Plasma Cortisol in Bos taurus and Bos indicus Heifers in Seasonal Tropical Climate

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Abstract
Cortisol concentration in blood plasma was measured in Brown Swiss, Holstein-Friesian and White Fulani heifers over 2 consecutive estrous cycles during each of the 4 quarters of the hot humid seasonal tropical climate of Ibadan, southern Nigeria. Basal cortisol in plasma was low (1-10 ng/ml). Differences between breeds were not significant. Averages and increases with proestrus and estrus were higher in the wet season (March-Aug.) than in the dry season (Nov.-Jan.). Packed cell volume was higher in the dry season than in the wet season. Seasonal changes in rhythmicity of the estrous cycle were not significant. The low cortisol in plasma probably reflects adaptation to the hot climate and a mechanism to prevent high metabolic heat.