The effect of temperature stress on dairy cows

J. Praks

When environmental temperatures move out of the thermoneutral zone (or comfort zone) dairy cattle begin to experience either heat stress or cold stress. Either stress requires the cow to increase the amount of energy used to maintain the body temperature and there is less energy available to produce milk. Thermoneutral zone is the range of environmental temperatures where normal body temperature is maintained and heat production is at the basal level. The ranges of thermoneutral zone are from lower critical temperature (LCT) to upper critical temperature (UCT). LCT is the environmental temperature at which an animal needs to increase metabolic heat production to maintain body temperature. UCT is the environmental temperature at which the animal increases heat production as a consequence of a rise in body temperature resulting for inadequate evaporative heat loss (Yousef, 1985). Thermoneutral zone depends on the age, breed, feed intake, diet composition, previous state of temperature acclimatization, production, housing and stall conditions, tissue (fat, skin) insulation and external (coat) insulation, and the behaviour of the animal. UCT is given as 25-26 ºC , LCT as a range from -16 to -37 ºC for dairy cows (Berman et al., 1985; Hamada, 1971). LCT for newborn calves is 10 ºC in dry and draught-free environment. LCT decreases to 0 ºC by the time the calf is 1 month old.

Temperature-humidity index (THI) could be used as an indicator of thermal climatic conditions. THI is determined by equation from the relative humidity and the air temperature and is calculated for a particular day according to the following formula (Kadzere et al., 2002):

THI=0.72 (W+D) +40.6 Where W – wet bulb temperature ºC D – dry bulb temperature ºC

The principle of THI is that as the relative humidity at any