Abstract
A relationship between luteal hypoplasia and ovulation was analyzed in cows. Adult cows of the luteal phase were injected with gonadotropin releasing hormone analogue (GnRH-A) 16 hr (group I of 4 head), 32 hr (group II of 4 head), or 48 hr (group III of 3 head) after injection with prostaglandin F2α-analogue (PGF2α-A), and then examined for the occurrence of luteal hypoplasia. Ovulation took place at 32-46 hr (37.5 hr on the average) in group I and 9-33 hr (20.0 hr on the average) in group II after injection with GnRH-A. Unsatisfactory luteinization and substantial shortening in estrous cycle were observed in all the cows of groups I and II. In group III, ovulation took place 12-32 hr (24.7 hr on the average) after injection with GnRH-A, and luteinization was satisfactory in one, but the corpora lutea formed were small in the other two cows. All the cows, except one in each of groups II and III were examined for changes in the plasma progesterone level after the induced ovulation. Low progesterone levels were observed in groups I and II, while in group III, the levels took almost the same course of changes as the normal estrous cycle, indicating the presence of a satisfactory luteinization. However, temporary increase was observed in another cow in this group. It is concluded that luteinization became unsatisfactory when ovulation was forced to take place in cows while the ovarian follicles were still developing.
