PSVI-5 Efficacy of fan and sprinkler cooling strategies in alleviating heat stress in lactating dairy cattle.

Abstract Heat stress exerts a substantial negative impact on the dairy industry with expected annual economic losses of approximately $40 billion in the United States. Dairy cows are highly susceptible to heat stress, reducing their milk production and reproductive efficiency. Previous studies reported that fans and sprinkler systems can mitigate heat stress to some extent. However, their effects on blood parameters, indicating systemic responses, are largely unexplored. In this study, 24 lactating Holstein cows were divided into heat stress (HS) group and heat stress abatement (HA) group, which had limited or full access to fan and sprinkler (automatically on at 74 °F, for 3 min on and 5 min off throughout the day) cooling system in the barn, with ad libitum access to feed during a 14-day period in summer. Respiration rate, feed intake, and milk yield were recorded regularly. Blood samples were collected on days 0 and 14 to assess the heat shock protein 70 (HSP70) and cortisol levels in the blood plasma. Regardless of fan and sprinkler system, average temperature-humidity indexes of both pens were above 72 (8am: 73.0±4.18 vs 72.5±3.61, 12pm: 77.9±3.28 vs 78.0±3.06, 8pm: 77.1±3.39 vs 76.6±3.09; p=0.46) throughout the daytime of the trial. Respiration rate showed no significant difference on day 0 between the two groups but was significantly higher in the HS group on day 14 compared to the HA group (84.4±8.36 breaths/min vs. 58.3±9.46 breaths/min, p< 0.01). Average feed intake was also significantly lower in the HS group compared to the HA group (44.7±9.36 kg/day vs 57.6±8.18 kg/day, p< 0.01). However, the average milk yield was not found to be significantly different within the 14-day period (15.8±4.62 kg vs 16.3±4.50 kg, p=0.27) between the two groups. There were no significant differences in plasma HSP70 and cortisol concentrations between the HS and HA group on both day 0 (HSP70, p=0.16; Cortisol, p=0.45) and day 14 (HSP70, p=0.97; Cortisol, p=0.87), respectively. Notably, at day 0, a significant positive correlation between respiration rate and HSP70 was observed in both HS (RPearson = 0.58, p=0.05) and HA groups (RPearson = 0.62, p=0.03), but this positive correlation was only observed in HS group on day 14 (RPearson = 0.57, p=0.05) compared to the HA group. Additionally, a significant positive correlation was also observed between HSP70 and cortisol concentration (RPearson = 0.73, p=0.007) in the HS group on day 14 but not in the HA group. These findings suggest that short-term fan and sprinkler cooling strategies can reduce physiological heat stress and mitigate systemic stress responses in certain animals but not efficiently for the overall HA group.