The effect of oestrus synchronization on ovarian function in the cow

The overall objectives of the present studies were to characterize ovarian follicular development in dairy cows and to relate this follicular development to the plasma progesterone levels during and after treatment with a progesterone intravaginal delivery device (CIDR, Eazibreed CIDR-B, AH I Plastic Moulding Company, Hamilton, New Zealand). Ovarian follicular development was monitored by using transrectal real-time, B-mode diagnostic ultrasonography, while the plasma progesterone levels were determined using a solid phase microtitre enzyme immunoassay. The accuracy of transrectal diagnostic ultrasonography for the assessment of the ovarian structures was examined by comparing the results of in vivo transrectal ultrasonography of the ovaries and in vitro slicing of the excised ovaries after slaughter. The results of the two types of determination of ovarian structures were highly correlated. Correlation coefficients of the diameter of the largest and second largest follicles using the two techniques were 0.94 and 0.91, respectively; the correlation coefficients for the number of small, medium and large follicles were 0.75, 0.67 and 0.82, respectively, while that of the size of the corpus luteum was 0.70. Imaging of the ovaries through the normal oestrous cycle in dairy cows (n - 18) using transrectal ultrasonography showed that most oestrous cycles (13/18 = 72.22 %) were characterized by the growth of 3 successive dominant follicles that occurred in waves. The first dominant follicle (non-ovulatory) was identified on Day 0 (day of ovulation), reached its maximum diameter on Day 8 and was detectable, on average, until Day 17. The second dominant follicle (non-ovulatory) was generally present between Days 5 and 20 of the oestrous cycle, and reached its maximum diameter on Day 14. The third dominant follicle (ovulatory) was first detected on Day 11, stayed relatively stable in diameter between Days 11 and 13, then increased in growth rate and reached its maximum diameter on Day 21, before ovulating. Some oestrous cycles (3/18 - 27.78 %) were characterized by the growth of only 2 dominant follicles. The dominant ovulatory follicles were larger than the dominant non-ovulatory follicles, and during mid-cycle the processes of growth and atresia of follicles were slower than at the beginning or end of the cycle. The development of the dominant follicle suppressed the further growth of smaller follicles in both ovaries. Changes in the size of the corpus luteum occurred concomitantly with changes in the plasma concentration of progesterone throughout the oestrous cycle. A study was undertaken to determine the ovarian follicular development using transrectal real-time, B-mode diagnostic ultrasonography during and after treatment with CIDRs. This follicular development was then related to plasma progesterone levels. The CIDR treatment regimes consisted of a 15 day CIDR treatment (treatment group A, n - 7), a 12 day CIDR treatment combined with an injection of 10 mg oestradiol benzoate at the time of CIDR insertion (treatment group B, n = 7), and a 9 day CIDR treatment combined with an injection of 25 mg prostaglandin F2a on the 8th day of treatment (treatment group C, n = 8). The ovaries were examined by transrectal ultrasonography and blood sample were collected daily throughout the experiment, commencing 3 days prior to the time of CIDR insertion until 3 days after CIDR removal. The CIDR proved to be a synchronizing agent with excellent retention rates and satisfactory synchrony, and an ovulatory follicle developed rapidly after CIDR removal. The development of the ovulatory follicle and the decline in the mean plasma concentration of progesterone in the short-term CIDR treatments combined with luteolytic agents was more marked than follicular development in the CIDR treatment alone, although there was no apparent difference in the incidence of oestrus or the interval from CIDR removal to the onset of oestrus among the treatment groups. Another study determined the capacity of the largest and second largest follicles to become ovulatory follicles following the CIDR treatment regimes. The CIDR treatments were exactly the same as previously described, and also consisted of treatment groups A (n - 4), B (n - 4) and C (n - 4). The animals were slaughtered approximately 48 hours of CIDR removal and the ovaries were immediately collected. The largest and second largest follicles and the corpus luteum of each cow of the treatment groups were dissected from the ovaries and then incubated in culture medium to measure cestradiol-17p and progesterone production by using a solid phase microtitre enzyme immunoassay. The hormonal status of the largest and second largest follicles from the excised ovaries of each cow was determined by oestrogen to progesterone ratios of their follicular fluid and culture medium. Results from the hormonal status determination confirmed that the largest or second largest follicles present in the ovaries collected 48 hours after CIDR removal in each of the treatment groups were oestrogen active and were potentially ovulatory follicles. The results of the present studies can be summarized as follows: 1). transrectal diagnostic ultrasonography is a highly reliable method for accurate examination of the ovarian structures of the cow; 2). the growth pattern of dominant follicles mostly consists of 3 waves of follicular development throughout the oestrous cycle in the cow; 3). the CIDR is a useful agent to synchronize oestrus in the cow, accelerating the development of an ovulatory follicle and ovulation after its removal. Its combination with either an injection of oestradiol benzoate or prostaglandin F2afor short-term treatments results in a more marked decline in the plasma progesterone concentration after CIDR removal than after CIDR treatment alone