Effects of gonadotropin-releasing hormone at initiation of the 5-d timed artificial insemination (AI) program and timing of induction of ovulation relative to AI on ovarian dynamics and fertility of dairy heifers

Abstract

Two experiments evaluated the effects of the first GnRH injection of the 5-d timed artificial insemination (AI) program on ovarian responses and pregnancy per AI (P/AI), and the effect of timing of the final GnRH to induce ovulation relative to AI on P/AI. In experiment 1, 605 Holstein heifers were synchronized for their second insemination and assigned randomly to receive GnRH on study d 0 (n = 298) or to remain as untreated controls (n = 307). Ovaries were scanned on study d 0 and 5. All heifers received a controlled internal drug-release (CIDR) insert containing progesterone on d 0, a single injection of PGF(2α) and removal of the CIDR on d 5, and GnRH concurrent with timed AI on d 8. Blood was analyzed for progesterone at AI. Pregnancy was diagnosed on d 32 and 60 after AI. Ovulation on study d 0 was greater for GnRH than control (35.4 vs. 10.6%). Presence of a new corpus luteum (CL) at PGF(2α) injection was greater for GnRH than for control (43.1 vs. 20.8%), although the proportion of heifers with a CL at PGF(2α) did not differ between treatments and averaged 87.1%. Progesterone on the day of AI was greater for GnRH than control (0.50 ± 0.07 vs. 0.28 ± 0.07 ng/mL). The proportion of heifers at AI with progesterone <0.5 ng/mL was less for GnRH than for control (73.8 vs. 88.2%). The proportion of heifers in estrus at AI did not differ between treatments and averaged 66.8%. Pregnancy per AI was not affected by treatment at d 32 or 60 (GnRH = 52.5 and 49.8% vs. control = 54.1 and 50.0%), and pregnancy loss averaged 6.0%. Responses to GnRH were not influenced by ovarian status on study d 0. In experiment 2, 1,295 heifers were synchronized for their first insemination and assigned randomly to receive a CIDR on d 0, PGF(2α) and removal of the CIDR on d 5, and either GnRH 56 h after PGF(2α) and AI 16h later (OVS56, n = 644) or GnRH concurrent with AI 72 h after PGF(2α) (COS72; n = 651). Estrus at AI was greater for COS72 than for OVS56 (61.4 vs. 47.5). Treatment did not affect P/AI on d 32 in heifers displaying signs of estrus at AI, but COS72 improved P/AI compared with OVS56 (55.0 vs. 47.6%) in those not in estrus at AI. Similarly, P/AI on d 60 did not differ between treatments for heifers displaying estrus, but CO S72 improved P/AI compared with OVS56 (53.0 vs. 44.7%) in those not in estrus at AI. Administration of GnRH on the first day of the 5-d timed AI program resulted in low ovulation rate and no improvement in P/AI when heifers received a single PGF(2α) injection 5 d later. Moreover, extending the proestrus by delaying the final GnRH from 56 to 72 h concurrent with AI benefited fertility of dairy heifers that did not display signs of estrus at insemination following the 5-d timed AI protocol.

Comparison of 4- versus 5-day Co-Synch + controlled internal drug release (CIDR) + timed artificial insemination protocols in dairy heifers

“…In a pilot study, dairy heifers treated with a 4-day Co-Synch þ CIDR protocol had an adequate P/TAI (66.7%, 10 of 15), also similar to that observed in the 5-day Co-Synch þ CIDR protocol (46.7%, 7 of 15) or AI after PGF 2a injection and heat detection (46.7%, 7 of 15) [16]. The pregnancy rate observed in the present trial using 4-or 5-day Co-Synch þ CIDR protocols without GnRH at the time of CIDR insertion or a second dose of PGF 2a 12 hours after CIDR withdrawal was similar to that observed in previous studies in dairy heifers treated with comparable 5-day protocols [12,14,15]. These previous studies revealed that a single dose of PGF 2a (compared to two PGF 2a injections) at the time of CIDR removal within a 5-day Co-Synch þ CIDR protocol was sufficient to obtain an adequate pregnancy rate (46.1% and 48.6%, respectively) and CL regression (86.9% and 92.8%, respectively) in dairy heifers [12].…”

“…The acceptable P/TAI obtained in this study using these short Co-Synch þ CIDR protocols was consistent with the concept that dairy heifers show more rapid turnover of follicles, have a common pattern of three follicular waves per estrous cycle, and display shorter periods of follicular dominance than cows [20]. Additionally, on the basis of these physiological events, Lima et al [15] stated that ovulation and recruitment of a new follicular wave at the beginning of a 5-day Co-Synch þ CIDR protocol in heifers might have less effect on fertility. Therefore, it is plausible that both the 4-and 5-day Co-Synch þ CIDR protocols were similarly able to synchronize the ovulation of a competent young follicle when applied at random times of the estrus cycle in dairy heifers.…”

“…These previous studies revealed that a single dose of PGF 2a (compared to two PGF 2a injections) at the time of CIDR removal within a 5-day Co-Synch þ CIDR protocol was sufficient to obtain an adequate pregnancy rate (46.1% and 48.6%, respectively) and CL regression (86.9% and 92.8%, respectively) in dairy heifers [12]. Moreover, Colazo et al [14] and Lima et al [15] reported that fertility of dairy heifers was not improved by administration of the first GnRH dose on Day 0 within the 5-day Co-Synch þ CIDR protocol, obtaining a similar pregnancy rate compared with heifers receiving GnRH at the beginning of the protocol (52.5% vs. 49.8%, respectively). Interestingly, a recent study in dairy and beef heifers showed that GnRH administration at the time of CIDR insertion was beneficial in beef heifers but not in dairy heifers to improve pregnancy when using the modified 5-day Co-Synch þ CIDR protocol [18].…”

“…Further studies revealed that fertility of dairy heifers was not improved by administration of the first GnRH on Day 0 within the 5-day Co-Synch þ CIDR protocol [14,15]. Therefore, when heifers are subjected to the 5-day Co-Synch þ CIDR TAI program with a single treatment of PGF 2a , the initial GnRH may not be necessary, obtaining a similar pregnancy rate compared to heifers receiving GnRH at the beginning of the protocol (52.5% vs. 49.8%, respectively) [15].…”

Synchronization of ovulation and fixed-time artificial insemination in beef cattle

“…Colazo and Ambrose (2011) and Lima et al (2011) showed that P/AI did not differ between dairy heifers receiving or not GnRH at the time of insertion of a progestin device. Rabaglino et al (2010) found no differences in P/AI when one or two doses of PGF were used in dairy heifers.…”