Plasma Corticoids of Angus Heifers in Programmed Circadian Temperatures of 17 to 21 C and 21 to 34 C

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Abstract
Total glucocorticoid levels were studied in eight 14-month-old Angus heifers maintained at control (17 to 21 C diurnal variation) and experimental (21 to 34 C diurnal variation) temperature regimes. Rectal temperatures were significantly (P<.01) increased by the experimental temperature regime, indicating that these conditions exceeded thermoneutrality. During exposure to the experimental temperature, corticoid levels were higher at 0800 and 1600 hr. for the first week and lower for the remainder of the estrous cycle as compared with the control temperature. Mean levels for the entire estrous cycle were only slightly increased by the experimental temperature regime (8.3 ± .4 vs. 7.6 ± .4 ng/ml, P>.05). At both temperature regimes corticoid levels were generally higher on the day of estrus and lower during the remainder of the estrous cycle. Increased ambient temperature resulted in greater variation of total corticoid levels. Days were the only significant (P<.05) main effect, presumably due to a combination of thermal effects and the increase of corticoid levels associated with estrus. Copyright © 1974. American Society of Animal Science . Copyright 1974 by American Society of Animal Science.

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