EFFECT OF SUMMER CLIMATIC CONDITIONS ON DIFFERENT HEAT TOLERANCE INDICATORS IN PRIMIPAROUS FRIESIAN AND JERSEY COWS

The effect of climatic conditions during summer on different heat tole rance indicators was determined in Friesian and Jersey cows which were kept in open camps with no protection against solar radiation. A comp lete diet (15.0% CP and 10.8 MJ ME/kg) was fed daily on an ad libitum basis in fence-line feeding troughs. Heart rates, respiration rates an d rectal temperatures of five primiparous Friesian and five primiparou s Jersey cows, which were accustomed to handling, were measured at two -hourly intervals from 07:00 to 19:00 over 15 days during which high ( greater-than-or-equal-to 27.1-degrees-C) maximum temperatures were exp ected. Rectal temperatures of Friesian cows were higher (P less-than-o r-equal-to 0.05) than those of the Jersey cows from 11:00 to 19:00. wi th the greatest difference (0.55-degrees-C) recorded at 15:00. The res piration rate of Friesian cows was higher (P less-than-or-equal-to 0.0 5) than that of Jersey cows at 15:00, 17:00 and 19:00, with the greate st difference (15.4 inhalations per minute) recorded at 15:00. The hea rt rate of the Friesian group was higher (P less-than-or-equal-to 0.05 ) than that of the Jersey cows at 15:00 and 17:00. Heart rate was not influenced by increasing ambient temperatures to the same extent as Te ctal temperature and respiration rate. Results suggest that Jersey cow s display a higher heat tolerance than Friesian cows, and that Jerseys should be more widely used in the warmer regions of South Africa.