The objective of this study was to assess the effects of the exposure to daily maximum and temperature-humidity index (THI) and to daily THI fluctuations (∆THI = maximum THI – minimum THI) at exposure periods comprising 2 d before birth to birth (-2 d), birth date (0 d), birth to 2 d of age (+2 d), and birth to 7 d of age (+7 d) on serum total proteins (STP), transfer of passive immunity (TPI), and the occurrence of scours and respiratory disease. A total of 841 Holstein heifer calves were retrospectively observed from -2 d until 65 days of age. Colostrum quality was assessed using a colostrometer to ensure a minimum globulin concentration of 52 mg/mL in the colostrum fed to the study calves. Two temperature and relative humidity sensors were installed at the calf yard. Maximum, minimum, and ∆THI values were obtained for each exposure period, and thermal exposure categories were defined as heat stress (HS: maximum THI>70 units; non-HS: THI≤70 units) and ∆THI (low <20 units, medium ≥20 to ≤30 units, high >30). The TPI was classified as poor (STP <5.1 g/dL), fair (5.1 and 5.7 g/dL), good (>5.7 and 6.1 g/dL), and excellent (≥ 6.1 g/dL). Associations between the thermal exposure categories and the study outcomes were examined using ANOVA, logistic regression, and survival analyses. No differences in STP at -2 d were observed between HS and non-HS calves (6.83±0.05 vs. 6.91±0.05 g/dL), whereas HS exposed calves at 0 d tended to have lower STP compared with non-HS calves (6.82±0.05 vs. 6.92±0.05 g/dL). Calves exposed to small ∆THI at 0 d had greater STP compared with calves exposed to medium ∆THI (7.00±0.06 vs. 6.75±0.05 g/dL). No association was found between HS, and ∆THI categories and the TPI category. The odds of scours were about two times greater in HS calves compared with non-HS calves at all exposure periods. In addition, HS calves were affected by scours between 9 and 15 days earlier than non-HS calves. Furthermore, high ∆THI favored the development of respiratory problems compared with medium and low ∆THI. Assessment of extreme THI values and THI fluctuations provides a research opportunity for assessing thermal stress in dairy heifer calves raised in dry climate.

中文翻译：

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围产期暴露于每日最大 THI 和 THI 波动对干燥气候下断奶前荷斯坦小母牛血清总蛋白和健康的影响


本研究的目的是评估暴露于出生前 2 天至出生期间的每日最高温度和湿度指数 (THI) 以及每日 THI 波动（ΔTHI = 最大 THI – 最小 THI）的影响（ -2天）、出生日期（0天）、出生至2天年龄（+2天）、出生至7天年龄（+7天）血清总蛋白（STP）、被动免疫转移（TPI） ），以及腹泻和呼吸道疾病的发生。对841头荷斯坦小母牛-2日龄至65日龄期间的情况进行回顾性观察。使用初乳计评估初乳质量，以确保喂养研究犊牛的初乳中球蛋白浓度最低为 52 mg/mL。犊牛场安装了两个温度和相对湿度传感器。获得每个暴露周期的最大、最小和 ΔTHI 值，热暴露类别定义为热应力（HS：最大 THI>70 单位；非 HS：THI≤70 单位）和 ΔTHI（低 <20 单位） ，中≥20 至≤30 单位，高>30)。 TPI 分为差（STP <5.1 g/dL）、一般（5.1 和 5.7 g/dL）、良好（>5.7 和 6.1 g/dL）和优秀（≥ 6.1 g/dL）。使用方差分析、逻辑回归和生存分析检查热暴露类别和研究结果之间的关联。在 HS 和非 HS 犊牛之间，-2 天时的 STP 没有观察到差异（6.83±0.05 与 6.91±0.05 g/dL），而与非 HS 犊牛相比，0 d 暴露于 HS 的犊牛的 STP 往往较低（ 6.82±0.05 与 6.92±0.05 克/分升）。与暴露于中等 ΔTHI 的犊牛相比，在 0 d 暴露于小 ΔTHI 的犊牛具有更高的 STP（7.00±0.06 与 6.75±0.05 g/dL）。 HS、ΔTHI 类别和 TPI 类别之间没有发现关联。 在所有暴露时期，HS 犊牛发生腹泻的几率大约是非 HS 犊牛的两倍。此外，HS 犊牛比非 HS 犊牛早 9 至 15 天受到腹泻影响。此外，与中、低 ΔTHI 相比，高 ΔTHI 有利于呼吸系统疾病的发生。 THI 极端值和 THI 波动的评估为评估干燥气候下饲养的奶牛犊的热应激提供了研究机会。