Physiological parameters for thermal stress in dairy cattle

ABSTRACT The objective of this study was to investigate changes in physiological parameters of dairy cows and understand which physiological parameters show greater reliability for verification of heat stress. Blood samples were collected for analysis and included hematocrit (Ht), erythrocyte count (ERY), and hemoglobin count (HEMO). In addition, physiological variables, including rectal temperature (RT), heart rate (HR), respiratory rate (RR), and panting score (PS) were recorded in 38 lactating cows. These varied according to genetic group (1/2, 3/4, and pure bred Holstein (HO)). Analysis of variance considering the effects of genetic group, days, and their interaction as well as linear and quadratic effect of the black globe humidity index (BGHI) was performed, as well as broken-line regression. These values were higher in pure HO than in 3/4 and 1/2 groups. The average BGHI during the morning was 74, when 70, 43, and 13% of pure HO, 3/4, and 1/2, respectively, presented RR above reference value. The RR was the best indicator of heat stress and its critical value was 116 breaths/min for 1/2, 140 for 3/4, and 168 breaths/min for pure HO cows. In the HO group, physiological variables increased linearly with BGHI, without presenting inflection in the regression. The inflection point occurred at a higher BGHI for the 1/2 group compared with the other groups. Hematocrit and HEMO were different among genetic groups and did not vary with BGHI, showing that stress was not sufficient to alter these hematological parameters. The 1/2 HO group was capable of maintaining normal physiological parameters for at least 3 BGHI units above that of HO and 1 to 3 units higher than 3/4 HO for RR and RT, respectively. Respiratory rate is the physiological parameter that best predicts heat stress in dairy cattle, and the 1/2 Holstein group is the best adapted to heat stress.

摘要 本研究旨在探究奶牛生理参数的变化规律，并明确哪些生理参数在热应激验证中具备更高的可靠性。本研究采集血液样本开展检测，涵盖血细胞比容（hematocrit, Ht）、红细胞计数（erythrocyte count, ERY）与血红蛋白计数（hemoglobin count, HEMO）三项指标。此外，对38头泌乳奶牛的多项生理指标进行了记录，包括直肠温度（rectal temperature, RT）、心率（heart rate, HR）、呼吸频率（respiratory rate, RR）及喘气评分（panting score, PS）。上述指标随遗传分组（1/2血统、3/4血统及纯繁荷斯坦（Holstein, HO））的不同而存在显著差异。本研究采用方差分析，考量遗传分组、采样天数及其交互效应，以及黑球湿度指数（black globe humidity index, BGHI）的线性与二次效应，同时开展折线回归分析。纯繁荷斯坦组的各项指标均显著高于3/4血统组与1/2血统组。晨间平均BGHI为74，此时纯繁荷斯坦、3/4血统及1/2血统组中呼吸频率超出参考范围的个体占比分别为70%、43%与13%。呼吸频率（RR）是热应激的最优预测指标，其临界值分别为：1/2血统组116次/分钟、3/4血统组140次/分钟、纯繁荷斯坦组168次/分钟。纯繁荷斯坦组的生理指标随BGHI呈线性增长，回归分析未发现拐点。与其余两组相比，1/2血统组的拐点对应BGHI值更高。血细胞比容与HEMO在不同遗传分组间存在差异，但不随BGHI发生变化，表明本次热应激强度不足以改变此类血液学参数。1/2血统荷斯坦奶牛维持正常生理参数的BGHI阈值，较纯繁荷斯坦组至少高出3个单位；就呼吸频率与直肠温度而言，其阈值较3/4血统组分别高出1~3个单位。呼吸频率是预测奶牛热应激效果最优的生理参数，而1/2血统荷斯坦组对热应激的适应性最强。