Effect of GnRH on ovulatory response after luteolysis induced by two low doses of PGF2α in lactating dairy cows

Highlights

• Ovulation occurred mostly between 28 and 30 h after GnRH injection.

• Cows in the Saline group ovulated later than 36 h post-injection.

• Ovulation rate within 48 h of GnRH injection tended to be increased (93.3%).

• Cows showed complete luteolysis following two low doses of PGF2α.

• This protocol was an effective alternative to TAI in cows.

Abstract

This study investigated the effect of gonadotropin-releasing hormone (GnRH) on ovulatory response after complete luteolysis induced by two low doses of prostaglandin (PG)F2α in lactating dairy cows. Cows (n = 18) ranging between 45 and 65 days in milk were recruited for synchronization by a modified Ovsynch-48 protocol (GnRH-7 days-375 μg PGF2α-1 day-250 μg of PGF2α-1 day-GnRH) over a total of 23 estrous cycles. Synchronized cows (n = 16) were randomly assigned to GnRH and Saline groups in stage 1 of the experiment after 9–10 days of ovulation in synchronization. On days 0 and 1 (day 0 = first PGF2α administration), cows were treated with 375 and 250 μg PGF2α, respectively. On day 2, cows in the GnRH and Saline groups were administered 250 μg GnRH or 2.5 mL of 0.9% saline, respectively. Serum progesterone (P4) levels were measured and changes in the corpus luteum (CL) were ultrasonically monitored daily from day 0–3 to assess complete luteolysis. Preovulatory follicle diameter and ovulatory response were evaluated by ultrasonography. In stage 2, cows were treated in a manner converse to that in stage 1. The synchronization rate was 69.6% (16/23). In stages 1 and 2, cows showed complete luteolysis with P4 concentration <1 ng/mL or remaining CL area <50%. Average ovulation time was 29.3 ± 0.5 h, which mostly occurred between 28 and 30 h after GnRH injection. However, all cows in the Saline group ovulated later than 36 h post-injection, with an average time of 52.7 ± 8.6 h. There was no difference in preovulatory follicle diameter between the two groups (16.8 ± 0.5 and 17.3 ± 0.5 mm for GnRH and saline groups, respectively). Although ovulation rate was not correlated with treatment, the rate within 48 h of GnRH injection (93.3%) tended to be higher compared with that in the Saline group (60.0%). Thus, GnRH administration increased ovulation rate following complete luteolysis induced by two low doses of PGF2α. These results indicate that this simple protocol for dairy cows is an effective alternative to timed artificial insemination programs in the field.

Introduction

Over the last decade, there has been an increasing interest in developing new synchronization protocols with timed artificial insemination (TAI) to improve reproductive efficiency in the dairy industry [1], [2]. Presynchronization strategies such as G6G [i.e., pre-prostaglandin (PG)F2α-2 days-pre-gonadotropin-releasing hormone (GnRH)-6 days] and Double-Ovsynch were developed to increase conception rates [3], [4]. It is widely acknowledged that GnRH and PGF2α play a critical role in the induction of ovulation. Under heat stress, their combined administration can even induce three successive and continuous follicular waves that enhance fertility [5].

GnRH-induced ovulation is typically determined by ultrasonography 7 days after GnRH injection in cows [4], [6], [7]; however, it is triggered by a surge of luteinizing hormone (LH) that causes the rupture of dominant follicles within 24–32 h after the GnRH surge [8]. It was previously reported that cows ovulated between 26 and 32 h after the second GnRH administration in the Ovsynch protocol, and that the rate of ovulation was highest at 28 h [1]. Another study found that ovulation mostly occurred between 28 and 30 h after GnRH administration [9]. Even in cows with high progesterone (P4) concentration that received 100 μg GnRH, the ovulatory response was only 75.0%. In the combined presynchronization/Breeding-Ovsynch protocol, the first GnRH pulse in Breeding-Ovsynch induced ovulation and the formation of an accessory corpus luteum (CL); however, the incidence of partial luteolysis in cows treated with a standard dose of PGF2α was concurrently increased [10]. The rate of complete luteolysis may be increased by administering a second dose of PGF2α in Breeding-Ovsynch [11], [12]. Two consecutive low doses of PGF2α were shown to efficiently enhance luteolysis and increase ovulation rate up to 90.0% [13], whereas 50% of cows ovulated between 96 and 120 h after the first PGF2a administration.

Previous reports described the ovulation time based on one standard dose of PGF2α followed by GnRH treatment in cows, in which ovulation occurred during a concentrated period [1], [9]; however, details of the ovulation process post-GnRH administration after two consecutive injections of PGF2α has yet to be described in TAI protocols. The objective of the present study was to evaluate the effect of GnRH on the induced ovulation time and rate after complete luteolysis following treatment with two low doses of PGF2α in lactating dairy cows as a useful addition to the standard TAI protocol. We hypothesized that GnRH treatment would advance ovulation time in cows and increase the ovulation rate as compared to those in non-treated animals.