Influence of Environmental Heat on Peripheral Plasma Progesterone and Cortisol During the Bovine Estrous Cycle

Abstract

Plasma progesterone and Cortisol were measured in jugular blood by competitive protein-binding assay. Six cycling, estrus-synchronized Guernsey heifers were followed for four consecutive estrous cycles under two controlled temperature (18.2C, 55% relative humidity; and 33.5C, 55% relative humidity) conditions. Prolonged exposure to heat of Guernsey heifers increased plasma progesterone on days 2 to 19 of the first cycle and only on days 2 to 8 of the second cycle. However, heat stress depressed plasma Cortisol in both cycles and reduced the correlation coefficient between these steroids relative to specific stages of the estrous cycle.