Follicular dynamics in progestagen treated cattle

A feature of all current progestagen-based systems is the administration of oestradiol at the start of the treatment to:

  • shorten the life span of the cow's corpus luteum
  • terminate the existing wave and induce the emergence of a new follicle.

Termination of the existing wave and induction of a new follicle is especially important as all progesterone/progestagen releasing systems create subluteal levels of progesterone in the circulation of treated cows. These levels are sufficient to create a negative feedback and prevent preovulatory LH surge, ovulation and heat. They cannot however block the LH release activity completely and a small pulsatile secretion is maintained allowing persistence of a dominant follicle should it be present on the ovary at the moment of treatment initiation. It has been known that when duration of dominance of the ovulatory follicle exceeds 4 days (persistent dominant follicle) there is a progressive decline in fertility. This has been attributed to a reduction in oocyte competence and an increase in embryonic loss (Diskin et al., 2002)

Figure 8: Estimation of pregnancy rate as duration of dominance of the pre-ovulatory follicle increases (Diskin et al., 2002).

estimation of pregnancy rate

Exogenous oestradiol when administered with progesterone, before or during emergence of the wave, suppresses formation of or decreases the diameter of DF. This is presumably due to suppression of FSH and perhaps LH.

When selection has occurred, this treatment results in a decrease in DF diameter without consistently changing the timing of the next wave emergence. Treatment of cows classified as anovulatory anoestrus with low doses of progestagens for 6-8 days rarely induces the formation of persistent dominant follicles as would be expected in cows that are in oestrus without the presence of a functional CL (McDougal et al., 2004).