The effect of repeated transport stress over a period of 60 hours on secretion of gonadotropins was investigated by transporting three cyclic mares in a trailer. The experiment involved ten 5-hour transport periods, each followed by one hour of rest. Ultrasound observation of ovaries and measurement of levels of circulating progesterone indicated that two mares (Horse Nos. 1 and 2) were in the luteal phase and the remaining one horse was in transition from the luteal phase to the follicular phase (luteolysis). Levels of plasma cortisol in all horses increased in response to each transport periods, but the cortisol response much leveled off during the last three transport periods (41 hr after the experiment began). Levels of plasma LH in Horses Nos. 1 and 2 remained low throughout the period of transport experiment. In Horse No. 3, however, levels of plasma LH increased during the first 2 transport periods, followed by a gradual decrease until the end of the experiment. There were no obvious changes in levels of plasma FSH in any of the animals. These results suggest that transport stress suppresses basal secretion of LH in cyclic mares and the pattern of this suppression may depend on the estrous cycle.
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