Effect of treatment with progesterone and oestradiol benzoate on ovarian follicular turnover in postpartum anoestrous cows and cows which have resumed oestrous cycles

Two experiments were carried out to determine the effect of a low dose of progesterone (P) with and without the addition of an injection of oestradiol benzoate (ODB) on ovarian follicle dynamics, oestradiol production and LH pulsatility in postpartum anoestrous cows, compared with cows which had resumed oestrous cycles (cycling cows). In the first experiment, anoestrous Jersey cows were treated with (AN+P, n=8) or without ( AN−3, n=3) a previously used intravaginal progesterone releasing (CIDR™) device for 10 days, commencing 3 or 4 days after emergence of a new dominant follicle (DF1) as determined by transrectal ultrasonography. Contemporary cycling cows (CYC+P, n=8) were similarly treated with used CIDR devices and injected with prostaglandin F 2α (PGF) at the time of device insertion. Follicle turnover was monitored by daily ultrasonography and pulsatile release of LH was measured on the ninth day after device insertion. During the period of CIDR device insertion, a second dominant follicle emerged in 4/8 of the CYC+P group and 7/8 of the AN+P group ( P=0.14). Maximum diameter of DF1 was greater in cows in the CYC+P compared with the AN+P group ( P=0.02), but did not differ between cows in the AN+P and AN− P groups ( P>0.1). Frequency of LH pulses was greater in cows in the CYC+P than AN+P group ( P=0.06), and in cows in the AN+P than AN− P group ( P=0.02). In the second experiment, anoestrous ( n=20) and cycling ( n=11) Friesian cows were treated with a new CIDR device for 6 days commencing 3 days after emergence of a new dominant follicle (DF1). Cycling cows were also injected with PGF on the day of device insertion. Half of the cows in each group were injected with 2 mg ODB on the day of device insertion. Daily ultrasonography was used to monitor follicular dynamics throughout the experimental period. Follicular turnover was increased by ODB in cycling (5/5 versus 1/6; P