Dairy heifers with normal oestrous cycles were treated with progestagen for 15 to 18 days beginning at the 15th day of the cycle. Daily oral doses of 0·25 to 8 mg of melengestrol acetate (6α-methyl-6-dehydro-16-methylene-17-acetoxyprogesterone: mga) inhibited oestrus and ovulation in all heifers except one receiving 0·5 mg. Daily intravenous injections of 0·4 mg inhibited ovulation in eight out of eight heifers. Lower doses by either route suppressed oestrus but did not uniformly inhibit ovulation. Orally, mga was about 300 to 900 times as potent as map (medroxyprogesterone acetate; Provera*) but only about ten to fifteen times as potent when both were compared by intravenous injection.
Groups of eight to ten heifers were fed doses of 0·2 to 2·0 mg daily beginning without regard to the stage of the oestrous cycle. Of seventy-two heifers, sixty-nine did not ovulate during treatment. The average interval from last feeding to oestrus or ovulation ranged from 2·7 days at 0·2 mg to 6·3 days at 2·0 mg. The conception rates for various groups varied from 25 to 88 % at first service. Of the total, 42 % conceived with one service and 82 % with two services.
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