Are crossbred cattle (F1, Bos indicus x Bos taurus) thermally different to the purebred Bos indicus cattle under moderate conditions?

In this study we compared the bio-thermal responses of a crossbred (F1, Nellore x Angus) and purebred Bos indicus (Nellore) cattle kept under moderate thermal conditions. Specifically, we assessed if they were different in their daily pattern of metabolic heat production, regulation of body temperature and use of evaporative heat transfer pathways. Six Nellore bulls with 669 +/- 65 kg of body weight and twelve crossbred bulls (F1, Nellore x Angus) with 606.5 +/- 25.09 kg of body weight, were evaluated in two experiments. Individual bulls were secured and attached to instruments in the biometeorology laboratory to enable recording of the proportion of respiratory gases in the exhaled air (oxygen, O-2; carbon dioxide, CO2), saturation pressure of the exhaled air (PS[T-EXP], kPa) and skin surface (PS[T-SKIN], kPa), respiratory rate (R-R, breath min(-1)), ventilation rate (VE, L s(-1)); skin (T-SKIN, degrees C), hair coat (T-S, degrees C), rectal (T-R, degrees C) and exhaled air temperatures (T-EXP, degrees C) from 0600 to 1800 h. The air temperature over the two experiments ranged from 16 to 30 degrees C. The mean of metabolic heat production was higher for crossbred (2.24 W kg(-1)) than Nellore cattle (1.09 W kg 1) (P <0.0001), as the crossbred had higher values for ventilation rate (P <0.0001), and proportion of the oxygen extracted from the inhaled air. Slight differences for thermal storage and daily fluctuation of daytime rectal temperature (0730 and 1630 h) were also observed, ranging from 39 to 39.30 degrees C for Nellore cattle and from 39.12 to 39.62 degrees C for the crossbred. Both genotypes increased heat loss through cutaneous evaporation with rising air temperature, which was slightly higher for the crossbred cattle. In conclusion, under the moderate thermal conditions of the present study, the crossbred cattle had higher levels of heat production, thermal storage and cutaneous evaporation than the purebred Nellore cattle. Further, it appears that both genotypes were able to maintain their thermal balance with relatively low energy demand for thermoregulation. (AU)