Heat stress in dairy cows : a review of the heat load effects on the animal response and relief strategies

A first part consists in a review of the available bibliographic sources on the influence of the micro-climatic parameters on the production and some physiological indicators of dairy cows in hot climate. The climatic parameters considered are: air temperature, air relative humidity and air speed; the animal indicators are: milk production, rectal temperature and respiratory rhythm. The data obtained in various experimental studies are examined and compared in order to find out some relationships of general validity. The influence of the climatic parameters has been investigated both separately and in various combinations, including the Temperature Humidity Index.. The results show a good agreement among the different authors especially for what’s concerning the influence of the air temperature and the THI on production. For these parameters the effect of the exposure to heat stress has been also put into evidence. Such an agreement shows the possibility to achieve the general relationships required in order to predict the animals response in hot climate for different housing and equipment design and management. The effect of animal related physiological and physical variables, such as behaviour, body weight and insulation, and the way they influence the capacity and pattern to resist heat stress and their relevance regarding practical aspects of housing, cooling systems and management are discussed. Practical aspects of housing, cooling system and cooling management as well as feeding management to provide the best condition for the animal to exercise optimally its thermoregulatory capacity are described together with some new approaches regarding the evaluation of facilities and management efficiency in reaching this goal.