# Tandem Administration of Prostaglandin F2α and Gonadotropin-Releasing Hormone in Beef Heifers and Cows as a Convergent Presynchronization Method in the 7 & 7 Synch Protocol

## TL;DR

This study tested a new method to synchronize estrus in beef cattle using hormones, but found it didn't improve pregnancy rates despite better ovarian responses.

## Contribution

A presynchronization strategy using GnRH and PG was evaluated to improve estrus synchronization in acyclic females within the 7 & 7 Synch protocol.

## Key findings

- Tandem administration of GnRH and PG generated a convergent ovarian response among females with varying pretreatment luteal status.
- Pregnancy rates following fixed-time artificial insemination did not differ between treatment groups.
- Multiparous females treated with 7 & 7 + G + P showed higher behavioral estrus expression prior to insemination.

## Abstract

In the cow–calf industry, estrus synchronization and artificial insemination are reproductive technologies that improve conception rates early in the breeding season. These technologies concentrate calving earlier in the season, increasing calf weaning weights and enhancing producer profitability. A recently developed estrus synchronization protocol, 7 & 7 Synch, has demonstrated improvements in pregnancy rates resulting from artificial insemination. However, improvements in pregnancy results using 7 & 7 Synch may be limited to cyclic females. Acyclic females (i.e., females not already undergoing regular estrous cycles prior to the initiation of the synchronization protocol) lack a functional corpus luteum, rendering them unresponsive to the prostaglandin F2α administered at the initiation of 7 & 7 Synch. Commercial cow–calf herds often consist of a mix of acyclic and cycling females, posing a challenge for successful estrus synchronization. Thus, differences in response of cyclic versus acyclic females to an estrus synchronization protocol limits overall protocol efficacy and negatively impacts producer profitability. This study aimed to evaluate a presynchronization strategy intended to enhance synchronization success in acyclic females while maintaining the results achieved in cyclic females using 7 & 7 Synch. Incorporating additional hormones during presynchronization facilitated a convergence in ovarian presentation among females with varying pretreatment estrous cyclicity status; however, it failed to improved P/AI.

An experiment was designed to evaluate modifications to the 7 & 7 Synch protocol, with the hypothesis that tandem administration of prostaglandin F2α (PG) and gonadotropin-releasing hormone (GnRH) during the presynchronization portion of the protocol would generate a convergent ovarian response among anestrous and estrous cycling females. Nulliparous, primiparous, and multiparous Angus and Angus-cross females were blocked based on parity and pre-treatment luteal status and, within a block, randomly assigned to one of three treatments. The females assigned to the 7 & 7 Synch treatment (n = 765) received an intravaginal progesterone-releasing insert (CIDR®) and an administration of PG on Day 0, GnRH on Day 7, and PG concurrent with CIDR removal on Day 14. Estrotect™ estrus detection aids were applied on Day 14 at the time of CIDR removal and PG administration. The females assigned to the 7 & 7 + G treatment (n = 769) received the same treatment schedule, with the modification that GnRH was administered in tandem with PG on Day 0. The females assigned to the 7 & 7 + G + P treatment (n = 786) received the same treatment schedule, with the modification that GnRH was administered in tandem with PG on both Days 0 and 7. Fixed-time artificial insemination (FTAI) was performed at 54 ± 2 h (nulliparous) or 66 ± 2 h (primiparous and multiparous) after CIDR removal. Transrectal ovarian ultrasonography was performed on Days 7 and 14 to characterize the largest follicle diameter (LFD) and the presence of corpora lutea (CL). Estrus prior to FTAI was characterized using estrus detection aids. Pregnancy status was determined via transrectal ultrasonography approximately 60–100 days after FTAI. The proportion of multiparous females with CL on Day 14 tended to be greater following treatment with 7 & 7 Synch or 7 & 7 + G (90% and 87%, respectively), as compared to females treated with 7 & 7 + G + P (82%; p = 0.09). A greater proportion of multiparous females expressed behavioral estrus prior to FTAI when treated with 7 & 7 + G + P (90%) as compared to multiparous females treated with 7 & 7 + G (79%; p < 0.002) or 7 & 7 Synch (78%; p = 0.0004); however, pregnancy rates resulting from FTAI did not differ based on treatment. In summary, tandem administration of GnRH and PG on Days 0 and 7 of the 7 & 7 Synch protocol effectively generated a convergent ovarian response among females with variable pretreatment luteal status but failed to enhance pregnancy rates to FTAI.