The effects of high ambient temperatures on production animals, once thought to be limited to tropical areas, has extended into northern latitudes in response to the increasing global temperature. The number of days where the temperature-humidity index (THI) exceeds the comfort threshold (>72) is increasing in the northern United States, Canada, and Europe. Compounded by the increasing number of dairy animals and the intensification of production, heat stress has become one of the most important challenges facing the dairy industry today.
Heat stress has negative effects on the health and biological functioning of dairy cows through depressed milk production and reduced reproduction performance.
Documented physiological coping strategies used by dairy cows include increased respiration rate, panting, and sweating, and reduced milk yield and reproductive performance. Behavioral coping strategies include modified drinking and feed intake (e.g., increased water intake and shifting feeding times to cooler periods during the day), increased standing time and shade seeking, and decreased activity and movement.
Heat Stress Decreases Milk Production
Lactating dairy cows have an increased sensitivity to heat stress compared with nonlactating (dry) cows, due to milk production elevating metabolis. Moreover, because of the positive relationship between milk yield and heat production, higher yielding cows are more challenged by heat stress than lower yielding animals.
Heat Stress Decreases Reproductive Success
The decrease in conception rates during summer seasons can range between 20 and 30%, with evident seasonal patterns of estrus detection. Elevated environmental temperatures negatively affect the cow's ability to display natural mating behavior, as it reduces both the duration and intensity of estrous expression.
Lameness, Pain, and Heat Stress
Cows under increased heat load change their behavior in an effort to improve cooling. Notably, heat-stressed cows have been reported to increase their standing time, and in turn decrease lying time and walking activity, to expose more surface area for heat abatement, sensible water loss, radiating surface area, and air movement via convection. Several studies examining the lying time of cows in freestalls report a range of 11 to 14 h; under thermo-neutral conditions, with a 30% reduction when ambient temperatures increase.
Extended periods of prolonged standing have been argued by some to be a major risk factor for lameness, which may also be associated with painful experiences. It is not surprising that heat stress is considered a major risk factor for lameness, but whether this association is a consequence of increased standing times or due to alterations in nutrient metabolism caused by a decrease in DMI is not known. Based on the available evidence to date, we speculate that heat stress may have a profound impact on behavior, biological functioning, and affective states of dairy cattle given that high ambient temperatures cause increased standing times, which in turn increases the risk of lameness and painful experiences.
Housing and Thermal Management of Dairy Cows
Various cooling options for dairy cows exist based on the principles of convection, conduction, radiation, and evaporation. Fan installations, which facilitate air movement and increase convection, have been used to reduce environmental temperatures and mitigate heat stress by decreasing respiratory rate and rectal temperature and increasing DMI. Other forms of evaporative cooling make use of high-pressure mist injected into fans (which function to cool the microclimate air that the cows inspire) or large water droplets from low-pressure sprinkler systems that completely wet the cow by soaking the hair coat. Both of these systems have been shown to reduce rectal body temperatures and improve DMI, conception rates, and live calf birth rate.
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