Thermostasis is the process by which cows attempt to keep their body temperature constant in spite of changes in environmental temperatures. Heat stress occurs when the cow is incapable of dissipating enough heat to maintain its core body temperature below 101.3°F. This increase in body temperature results from the combination of heat from the environment and that produced internally during rumen fermentation and nutrient metabolism. In addition, heat stress leads to an increase in respiratory frequency (panting) in an attempt to increase heat dissipation. This increase in physical activity is the reason why cows in hot environments produce more heat than in cold environments. Internal heat production increases at higher feed intakes and milk production, which is why high-producing cows are more sensitive to heat stress than lower-producing cows. One mechanism the organism has to reduce heat production is to reduce intake and thus less heat needs to be dissipated. Knapp and Grummer (1991) reported that when a cow’s body temperature increased from 101.3° to 104°F, dry matter intakes decreased from 45 to 31 pounds. Berman (2005) showed cows started to suffer from heat stress at temperatures 9°F cooler when production increased from 77 to 99 pounds per day. They also reported that air velocity increased the maximum threshold, suggesting that cows housed in facilities with forced air can tolerate higher ambient temperatures.
Temperature and Humidity
Cows during lactation start to drop feed intake when ambient temperatures exceed 77° to 79°F, with marked reductions above 86°F (NRC, 1989). Aside from the temperature, however, there are other environmental factors that need to be factored in. Cornell researchers have developed an equation to correct dry matter intake depending on ambient temperature, relative humidity (RH), wind speed, and direct exposure to solar radiation (Table 1). At 68°F and 50% RH, there is no effect on intake; however, when ambient temperatures exceed 75°F, intake drops considerably even at 50% RH. This is explained because when RH is high, the capacity to dissipate heat through the lungs is reduced during respiration and panting, which causes an increase in body temperature and a reduction in intake. In an experiment by Knapp and Grummer (1991), cows subjected to heat stress increased their body temperature, and both their intake and milk production decreased by 14 pounds per day. These results agree with a University of Arizona experiment in high-producing cows (more than 77 pounds per day) which showed production started to drop when rectal temperature exceeded 101.3°F. Collier et al. (2012) reported that when environmental temperatures were at 71.6°F, this increase in body temperature occurred at 45% RH or higher.