GnRH treatment at artificial insemination in beef cattle fails to increase plasma progesterone concentrations or pregnancy rates.

Treatment with GnRH at the onset of standing estrus increased pregnancy percentages and circulating concentrations of progesterone in repeat breeder dairy cows. The objective of this study was to determine the effect of treatment with GnRH at AI on concentrations of progesterone and conception rates in beef cattle that exhibited estrus. Two hundred ninety-three heifers at four locations were synchronized with the Select Synch plus CIDR protocol (given GnRH and a CIDR was placed into the vagina, and 7 d later, given PGF(2alpha) and CIDR removed; n=253) or the 14-19 melengestrol acetate (MGA) protocol (MGA fed at 0.5mg/head/d for 14 d, with PGF(2alpha) 19 d after MGA withdrawal n=40) and AI was done after detection of estrus. At Location 1, blood samples were collected on Day 2, 4, 6, 10, 15, and 18 after AI (Day 0=AI). Two hundred and fifty postpartum cows at two locations were synchronized with the Select Synch plus CIDR protocol, and AI was performed after detection of estrus. At AI, cattle were alternately assigned to one of two treatments: (1) treatment with GnRH (100microg) at AI (n=127 heifers and n=108 cows); or (2) non-treated control (n=120 heifers and n=119 cows). Concentrations of progesterone tended to be greater in control heifers compared to GnRH-treated heifers on Days 6 (P=0.08), 10 (P=0.07), and 15 (P=0.11). Overall conception rates were 68% and 66% for GnRH treated and control, respectively, and were not different between treatments (P=0.72). In summary, treatment with GnRH at time of AI had no influence on conception rates in cattle that had exhibited estrus.