Ovarian Response in Heifers to Progesterone Injections1

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Abstract
Five different dosages of progesterone were injected daily into 12–24-month old dairy heifers (50; 25; 12.5; 6.25 and 3.125 mg.). The beginning of the injection interval varied from day-15 of the estrual cycle to the time when the animal was first noticed in heat. The numbers of injections varied from one to twenty-eight. The interval of time between the end of injections to beginning of heat decreased as the dosage level was decreased. Fifty mg. daily is capable of inhibiting heat and ovulation if started before heat occurs. However, in order to inhibit most of the follicular development, the injection interval must be started earlier in the cycle (day-15). Daily dosages of 25 or 12.5 mg. usually prevent heat and ovulation but follicles in the 20–30 mm. range will develop during treatment. Heat will occur with these follicles ovulating when the injection period is stopped. It was shown that follicles under the influence of prolonged daily injections of 12.5 mg. would regress and be replaced by another follicle at 2–3 week intervals. Dosages lower than 12.5 mg. had little, if any, effect. The data can be interpreted in accord with the theory that progesterone inhibits the action of the gonadotrophic complex, mainly LH, from acting upon the ovary. Copyright © . .