and IU-CLPF calves. Neutrophil-lymphocyte ratio was higher in IU-HS calves compared to IU-CLPF and IU-CL calves. The mRNA expression of TNFα of IU-HS calves was downregulated compared with IU-CL and IU-CLPF calves. In summary, maternal HS during late gestation reduces calf birth weight and dramatically alters blood hormones and metabolites, but its effect on immune system function was not independent of maternal reduced feed intake.

这项研究的目的是评估妊娠后期热应激的影响，而与母体减少采食量无关，从出生到断奶，犊牛的生产性能，血液激素和代谢产物以及免疫反应均受到影响。在预期产犊前45天，将总共30头荷斯坦产奶牛随机分为以下3组之一：（1）热中性（CL，n = 10）条件，随意采食（饲喂时拒食的10％）基础）; （2）成对喂养的热中性（CLPF，n = 10）条件，以在热中性条件下（CL组的80％）饲养时将采食量减少到与热应激（HS）组相似的水平；或（3）随意采食热应激（HS，n = 10）条件。进行配对喂养以量化不同采食量的混杂影响。从出生到断奶，使用暴露于降温（IU-CL），成对喂养（IU-CLPF）或热应激（IU-HS）的母牛出生的犊牛（每组10只）。出生后，所有小牛均在相同条件下进行处理。IU-HS犊牛的出生体重较低，出生时和14 d时髋关节高度较低。与IU-CL和IU-CLPF犊牛相比，IU-HS犊牛的IgG血清浓度较低，且IgG吸收效率明显，但血清胰岛素浓度较高。IU-HS和IU-CLPF犊牛的血清皮质醇浓度高于IU-CL犊牛。IU-CL犊牛的中性粒细胞百分比低于IU-HS和IU-CLPF犊牛。与IU-CLPF和IU-CL犊牛相比，IU-HS犊牛中性粒细胞-淋巴细胞比率更高。与IU-CL和IU-CLPF犊牛相比，IU-HS犊牛TNFα的mRNA表达下调。