Effect of increasing GnRH and PGF_2α dose during Double-Ovsynch on ovulatory response, luteal regression, and fertility of lactating dairy cows

Abstract

Ovsynch-type synchronization of ovulation protocols have suboptimal synchronization rates due to reduced ovulation to the first GnRH treatment and inadequate luteolysis to the prostaglandin F_2α (PGF_2α) treatment before timed artificial insemination (TAI). Our objective was to determine whether increasing the dose of the first GnRH or the PGF_2α treatment during the Breeding-Ovsynch portion of Double-Ovsynch could improve the rates of ovulation and luteolysis and therefore increase pregnancies per artificial insemination (P/AI). In experiment 1, cows were randomly assigned to a two-by-two factorial design to receive either a low (L) or high (H) doses of GnRH (Gonadorelin; 100 vs. 200 μg) and a PGF_2α analogue (cloprostenol; 500 vs. 750 μg) resulting in the following treatments: LL (n = 263), HL (n = 277), LH (n = 270), and HH (n = 274). Transrectal ultrasonography and serum progesterone (P4) were used to assess ovulation to GnRH1, GnRH2, and luteal regression after PGF_2α during Breeding-Ovsynch in a subgroup of cows (n = 651 at each evaluation). Pregnancy status was assessed 29, 39, and 74 days after TAI. In experiment 2, cows were randomly assigned to LL (n = 220) or HH (n = 226) treatment as described for experiment 1. For experiment 1, ovulation to GnRH1 was greater (P = 0.01) for cows receiving H versus L GnRH (66.6% [217/326] vs. 57.5% [187/325]) treatment, but only for cows with elevated P4 at GnRH1. Cows that ovulated to GnRH1 had increased (P < 0.001) fertility compared with cows that did not ovulate (52.2% vs. 38.5%); however, no effect of higher dose of GnRH on fertility was detected. The greater PGF_2α dose increased luteal regression primarily in multiparous cows (P = 0.03) and tended to increase fertility (P = 0.05) only at the pregnancy diagnosis 39 days after TAI. Overall, P/AI was 47.0% at 29 days and 39.7% at 74 days after TAI; P/AI did not differ (P = 0.10) among treatments at 74 days (LL, 34.6%; HL, 40.8%; LH, 42.2%; HH, 40.9%) and was greater (P < 0.001) for primiparous cows than for multiparous cows (46.1% vs. 33.8%). For experiment 2, P/AI did not differ (P = 0.21) between H versus L treatments (44.2% [100/226] vs. 40.5% [89/220]). Thus, despite an increase in ovulatory response to GnRH1 and luteal regression to PGF_2α, there were only marginal effects of increasing dose of GnRH or PGF_2α on fertility to TAI after Double-Ovsynch.

Introduction

A strategy to maximize insemination risk after the end of the voluntary waiting period and fertility at first artificial insemination (AI) in lactating dairy cows is to combine a program for presynchronization of the estrous cycle, such as Presynch, with a synchronization of ovulation program, such as Ovsynch [1], [2], [3]. Depending on the management approach, presynchronization programs are initiated at specific days in milk (DIM) postpartum to allow completion of the program and first AI service either by the end of voluntary waiting period or shortly thereafter. The purpose of presynchronization is to initiate the Ovsynch protocol at the ideal time in the estrous cycle to optimize fertility [2], [3], [4], [5], [6].

Recently, a presynchronization program, termed Double-Ovsynch (DO), has been developed and tested [6]. The DO protocol consists of a modified Ovsynch protocol 7 days before a regular Ovsynch-56 hour-timed artificial insemination (TAI) program (Pre-Ovsynch; GnRH-7 days-prostaglandin F_2α (PGF_2α)-3 days-GnRH and Breeding-Ovsynch; GnRH-7 days-PGF_2α-56 hour-GnRH-16 to 20 hour-TAI). A number of studies have reported results with the DO protocol at first service [6], [7], [8], [9], [10] or later services [11]. Comparison of the DO protocol with the Presynch-Ovsynch protocol with a 12-day interval from the second PGF_2α of Presynch to initiation of Ovsynch found greater fertility with DO [6], [9]. Nevertheless, improved responses to each of the hormonal treatments in the DO protocol would be expected to further improve the fertility of lactating dairy cows treated with this protocol before TAI.

