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

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Abstract

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 (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 ANP 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 ANP 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<0.05), but not anoestrous cows (5/9 versus 4/11). Persistence of DF1 was reduced by ODB treatment in both cycling and anoestrous cows (P<0.001). Maximum diameter of DF1 was influenced by ODB treatment and reproductive status (P<0.05). In anoestrous cows in which a second dominant follicle did not emerge during the period of device insertion, the interval from emergence of DF1 to emergence of a second dominant follicle was significantly delayed by treatment with ODB (P=0.04).

In conclusion, P treatment of anoestrous cows increased pulsatile release of LH, but did not induce the development of persistent follicles. Injection of ODB in association with P treatment reduced the persistence of dominant follicles in both cycling and anoestrous cows, but delayed subsequent follicular development in a proportion of anoestrous cows.

Introduction

A period of lactational anoestrus of varying duration is observed in both suckled beef and milked dairy cattle following parturition. This period is characterised by an absence of oestrous behaviour and a lack of progesterone (P) secretion by the ovary. Nevertheless, ovarian follicular turnover occurs, similar to that observed during normal oestrous cycles, and commences around 10 days postpartum (Murphy et al., 1990, Savio et al., 1990, McDougall et al., 1995a). The first postpartum ovulation often occurs without any expression of oestrous behaviour and is followed by a luteal phase of short duration (Murphy et al., 1990, McDougall et al., 1995a).

Oestrus and/or ovulation may be induced in anoestrous cattle or sheep by hormonal treatment. Injection of GnRH or hCG results in ovulation in postpartum anoestrous cows or seasonally anoestrous ewes (McNeilly et al., 1981, Crowe et al., 1993, McDougall et al., 1995b). Oestradiol benzoate (ODB) may also be used to induce oestrus with or without concurrent ovulation (McDougall et al., 1992). Regardless of treatment, such induced ovulations are usually followed by luteal phases of short duration, similar to that observed after spontaneous resumption of oestrous cycles. Early studies demonstrated that the first normal length oestrous cycle postpartum was preceded by a short period of elevated circulating concentrations of P (Henricks et al., 1972, Webb et al., 1980). If treatments to induce ovulation are preceded by a period of P treatment, then the majority of subsequent oestrous cycles are of normal duration (Ramirez-Godinez et al., 1981, Garcia-Winder et al., 1986, Breuel et al., 1993).

Pre-treatment with P prior to an injection of ODB increases pregnancy rates in postpartum anoestrous cows, but fertility is less as compared with that of cows which have spontaneously resumed oestrous cycles (cycling cows). In New Zealand anoestrous dairy cows a combined treatment regimen of intravaginal P and an injection of ODB results in 87% of cows being inseminated on detection of oestrus within 7 days of the end of the treatment period. However, conception rates to this first insemination average 45% compared with 71% in cycling cows (Xu and Burton, 1997). In cows displaying normal oestrous cycles, treatment with low concentrations of P results in the development of persistent ovarian dominant follicles. These follicles are characterised by an increase in size and production of oestradiol, suppression of the development of subordinate follicles and are associated with an increase in frequency of release of LH pulses (Sirois and Fortune, 1990, Savio et al., 1993, Stock and Fortune, 1993). However, fertility following ovulation of dominant follicles which have persisted for ≥10 days is significantly reduced (Mihm et al., 1994). The development of persistent dominant follicles may be one reason for the lesser fertility of anoestrous cows treated with P; however, the effect of treatment with small doses of P on follicular turnover and release of LH in anoestrous postpartum dairy cows has not been fully investigated.

The development of persistent ovarian follicles may be prevented by treatments which cause regression of the dominant follicle. A strategically timed injection of 1 mg ODB during the luteal phase of cycling cows has been demonstrated to be effective in synchronising emergence of a wave of ovarian follicles (Burke et al., 2000). Injection of 2 mg ODB at the time of P treatment has also been found to result in greater synchrony of oestrus and normal fertility in postpartum cows which have resumed oestrous cycles (Day et al., 2000). The aim of the first experiment described here was to test the hypothesis that small doses of P result in development of persistent ovarian follicles and increased release of LH in postpartum anoestrous cows, similar to the effect in cycling cows. The aim of the second experiment was to test the hypothesis that treatment of anoestrous cows with 2 mg ODB at the time of P treatment would result in emergence of a new follicular wave, similar to the responses observed in cycling cows.

Section snippets

Experiment 1

Commencing between 14 and 21 days postpartum, the ovaries of 16 primiparous Jersey cows were examined daily using transrectal ultrasonography (Aloka DX210 with 7.5 MHz probe, Medtel, New Zealand). Animals which had not displayed oestrus and had no detectable corpus luteum were randomly allocated to two groups balanced for calving date. A previously used controlled intravaginal drug releasing device (CIDR™, DEC-InterAg, New Zealand) was inserted in eight cows (AN+P group) for 10 days, commencing

Experiment 1

Mean±S.E.M. interval between calving and day 0 did not differ between cows in the AN+P and ANP groups (P>0.1, 20±2 and 24±6 days, respectively), but was significantly longer for cows in the CYC+P group (P<0.001, 59±2 days). Diameter of DF1 at device insertion did not differ between cows in the three groups (P>0.1; Table 1).

In cows treated with a used CIDR device, a second dominant follicle emerged during the period of device insertion in 4/8 of the CYC+P group and 7/8 of the AN+P group (P

Discussion

The results from these two experiments demonstrate that the ovarian response to treatment with low doses of P is similar in postpartum anoestrous cows and cows which have resumed oestrous cycles. In both experiments emergence of a second dominant follicle during the period of treatment with a used (the first experiment) or new (the second experiment) CIDR device occurred at a similar rate in both anoestrous and cycling cows, although there was a tendency for follicular turnover to be greater in

Conclusions

These results demonstrate that P treatment of anoestrous cows stimulated pituitary release of LH, but not sufficiently to induce the development of dominant ovarian follicles with increased size or oestradiol production as seen in cycling cows. A combination treatment of P and ODB reduced the persistence of ovarian follicles in both cycling and anoestrous cows, but delayed subsequent follicular development in a proportion of anoestrous cows.

Acknowledgements

The technical assistance of Gwyneth Verkerk, David Nation and Mark Ashcroft and laboratory support of Glenys Parton, Rachel Stevenson, Trish O’Donnel and Eleneor Smith is gratefully acknowledged. Antibody to LH was kindly donated by AgResearch, Invermay, New Zealand and purified LH (AFP-7071B) by NIDDK, Bethesda. DEC-InterAg, Hamilton, New Zealand provided the CIDR devices and CIDIROL. Funding was from The Foundation for Science, Research and Technology, New Zealand, contract number DRC 601.

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    Present address: University of Melbourne, Veterinary Clinical Centre, 250 Princes Highway, Werribee, Vic. 3030, Australia.

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