Fertility of Angus cross beef heifers after GnRH treatment on day 23 and timing of insemination in 14-day CIDR protocol

This study compared artificial insemination pregnancy rate ( AI-PR) between 14-day CIDR-GnRH-PGF2a-GnRH and CIDR-PGF2a-GnRH synchronization protocol with two fixed AI times ( 56 or 72 hr after PGF2a). On day 0, heifers ( n = 1311) from nine locations assigned body condition score ( BCS: 1, emaciated; 9, obese), reproductive tract score ( RTS: 1, immature, acyclic; 5, mature, cyclic) and temperament score ( 0, calm; and 1, excited) and fitted with a controlled internal drug release ( CIDR, 1.38 g of progesterone) insert for 14 days. Within herd, heifers were randomly assigned either to no - GnRH group ( n = 635) or to GnRH group ( n = 676), and heifers in GnRH group received 100 mu g of GnRH ( gonadorelin hydrochloride, IM) on day 23. All heifers received 25 mg of PGF2a ( dinoprost, IM) on day 30 and oestrous detection aids at the same time. Heifers were observed for oestrus thrice daily until AI. Within GnRH groups, heifers were randomly assigned to either AI-56 or AI-72 groups. Heifers in AI-56 group ( n = 667) were inseminated at 56 hr ( day 32 PM), and heifers in AI-72 group ( n = 644) were inseminated at 72 hr ( day 33 AM) after PGF2a administration. All heifers were given 100 mu g of GnRH concurrently at the time AI. Controlling for BCS ( p <.05), RTS ( p <.05), oestrous expression ( p <.001), temperament ( p <.001) and GnRH treatment by time of insemination ( p <.001), the AI-PR differed between GnRH treatment [ GnRH ( Yes - 60.9% ( 412/ 676) vs. No - 55.1% ( 350/ 635); p <.05)] and insemination time [ AI-56 - 54.6% ( 364/ 667) vs. AI- 72 - 61.8% ( 398/ 644); ( p <.01)] groups. The GnRH treatment by AI time interaction influenced AI- PR ( GnRH56 61.0% ( 208/ 341); GnRH72 - 60.9% ( 204/ 335); No- GnRH56 - 47.9% ( 156/ 326); No-GnRH72 - 62.8% ( 194/ 309); p <.001). In conclusion, 14-day CIDR synchronization protocol for FTAI required inclusion of GnRH on day 23 if inseminations were to be performed at 56 hr after PGF2a in order to achieve greater AI-PR.