Cows that ovulate to the first GnRH treatment of the Ovsynch protocol have greater fertility than cows that do not ovulate to this GnRH treatment [5], [12], [13]. Ovulation to the first GnRH treatment of Ovsynch (GnRH1) results in the formation of a new CL, which, in turn, increases circulating progesterone (P4) during the Ovsynch protocol. Furthermore, ovulation to GnRH1 results in the initiation of a new follicular wave within ∼2 days of treatment [14], thereby increasing the overall response of cows to the protocol [11], [15] and reducing the variability in follicle size [4], [5], [16]. The DO protocol is designed to optimize ovulation to the first GnRH treatment of Breeding-Ovsynch by initiating the protocol on Day 7 of the estrous cycle, a time when the dominant follicle of the first follicular wave should be present and likely at an optimal stage of development for ovulation in response to a GnRH treatment. Nevertheless, ovulation was not detected in all cows treated with GnRH on Day 7 after a GnRH treatment [6], [11], [17]. This may be, at least in part, because of a suboptimal LH surge when GnRH treatment is applied in the presence of high circulating P4 as would be expected on Day 7 of the estrous cycle [18]. In lactating dairy cows, the use of a higher dose of GnRH can dramatically increase the magnitude of the LH surge, either in the presence or absence of circulating P4 [17], [18]; hence, this may be a viable alternative to increase ovulatory response to the first GnRH treatment of Breeding-Ovsynch to optimize synchronization and fertility.

Optimal response to the PGF_2α treatment before TAI of Ovsynch is desirable because lack of complete luteolysis after this treatment reduces fertility during Ovsynch-type programs in lactating dairy cows [7], [11], [19]. This may be particularly problematic in a program such as DO in which increased ovulation to the first GnRH treatment of Breeding-Ovsynch is expected to increase the proportion of cows with a CL that is less mature and potentially less responsive to PGF_2α. For example, Brusveen et al. [7] reported that after the use of DO for first service, ∼15% of cows did not undergo complete luteolysis (P4 < 0.4 ng/mL 56 hours after GnRH), a problem which was almost completely eliminated after a second treatment with PGF_2α 24 hours after the first treatment. Similarly, Giordano et al. [11], using the same criteria to determine CL regression, reported that when using the DO protocol for resynchronization of ovulation, ∼10% of cows failed to regress their CL. Giordano et al. [11] also reported a lower rate of luteal regression for multiparous (83.9%) than for primiparous cows (89.7%) and for cows that ovulated (79.4%) versus those that did not ovulate (93.0%) in response to the first GnRH treatment of Breeding-Ovsynch. In agreement, other investigators have reported lower rates of luteal regression in multiparous than in primiparous lactating dairy cows [19]. Thus, optimizing CL regression in specific groups of cows may be particularly challenging; hence, new alternatives should be explored to increase CL regression during the DO protocol. As opposed to administering two doses of PGF_2α 24 hours apart as performed by Brusveen et al. [7], it is possible that increasing the dose of PGF_2α may be a simple alternative to maximize the percentage of cows that undergo complete CL regression before TAI.

This research focused on increasing the doses of the first GnRH and the PGF_2α treatments of the Breeding-Ovsynch portion of the DO protocol in an attempt to increase ovulatory response to GnRH1 and the rate of luteolysis after PGF_2α and before TAI. Thus, the specific objectives of the two studies presented herein were the following: (1) to assess ovulatory response after treatment with 100 μg versus 200 μg of GnRH at the time of the first GnRH of the Breeding-Ovsynch portion of the DO protocol and (2) to determine the rates of CL regression in response to 500 μg versus 750 μg of a synthetic PGF_2α analogue during the Breeding-Ovsynch portion of the DO protocol in lactating dairy cows. In addition, the fertility of cows treated with the different GnRH and PGF_2α doses was evaluated. We hypothesized that cows receiving the greater dose of GnRH would have a greater ovulatory response and that more cows receiving the greater dose of PGF_2α would undergo complete luteal regression after the PGF_2α treatment. We also hypothesized that because of the greater ovulatory response and percentage of cows with complete luteal regression, cows treated with the greater doses of GnRH and PGF_2α would have superior pregnancies per artificial insemination (P/AI) than cows receiving the lower doses.