The effects of gonadotrophin releasing hormone in prostaglandin F-2 alpha-based timed insemination programs for beef cattle

Trials were conducted in the spring (May; n = 324) and fall (October; n = 1 32) with crossbred continental-type beef cows assigned on the basis of pari ty and postpartum interval to one of three timed-AI treatments and one of t wo post-AI treatments. The timed-AI treatments were: (DPG) double (14 d apa rt) PGF(2 alpha) (Lutalyse (R)) and AI (day = 0) 72 h after the second PGF( 2 alpha) (day -3); (OVS) Ovsynch (R) with the second GnRH (Factrel (R)) at 48 hand AI at 66 h; and (BRC) the same as OVS except that the second GnRH w as given at the time of AI. Half of the cows within each treatment were giv en GnRH on day 14. Plasma progesterone concentrations were determined for t he day of the first injection and on days -3, 0, 14, and 21. Timed-AI pregn ancy was diagnosed by ultrasonography at day 42 and confirmed at calving. F or DPG, OVS and BRC, PGF(2 alpha) responder rates were 75.9, 51.4 and 71.3% , respectively, in spring (P < 0.05) and 70.4, 70.4 and 59.1% in fall (P > 0.05), and AI pregnancy rates were 28.7, 44.9 and 44.4% in spring (P < 0.05 ) and 25.0, 40.9 and 43.2% in fall (P > 0.05). Post-AI GnRH had no signific ant effect on pregnancy or conception rates or day 21 progesterone. The use of GnRH in the PGF(2 alpha) based timed-AI program improved pregnancy rate s and the BRC treatment was as effective as OVS. Neither postpartum interva l nor initial progesterone concentration influenced (P > 0.05) the effect o f GnRH on AI pregnancy rate, and GnRH had no effect (P > 0.05) on twinning rate or gender ratio.