The effect of prostaglandin F2 alpha during various stages of the oestrous cycle of beef heifers.

The effects of prostaglandin F2\g=a\on peripheral plasma progesterone and oestrogen levels, incidence of oestrous cycle alteration, and fertility were examined in thirty beef heifers. The PGF2\g=a\was administered by insertion of an insemination pipette into the lumen of the uterus during three periods of the oestrous cycle: Days 3 to 5, Days 9 and 10, and Days 16 and 17. Oestrus occurred within 2 to 5 days in all animals treated except those treated on Days 3 and 4. Plasma progesterone levels had fallen significantly by the day following treatment and continued to fall in those heifers returning to oestrus. As progesterone fell, plasma oestrogen increased to a peak level on the day before or the day of oestrus. The hormone patterns were quite similar to those of the untreated animals as they approached oestrus and during the 16-day period after mating. The pregnancy rate was significantly lower in the treated heifers than in the untreated heifers. This was probably due to uterine infections rather than to the drug per se.