Evaporative heat loss in Bos taurus: Do different cattle breeds cope with heat stress in the same way?

The aim of this study was to compare two Portuguese (Alentejana and Mertolenga) and two exotic (Frisian and Limousine) cattle breeds in terms of the relationship between the increase in ambient temperature and the responses of the pathways of evaporative heat loss and the effects on homeothermy. In the experiment, six heifers of the Alentejana, Frisian, and Mertolenga breeds and four heifers of the Limousine breed were used. The animals were placed in four temperature levels, the first one in thermoneutral conditions and the other ones with increase levels of thermal stress. The increase in chamber temperatures had different consequences on the animals of each breed. When submitted to severe heat stress, the Frisian animals developed high thermal tachypnea (125 mov/min) and moderate sweating rates (117 g.m-2.h-1), which did not prevent an increase in the rectal temperature (from 38.4°C to 40.0°C). The Alentejana and Limousine breeds showed intermediate thermoregulatory reactions, with moderate increases in rectal temperature, especially during the period of greatest heat stress (from 38.6°C to 39.4°C and from 38.87°C to 39.4°C for Limousine and Alentejana, respectively). However, the importance and the intensity of evaporative heat loss pathways were different. The Limousine breed showed moderate levels of tachypnea (101 mov/min) while showing the lowest rates of sweating (95 g.m-2.h-1), which had a tendency for early stabilization. The Alentejana breed showed significant increases in sweating rate (156 g.m-2.h-1) that played a major role in homeothermy under conditions of moderate heat stress. With the tendency for higher heat storage, the involvement of tachypnea (112 mov/min) became clearer. The Mertolenga breed showed a superior stability of body temperature, even in the most severe conditions of heat stress (from 38.5°C to 39.1°C). However, the evaporative heat loss occurred in an uncommon way. The maintenance of homeothermy during moderate heat stress was achieved primarily by intense tachypnea (122 mov/min). This response is quite unusual in cattle, particularly when associated with low sweating rate and low heat storage. The sweating rate remained abnormally low in conditions of moderate heat stress, rising significantly when ambient temperatures led to marked tendency for hyperthermia (110 g.m-2.h-1). However, the sweating rate did not reveal any evidence of stabilization. This change in the protagonist of the evaporative heat loss pathways suggests a different thermoregulatory strategy, suggesting a different adaptation to semi-arid environment and strong association with water metabolism