The Use of Gonadotropin-Releasing Hormone in a Progesterone-Based Timed Artificial Insemination Protocol in Replacement Beef Heifers

The objective of this experiment was to determine the effect of supplemental gonadotropin-releasing hormone (GnRH) at the time of progesterone controlled internal drug-release (CIDR) insertion, when incorporated into a CIDR-prostaglandin F2α (PGF2α)-GnRH timed AI (TAI) protocol on pregnancy per AI of beef heifers. One hundred ninety-two beef replacement heifers in 2 locations were paired by BCS and BW and received a CIDR on d 0. At the time of CIDR insertion, heifers were assigned randomly to receive either GnRH (100 μg, i.m.) or physiological saline. On d 7, the CIDR was removed and all heifers received 25 mg PGF2α On d 9, all heifers were administered GnRH (100 μg, i.m.), followed immediately by TAI by a single inseminator. Pregnancy status was determined by ultrasound on d 35 after AI. Blood samples were collected on d −8, 0, and 9 to measure progesterone concentrations. There was no effect of location or a location × treatment interaction on pregnancy incidence (P = 0.47), and 97.8% of heifers were cyclic. Pregnancy per AI did not differ between treatments (P > 0.5). Body weight and BCS had no effect on the odds ratio of pregnancy (P > 0.5). Administration of GnRH at CIDR insertion did not improve pregnancies per AI when a 7-d CIDR-PGF2α-GnRH TAI protocol was used in cyclic beef heifers.