Heat stress in lactating dairy cows: a review

Our objective was to provide a review of factors influencing heat stress in lactating dairy cows and how it affects milk production. In warmer parts of the world, during summer months in the United States, and in other temperate regions, reduced milk production resulting from heat stress counteracts tremendous genetic progress achieved in increasing milk production. Genetic progress in milk production is closely related to increased feed intake. High feed intake results in raised metabolic heat increment. High metabolic heat increment requires effective thermoregulatory mechanisms to maintain body temperature in a thermoneutral zone and in physiological homeostasis. Cows can succumb to hyperthermia if they fail to maintain thermoneutrality. Accurate measurement of when cows enter heat stress is complicated because the responses to heat stress affect not only the energy balance, but also water, sodium, potassium and chlorine metabolism. Water, sodium, potassium and chlorine are important constituents of sweat, and sweating is a major, if not the most important, thermoregulatory mechanism used to dissipate excess body heat. Due to high metabolic heat increment, and especially in the warmer months, high-producing dairy cows may enter heat stress much earlier than their lower-producing counterparts and than currently thought, or extra heat has been accommodated by physiological adaptations. Should this be the case, then strategies to reduce heat stress must be developed to enable cows to express their full genetic potential. The thermoneutral zone, heat production and heat gain, heat dissipation mechanisms, and how the lactating cow responds to heat stress are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.