It is well documented that heat stress (HS) causes subfertility in dairy cows. However, during the last ten years we have been observing that, under high temperature–humidity index (THI ≥ 75), despite the overall reduced fertility, some cows conceive at the first artificial insemination (AI). Here, we examined distinctive features of cows with conserved fertility under severe HS. From the databases of three herds, 167 lactating Holstein cows were selected; group TT cows (n = 57) conceived in the previous summer (THI ≥ 75) at the 1st AI, and group TS (n = 110) failed to conceive at the same period after at least 2 consecutive AIs. The animals calved in spring, and in August, blood samples were collected during a hot day (THI ≥ 81) for the determination of cortisol and HSP70 concentrations. In one farm, the validity of fertility data of the previous year was re‐examined. In 28 cows from group TT and in 39 cows from group TS, the conception rate was examined during July and August. In 6 cows from each group (TT and TS) the oestrous cycles were synchronized, ovulation was induced with GnRH (THI = 80), and the concentration of the pre‐ovulatory LH surge was determined in 9 blood samples. The progesterone concentration in the ensuing cycle was determined in blood samples collected every other day. Overall, cortisol and HSP70 were significantly lower in TT group compared to TS. More (p < .05) animals from group TT conceived at the first AI compared with those from group TS. The induced pre‐ovulatory LH surge peaked at higher level (p < .002) in group TT than in group TS, while no difference was recorded among groups either in mean progesterone concentrations or in the duration of the ensuing oestrous cycle. These results are highly suggestive that thermotolerance in some dairy cows is an inherent characteristic, warranting further genetic investigation.

中文翻译：

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表型耐热和热敏奶牛夏季热应激期间的胁迫响应和生育参数的研究

有充分的证据表明，热应激（HS）会导致奶牛生育力下降。然而，在过去的十年中，我们一直观察到，尽管高温-湿度指数（THI≥75），尽管生育力总体下降，但仍有一些母牛在第一次人工授精（AI）时受孕。在这里，我们研究了重度HS下具有保守生育力的母牛的显着特征。从三头牛的数据库中，选择了167头泌乳的荷斯坦奶牛。 第一组AI在去年夏天（THI≥75）怀胎的TT组（n = 57），TS组（n = 110）在至少连续2次AI之后的同一时间未能怀孕。动物在春季产犊，八月在炎热的白天（THI≥81）收集血液样本，用于测定皮质醇和HSP70的浓度。在一个农场中，重新检查了上一年的生育率数据的有效性。在7月和8月检查了TT组的28头母牛和TS组的39头母牛。每组（TT和TS）的6头母牛的卵巢周期同步，用GnRH诱导排卵（THI = 80），并在9个血液样本中确定排卵前LH激增的浓度。每隔一天收集一次血样，确定随后周期中的孕酮浓度。总体而言，TT组的皮质醇和HSP70显着低于TS。更多 （p  <.05）与第一次实验组相比，第一次实验组来自TT组的动物。 TT组诱导的排卵前LH高峰比TS组更高（p <.002），而各组之间的孕酮平均浓度或随后的雌性周期均无差异。这些结果高度暗示某些奶牛的耐热性是其固有特征，值得进一步的基因研究。