The objective of this study was to determine if the omission of GnRH at controlled internal drug release device (CIDR) insertion would impact pregnancy rates to timed AI (TAI) in beef heifers enrolled in a 5-d CO-Synch + CIDR protocol that used 1 PGF 2α dose given at CIDR removal. Yearling beef heifers in Ohio in 2 consecutive breeding seasons (2011, n = 151, and 2012, n = 143; Angus × Simmental), Utah (2012, n = 265; Angus × Hereford), Idaho (2012, n = 127; Charolais), and Wyoming (2012, n =

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... 7; Angus) were enrolled in the 5-d CO-Synch + CIDR protocol. At CIDR insertion (d -5), heifers were randomly assigned either to receive 100 μg GnRH (GnRH+; n = 408) or not to receive GnRH (GnRH-; n = 415). At CIDR removal (d 0 of the experiment), 25 mg PGF 2α was administered to all heifers. All heifers were inseminated by TAI and given 100 μg GnRH 72 h after PGF 2α (d 3). In heifers at the Ohio locations (n = 294), presence of a corpus luteum (CL) at CIDR insertion (d -5) was determined via assessment of progesterone concentrations (2011) and ovarian ultrasonography (2012). Subsequently, in both