Rectal Temperature and Respiratory Responses of Jersey and Sindhi-Jersey (F1) Crossbred Females to a Standard Hot Atmosphere

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Abstract
Jersey and Red Sindhi-Jersey (F1) crossbred female cattle were subjected to a standard hot atmosphere at 105[degree]F with a vapor pressure of 34 mm Hg (wet bulb 92[degree]F) for 6 hours, at approximately 2-month intervals from 6 months of age onward except during the 1st lactation, which was undisturbed, and during the 45 days preceding each expected parturition. Data show that the mean response of rectal temperature was less in the crossbred animals than in the Jerseys, whether the comparison be made as young heifers, older heifers, dry cows, or lactating cows, or between animals at the same level of milk production. Both the initial temperature and temperature during exposure were higher in the younger animals, but the rise in rectal temperature was similar for all ages. Stage of lactation had little effect on the response of rectal temperature to heat, but level of milk production had considerable effect. Rectal temperature during exposure tended to increase steadily with production in Jerseys, but in the F1 crosses it reached a level (30 lb. fat corrected milk) which did not rise with further increases in production. There was a marked seasonal variation in rectal temperature response. The repeatability of response was high in heifers 8-22 months of age when the results were sorted by season, but low in the lactating cows. In heifers the respiratory response of the Jerseys was greater than in the crossbreds, but in lactating and dry cows the crossbreds showed the greatest rise in respiratory rate. The view is confirmed that differences in respiratory rate are the result and not the cause of differences in heat tolerance.