The effect of a high ambient temperature on the physiological responses of Friesian and Brahman x Friesian calves fed on two rations : a thesis presented in partial fulfilment of the rquirements for the degree of Master of Agricultural Science in Animal Science at Massey University
Eight Friesian and eight Brahman x Friesian (F1) steer calves were kept at 17 or 34.5°C in temperature controlled rooms and fed on hay or concentrate diets in a 2 x 2 x 2 factorial design for eight weeks. Then eight of the calves were used to study the effect of coat clipping at the higher air temperature, and later they were exposed each day to eight hours of artificial radiant heat load for two weeks. In each trial voluntary feed and water intake, liveweight gain, rectal temperature and respiration rate were assessed. In addition dietary digestibility and hair coat cover were estimated in the factorial trial, while radiant heat load and sweating rates were measured in the radiation trial. Data for feed and water intake, digestibility and liveweight gain are the subject of another thesis (King, 1978). There were no significant differences between breeds at the cool ambient temperature of 17°C, but at the high air temperature (34.5°C) both breeds had reduced their voluntary feed intake and had significantly increased their rectal temperatures (P¼0.01) and respiration rates (P¼0.005). However, although the F1, calves ate more feed at the high ambient temperature than the Friesians, rectal temperatures and respiration rates of Friesians (40.50°C, 100 counts per minute) were higher than those for the F1 calves (39.80°C and 93 counts per minute respectively). Nevertheless, only rectal temperatures showed significant (P¼0.05) interactions between the effects of breed and air temperature. At the high ambient temperature concentrate fed calves consuxed more DM and had slightly higher respiration rates but slightly lower rectal temperatures (40.11°C) than hay fed calves (40.20°C); F1 calves fed this diet had the highest DMI and liveweight gain but lowest rectal temperature (39.63°C). Coat cover was related to level of feeding, breed and air temperature. Generally F1, calves consumed more of the concentrate diet and had significantly (P¼0.05) less coat cover than the Friesians. Hay fed calves had significantly (P¼0.01) heavier coats than concentrate fed calves. Coat weights were substantially heavier at 17°C (177 g/m2) than at 34.5°C ambient (150 g/m2), but this difference was not significant. Although coat cover was positively correlated with body temperature at the high ambient temperature, clipping did not affect the DMI, body temperature or respiration rate of either of the genotypes. Exposure to radiant heaters resulted in a net gain of radiant heat by the animals of 130 - 140 kcal/m2 hr; this caused significant increases (P¼0.05) in respiration rate and rectal temperature measured after 8 hours of exposure to the radiant load. However, F1 calves were significantly (P¼0.05) less stressed by the radiation than the Friesians. The measured sweating rates of 153 and 60 g/m2 hr for F1 and Friesian calves respectively suggest that the most important cause for the genotype differences in heat tolerance is due to the superior ability of the F1 to discharge moisture on to the skin surface for evaporative cooling.