Higher estrus associated temperatures (HEAT) are a hallmark feature in sexually active females. The overarching aim of this study was to characterize the variability, magnitude, and persistence of HEAT in heifers and suckled beef cows as well as identify associated factors when occurring during thermoneutral conditions at the onset of the spring breeding season. In both heifers and cows, estrus was induced using a seven-day controlled internal drug release (CIDR)-PGF2α protocol. Vaginal temperature after prostaglandin F2α administration was recorded every 5 min using a Thermochron iButton affixed to a blank CIDR (containing no progesterone). Estrus was defined as when a heifer first stood to be mounted or when a cow had an Estrotect patch score of 3 or 4. Level of HEAT varied among individual animals. When comparing common HEAT variables using a mixed model with date nested within year, maximum HEAT (39.9 ± 0.1 C and 40.0 ± 0.1°C) and duration (15.5 ± 0.8 and 15.4 ± 0.7) were similar in heifers and cows, respectively. However, the magnitude and persistence of HEAT differed. Total area under the HEAT curve was 117.1 ± 13.5 and 158.7 ± 12.3 for heifers vs cows, respectively (P = 0.0571). Further, 42.9% of heifers and 49% of cows had maximum HEAT ≥ 40°C which persisted up to 6.5 and 10 h, respectively. When ambient conditions were predominantly thermoneutral, temperature humidity index had minimal impact on HEAT (mixed model, repeated measures over time). Towards identifying associated factors with different aspects of HEAT using best fit hierarchical linear regression models, baseline vaginal temperature and baseline duration were the most highly associated independent variables. Follicle size, estradiol and progesterone levels, and other available animal related variables (e.g., age, weight, hair coat score) explained only a small amount of variation in HEAT. In summary, level of HEAT varies in estrus females even under thermoneutral conditions. Because HEAT can persist for an extended time, direct effects on fertility important components are unavoidable. Whether HEAT is a good or bad component of the periovulatory microenvironment is the basis of ongoing and future studies.

中文翻译：

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肉牛和乳牛发情相关温度升高的幅度和持续时间

较高的发情相关温度（HEAT）是性活跃女性的一个标志特征。本研究的总体目标是表征小母牛和哺乳肉牛中 HEAT 的变异性、强度和持续性，并确定春季繁殖季节开始时热中性条件下发生的相关因素。在小母牛和奶牛中，使用为期 7 天的受控内部药物释放 (CIDR)-PGF2α 方案诱导发情。使用固定在空白 CIDR（不含黄体酮）上的 Thermochron iButton 每 5 分钟记录一次施用前列腺素 F2α 后的阴道温度。发情期的定义是小母牛首次站立待骑时或母牛的 Estrotect 斑块评分为 3 或 4 时。个体动物之间的 HEAT 水平各不相同。当使用具有嵌套日期的混合模型比较常见的 HEAT 变量时，小母牛和奶牛的最大 HEAT（39.9 ± 0.1 C 和 40.0 ± 0.1°C）和持续时间（15.5 ± 0.8 和 15.4 ± 0.7）分别相似。然而，高温的强度和持续时间有所不同。小母牛与奶牛的 HEAT 曲线下总面积分别为 117.1 ± 13.5 和 158.7 ± 12.3 (P = 0.0571)。此外，42.9% 的小母牛和 49% 的奶牛的最高 HEAT ≥ 40°C，分别持续长达 6.5 和 10 小时。当环境条件主要为热中性时，温度湿度指数对 HEAT 的影响最小（混合模型，随时间重复测量）。为了使用最佳拟合分层线性回归模型识别与 HEAT 不同方面的相关因素，基线阴道温度和基线持续时间是最相关的自变量。卵泡大小、雌二醇和黄体酮水平以及其他可用的动物相关变量（例如年龄、体重、毛皮评分）仅解释了 HEAT 的少量变化。总之，即使在热中性条件下，发情期雌性的 HEAT 水平也会有所不同。由于高温可以持续较长时间，因此对生育重要组成部分的直接影响是不可避免的。 HEAT 是排卵期微环境的好还是坏组成部分是正在进行和未来研究的基础。