Heat stress – temperature-humidity index

Heat stress can have major detrimental effects on animal health and productivity and poses particular challenges to herds in ‘hot’ regions. As well as temperature, relative humidity has a significant effect on heat stress, with the temperature-humidity index (THI) often used to indicate the degree of stress on dairy cattle (see Figure 1). Optimal temperature for producing animals is determined by their ‘thermoneutral zone’, which for lactating dairy cows is between 5°C and 20°C. This represents the temperature zone within which no additional energy above maintenance is expended to heat or cool the body.

Figure 1      Temperature-humidity index chart (University of Arizona)