Ovarian responses to progesterone and oestradiol benzoate administered intravaginally during dioestrus in cattle

This study examined the effects of administering progesterone and oestradio l benzoate (ODB) during mid-dioestrus, on ovarian follicular dynamics in ca ttle. Twelve cycling cows were used in a 4 x 4 latin square design, with th e 4 treatments being initiated on Day 13 of the cycle (oestrus = Day 0) and comprising intravaginal insertion for 5 days of: (i) a progesterone releas ing device (CIDR (TM); 'P4'); (ii) a CIDR device with a gelatin capsule con taining 10 mg ODB and 1 g lactose (CIDIROL (TM);'P4/ODB') attached; (iii) a placebo CIDR device with the 10 mg ODB capsule (ODB); and, (iv) a placebo CIDR device alone (CTRL). The ovaries of each cow were examined daily by tr ansrectal ultrasonography from Day 7 of the cycle until subsequent ovulatio n. Blood samples were collected daily from Day Il, and at intervals of 2-4 h during the 24 h period either side of treatment initiation. The second do minant follicle (DF2) emerged on Day 10.7 +/- 0.2 (mean +/- SEM), and was 8 .5 +/- 0.2 mm in diameter by Day 13. The DF2 developed through to ovulation (2-wave cycles) in half of the animals in the CTRL group; while in the oth er half of cases, the ovulatory follicle originated from the third follicle wave that emerged on Day 17.2 +/- 0.4. Administration of a CIDR device alo ne (P4 group) did not alter the 1:1 ratio of 2 and 3-wave cycles, but the t hird dominant follicle (DF3) in those cows with 3-wave cycles emerged earli er on Day 15.6 +/- 0.2. In contrast, the DF2 of every animal in the ODB and P4/ODB groups became atretic and was replaced by a DF3 which emerged 4.0 /- 0.3 days later. The effects of ODB on luteal function were limited to an earlier decline in plasma progesterone concentrations from 2 to 4 days aft er device insertion and a reduction in diameter of the corpus luteum when a dministered concurrently with progesterone. intravaginal administration of 10 mg ODB on Day 13 of the oestrous cycle, with or without progesterone, wa s effective in promoting follicle wave turnover. In the absence of ODB, pro gesterone administration alone (P4 group) did not alter the ratio of animal s with 2 of 3-wave cycles from that observed in animals in the CTRL group, but did advance the timing of subsequent follicle wave emergence in those a nimals with 3-wave cycles. (C) 1999 Elsevier Science B.V. All rights reserv ed.