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 d 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 2 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 d 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 波动对干燥气候下饲养的断奶前荷斯坦小母牛血清总蛋白和健康状况的影响


本研究的目的是评估在出生前 2 天（-2 天）、出生日期 （0 d）、出生至 2 天（+2 天）和出生至 7 天（+7 天）的暴露期内暴露于每日最大值和温度湿度指数 （THI） 以及每日 THI 波动（∆THI = 最大 THI-最小 THI））对血清总蛋白 （STP） 的影响， 被动免疫 （TPI） 的转移，以及腹泻和呼吸系统疾病的发生。回顾性观察了 841 头荷斯坦小母牛，从 -2 d 到 65 d。使用初乳计评估初乳质量，以确保饲喂研究犊牛的初乳中的最低球蛋白浓度为 52 mg/mL。在犊牛场安装了两个温度和相对湿度传感器。获得每个暴露期的最大、最小和 ∆THI 值，热暴露类别定义为热应激（HS：最大 THI > 70 个单位;非 HS：THI ≤ 70 个单位）和 ∆THI（低 < 20 个单位，中≥ 20 至 ≤30 个单位，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） 和极好 （≥6.1 g/dL）。使用方差分析、 logistic 回归和生存分析检查热暴露类别与研究结果之间的关联。HS 和非 HS 犊牛在 -2 d 时的 STP 没有差异（6.83 ± 0.05 vs. 6.91 ± 0.05 g/dL），而与非 HS 犊牛相比，HS 暴露犊牛在 0 d 时的 STP 往往较低（6.82 ± 0.05 vs. 6.92 ± 0.05 g/dL）。与暴露于中等 ∆THI 的犊牛相比，0 d 暴露于小 ∆THI 的犊牛具有更高的 STP（7.00 ± 0.06 vs. 6.75 ± 0.05 g/dL）。未发现 HS 和 ∆THI 类别与 TPI 类别之间存在关联。 在所有暴露期间，HS 犊牛的腹泻几率大约是非 HS 犊牛的 2 倍。此外，HS 犊牛比非 HS 犊牛早 9 至 15 d 受到腹泻的影响。此外，与中低 ∆THI 相比，高 ∆THI 有利于呼吸问题的发展。评估极端 THI 值和 THI 波动为评估在干燥气候下饲养的奶牛犊的热应激提供了研究机会。