**5. Effect of evaporative cooling system on the environmental condition and physiological response of dairy cows**

The upper critical temperature for heat stress to begin was between 25 and 26C [47]. Climatic conditions in the present study are such that the hot season is relatively long, and generally accompanied by high relative humidity. Thus heat stress is chronic in nature and there is little relief from the heat during the evening through to the morning, and also includes intense bursts of combined heat and humidity which further depresses performance.

Environmental modifications by evaporative cooling system equipped with tunnel ventilation in this study led to a decrease in the ambient temperature and an increase in the relative humidity, during the day. The air temperature in the tunnel barn was up to 6 C

cooler (P<0.05) during the daytime than that in the outside barn, while the relative humidity increased by up to 16%. As air was drawn through the wet cooling pads, water was evaporated into the air causing the temperature to be reduced while increasing the air moisture level. The amount of initial moisture in the outside air will directly impact on how much reduction in air temperature might be expected. Therefore, when the outside air is initially very high humid (relative humidity greater than 70%), the reduction in air temperature will be minimal (less than 2.8 to 5.6C).

The average daily minimum and maximum temperatures were 24.2±0.1 and 28.4±0.1C, respectively in the evaporative, cooled tunnel, ventilated barn and 24.7±0.1 and 34.4±0.1C, respectively in the outside barn, indicating less variation and more consistency in this cooling system. This evaporative, cooled tunnel, ventilated barn reduced daily fluctuation in the ambient temperature, relative humidity and THI during hot and humid climatic conditions. Although, this system reduced (P<0.05) afternoon barn temperature, but relative humidity increased (P<0.05) when compared to the barn without a supplemental cooling system.

Although, high environmental relative humidity reduced the cooling capacity of the evaporative cooling, but on the days when animals were observed it reduced heat stress of the lactating dairy cows. This study shows that evaporative cooling with tunnel ventilation reduced the severity of afternoon heat stress in dairy cows. THI in the tunnel ventilated barn was decreased when compared to the outside barn. There was no thermoneutal zone (THI72) during the study but exposure to conditions of moderate heat stress (THI79) that occurred outside was decreased by utilizing the evaporative cooling system. This difference in environmental conditions had a dramatic effect on the physiological response of the cows, as THI was highly correlated with both rectal temperature and respiration rate. The average rectal temperature and respiration rate in the cooled cows was lower (P<0.05) than in the uncooled cows. Changing in cow body temperatures was most sensitive to same day climatic factors [48]. These results suggest that evaporative cooling and tunnel ventilation has the potential to decrease exposure to heat stress and alleviate the symptoms of heat stress.

The average daily THI in the morning and afternoon in the evaporative, cooled tunnel, ventilated barn were lower (P<0.05) than that in the outside barn. However, the mean THI exceeded the critical point of 72 at daytime and nighttime, suggesting that the cows were exposed continuously to conditions conducive to mild heat stress for the cooled cows and moderate heat stress for the uncooled cows. In hot, arid conditions this system would work well and evaporative cooling has already been used very successfully to cool such dairy cows [46], but in high humidity locations its effectiveness would be limited by the evaporation potential. In this study the air temperature increases during the day and decreases in the evening until the next morning. As the air temperature rises during the day, the relative humidity will decrease. Accordingly during the hottest portion of the day, the outside relative humidity dropped to a level that allowed for maximum evaporation potential, making the system effective for reducing the severity of heat stress.


\* Means differ (*P*<0.05) between groups

314 Milk Production – An Up-to-Date Overview of Animal Nutrition, Management and Health

temperature will be minimal (less than 2.8 to 5.6C).

heat stress and alleviate the symptoms of heat stress.

a supplemental cooling system.

cooler (P<0.05) during the daytime than that in the outside barn, while the relative humidity increased by up to 16%. As air was drawn through the wet cooling pads, water was evaporated into the air causing the temperature to be reduced while increasing the air moisture level. The amount of initial moisture in the outside air will directly impact on how much reduction in air temperature might be expected. Therefore, when the outside air is initially very high humid (relative humidity greater than 70%), the reduction in air

The average daily minimum and maximum temperatures were 24.2±0.1 and 28.4±0.1C, respectively in the evaporative, cooled tunnel, ventilated barn and 24.7±0.1 and 34.4±0.1C, respectively in the outside barn, indicating less variation and more consistency in this cooling system. This evaporative, cooled tunnel, ventilated barn reduced daily fluctuation in the ambient temperature, relative humidity and THI during hot and humid climatic conditions. Although, this system reduced (P<0.05) afternoon barn temperature, but relative humidity increased (P<0.05) when compared to the barn without

Although, high environmental relative humidity reduced the cooling capacity of the evaporative cooling, but on the days when animals were observed it reduced heat stress of the lactating dairy cows. This study shows that evaporative cooling with tunnel ventilation reduced the severity of afternoon heat stress in dairy cows. THI in the tunnel ventilated barn was decreased when compared to the outside barn. There was no thermoneutal zone (THI72) during the study but exposure to conditions of moderate heat stress (THI79) that occurred outside was decreased by utilizing the evaporative cooling system. This difference in environmental conditions had a dramatic effect on the physiological response of the cows, as THI was highly correlated with both rectal temperature and respiration rate. The average rectal temperature and respiration rate in the cooled cows was lower (P<0.05) than in the uncooled cows. Changing in cow body temperatures was most sensitive to same day climatic factors [48]. These results suggest that evaporative cooling and tunnel ventilation has the potential to decrease exposure to

The average daily THI in the morning and afternoon in the evaporative, cooled tunnel, ventilated barn were lower (P<0.05) than that in the outside barn. However, the mean THI exceeded the critical point of 72 at daytime and nighttime, suggesting that the cows were exposed continuously to conditions conducive to mild heat stress for the cooled cows and moderate heat stress for the uncooled cows. In hot, arid conditions this system would work well and evaporative cooling has already been used very successfully to cool such dairy cows [46], but in high humidity locations its effectiveness would be limited by the evaporation potential. In this study the air temperature increases during the day and decreases in the evening until the next morning. As the air temperature rises during the day, the relative humidity will decrease. Accordingly during the hottest portion of the day, the outside relative humidity dropped to a level that allowed for maximum evaporation

potential, making the system effective for reducing the severity of heat stress.

**Table 1.** The environmental measurement and physiological responses of dairy cows in the experimental housing. (MeanS.E.M.)

Air temperature had the greatest impact on physiological measurement, while radiation was second in importance, followed by vapor pressure and air movement [49]. Increasing the air temperature reduces the temperature differential between the cow's body temperature and the ambient temperature and decreases the transfer of heat to the environment. As ambient temperatures increase in the presence of low or high relative humidity, the cooling mechanisms employed by the cows shifted from a nonevaporative processes (convective, conductive, and radiation) to evaporative (sweating and panting) [48], this demonstrate that the percentage of cooling originating from the non evaporative processes declines as ambient temperature increases, while the evaporative process increase. As a result the uncooled cows had greater (P<0.05) rectal temperatures and respiration rates than the cooled cows.
