Impact of Heat Stress on Blood, Production, and Physiological Indicators in Heat-Tolerant and Heat-Sensitive Dairy Cows

Abstract

Heat stress affects production and health in cows severely. Since it is difficult to define heat-tolerant animals, studies of response to heat stress are important for understanding dairy cows’ health and production. However, information on the impact of heat stress on various indicators in heat-tolerant and heat-sensitive cows is sparse. This study aimed to investigate the effects of heat stress (HS) on blood, production, and physiological indicators in heat-tolerant and heat-sensitive cows. A total of 43 dairy cows were used from 9 May to 7 August 2021, under Temperature–Humidity Index (THI) measurements that ranged from 65.9 to 86.7. We identified cows that were tolerant or sensitive to HS based on the slope of the response of physiological and production traits against THI during the HS period by using a clustering method. After HS, serum glucose (Glu), cortisol (COR), 5-hydroxytryptamine (5-HT), and interleukin-6 (IL-6) levels of cows in the heat-tolerant group were lower than in the heat-sensitive group (p < 0.05). With THI as the predictor, the R2 for predicting respiration rate (RR) and body surface temperature (BT) in heat-tolerant cows was 0.15 and 0.16, respectively, whereas the R2 for predicting RR and BT in heat-sensitive cows was 0.19 and 0.18, respectively. There were low to moderate, positive correlations between RR, BT, and MY with THI, with Pearson correlation coefficients ranging from r = 0.11 to 0.4 in the heat-tolerant group, and from r = 0.24 to 0.43 in the heat-sensitive group. There was a low positive correlation between VT and THI, with a Spearman correlation coefficient of r = 0.07 in the heat-sensitive group. The heat-tolerant dairy cows had lower MY losses and had lower MY (p = 0.0007) in mixed models. Heat-tolerant cows with low-stress levels, through upregulating RR rapidly, increased their adaptability to thermal environments. They have better thermoregulation capability; the hypothalamic–pituitary–adrenal (HPA) axis regulated the thermoregulatory in animals by releasing a variety of neurotransmitters and hormones.

Effects of heat stress on endocrine, thermoregulatory, and lactation capacity in heat-tolerant and -sensitive dry cows

“…This categorization was achieved through hierarchical cluster analysis after standardizing the individual slopes. Consistent with our previous study ( 19 ), the group characterized by fewer numbers and relatively steeper slopes after clustering was identified as heat-tolerant cows, while the opposite was heat-sensitive cows. The present study expands upon screening methods for selecting heat tolerance in dry cows under thermal environmental conditions.…”

“…RR and BT were used as physiological indicators. RR was assessed by two trained postgraduates with a high level of agreement (intragroup correlation coefficient: 0.91), and was achieved by timing 15 flanking movements and converting them to breaths per minute (bpm) ( 19 ). BT was evaluated by IRT using a portable thermal imaging infrared camera (Fluke TiS60+ Thermal Imaging Camera, Fluke, Everett, Washington, USA) equipped with 320 × 240 pixel detector and adjusted to thermal sensitivity <45 mK (< 0.045°C at the ambient temperature of 30°C) and temperature range from −20 to 400°C, in the manual focus option.…”

“…This study used PROC MIXED to fit a mixed-effects model that uses data obtained during HS to quantify individual differences in dry cows’ heat tolerance and sensitivity. HTDC and HSDC were obtained from a hierarchical cluster analysis based on the Ward method using PROC CLUSTER after separate slopes for RR and BT of the THI responses were derived from the mixed-effects model ( 19 ). Levene’s test was employed to examine the data, and residual normality was assessed using the Shapiro–Wilk statistic through the UNIVARIATE procedure.…”

“…Gaughan ( 18 ) demonstrated that the influence of the ambient temperature on RR. Synthesizing results of these studies, we propose that the slopes of RR and BT to THI, determined by analyzing RR, BT, and cows background information using a mixed-effects model, be used as selection criterion for determining heat tolerance ( 19 ). Carabaño ( 20 ) reported a similar conclusion, they assessed the heat tolerance of cows by the slope of production traits and somatic cell scores relative to ambient temperature.…”

“…Carabaño ( 20 ) reported a similar conclusion, they assessed the heat tolerance of cows by the slope of production traits and somatic cell scores relative to ambient temperature. In our study with lactating cows ( 19 ), we selected heat-tolerant and heat-sensitive cows by using mixed models and studied the effect of HS on these two types of cows. Our study indicated that heat-tolerant cows generally showed steeper slope in RR and BT, and conversely, the decay in milk yield was lower than heat-sensitive cows.…”