Ultrasound-monitored ovarian responses in normal and superovulated cattle given exogenous progesterone at different stages of the oestrous cycle

In two experiments with female cattle, responses to synchronisation and superovulation were monitored by transrectal ultrasonography and embryo recovery. Each experiment had both a synchronisation phase to establish a reference oestrus and a superovulatory phase with the oestrous cycle controlled by exogenous progesterone commencing at two specific times. The reference oestrus was controlled using a progesterone releasing intravaginal device (PRID) applied for 12 days with prostaglandin F2α given 1 day before removal. Experiment 1 had two treatments which differed by the absence (A) or presence (P) of a 10mg oestradiol benzoate capsule on the PRID, while in Experiment 2 all animals were on treatment P. In the superovulatory phase of both experiments treatment P commenced on Day 7 (PRID 7 treatment) or Day 14 (PRID 14 treatment) of the oestrous cycle (oestrus designated Day 0). Superovulation, using equine chorionic gonadotrophin in Experiment 1 and oFSH in Experiment 2, commenced 3 days before PRID removal. Treatment P caused rapid regression of the dominant follicle and corpus luteum (CL) irrespective of when treatment commenced. A second wave of follicular growth was detected after 6–8 days and the dominant follicle grew at 1.1 mm day−1 in the 7 days before oestrus. In contrast, in treatment A of Experiment 1, the dominant follicle either grew slowly and eventually ovulated for cows in the mid-luteal phase, or the dominant follicle regressed and a second wave follicle ovulated if cows were early luteal at PRID insertion. In the superovulatory phase of both experiments the dominant follicle of PRID 7 animals increased in size and then regressed, but in PRID 14 cows, the dominant follicle was regressing before PRID insertion. During superovulation, the number of 7–10 mm follicles was significantly (P<0.001) greater in PRID 7 animals in Experiment 2. In both experiments, half the animals on the PRID 14 treatment maintained a large follicle during the superovulatory phase in contrast to the even sized follicles in animals on PRID 7 treatment. In Experiment 1, the number of grade 1 embryos recovered was significantly (P<0.05) higher for PRID 7 than PRID 14 treatments. In Experiment 2, there were significant differences (P<0.001) in the number of corpora lutea, total ova plus embryos and grade 1 embryos in favour of PRID 7 animals following superovulation. We conclude that the initiation of control of the oestrous cycle with a PRID and subsequent superovulating regime should take account of normal follicular wave status for effective superstimulation and production of viable embryos, and that ultrasonography may usefully be applied to the process.