Peripheral plasma levels of progesterone and fertility after prostaglandin-2alpha induced oestrus in heifers.

Sixteen heifers of the Swedish Red and White Breed were given 25 mg PGF2 alpha as tromethamine salt intramuscularly or subcutaneously either on day 8 or day 14 of the oestrus cycle. The injection of PGF2 alpha caused a marked reduction of the size of the corpus luteum in 24 hours. Irrespective of day of injection standing heat occurred 48-96 hours after administration of PGF2 alpha. In the heifers injected on day 8 of the oestrus cycle the midcycle follicle continued to grow and appeared to be the one which was ovulated. Inheifers injected on day 14 a new follicle could be palpated within two days. Eleven of the 16 heifers (69%) became pregnant during the induced heat period. All heifers showed a marked decrease of the peripheral plasma level of progesterone on the day after treatment. The mean progesterone level at the day of first insemination was 80 pg/ml and at the day of second insemination where applied 46 pg/ml. It is concluded that intramuscular or subcutaneous administration of PFG2 alpha on days 8 and 14 of the oestrus cycle results in luteolysis of the cyclic corpus luteum, follicular growth, ovulation and normal pregnancy result after artificial insemination.