Control and induction of ovulation in cattle.

The control and induction of ovulation in cattle are discussed with particular reference to use of progesterone-impregnated coils in heifers and beef cows. Progesterone treatment for 14 days was required to obtain precise onset of oestrus. With 7, 9 or 12 days of progesterone treatment a luteolytic agent in the form of a prostaglandin (PG) or oestradiol benzoate had to be used. Fertility was normal after treatment durations of 7, 9 or 12 days, but fertility after 14-day treatment requires further testing. The progesterone coil was not effective in maintaining luteal-phase levels of progesterone throughout a 12-day treatment and increasing the concentration of progesterone in the coil from 4 to 20% was not effective in elevating the progesterone concentrations in blood. When progesterone concentrations dropped below approximately 1.5 ng/ml the basal level of LH began to rise before removal of the coil. A 2-fold rise in the basal level of LH was observed following the removal of the progesterone coil. This early rise in LH was absent in cows which did not ovulate after they were given a 12-day progesterone treatment and GnRH 24-36 h after removal of coils to induce the main LH peak. Absence of this early rise suggests that frequency and amplitude of episodic LH release were inadequate in the post-partum anovulatory period. Ovariectomy in the early post-partum period was not followed by an abrupt LH release.