Administration of GnRH on day 9 of a 14-day CIDR with CO-Synch 72 h in lactating beef cows

§%§Most progestin-based estrus synchronization protocols focus on inducing one new follicular wave before progestin removal. Our objective was to determine effectiveness of an extended controlled internal drug release (CIDR) protocol with 2 induced follicular waves. Lactating beef cows (~25% primiparous) at 3 locations (n=247, n=395, n=137) were randomly assigned to 3 treatment groups. Cows in the 14-d 50 PG group received a CIDR (1.38 g progesterone) insert and 100 μg GnRH im on d 0, 100 μg GnRH im on d 9, CIDR removal and 50 mg PGF2α im on d 14, and 100 μg GnRH im with TAI 72 _ 3 h later. Cows in the 14-d 6h PG group were assigned to the same protocol as the first group except that 25 mg PGF2α im was given at CIDR removal on d 14, plus 25 mg PGF2α 6 _ 1 h later. Cows in the control group (5-d COSynch+CIDR) received a CIDR insert and 100 μg GnRH im on d 0, CIDR removal and 25 mg PGF2α im on d 5, 25 mg PGF2α 6 _ 1 h later, and 100 μg GnRH im with timed-AI 72 _ 3 h after CIDR removal. Body condition scores for Tx 1, 2, and 3 averaged 4.9 _ 0.73, 4.9 _ 0.65, and 4.9 _ 0.72 (SD). Post partum interval at TAI ranged from 38 to 115 d. Pregnancy status was determined on d 37 to 40 by ultrasonography. Averaged over all locations, conception rates for 14-d 50 PG (n=278) 14-d 6h PG (n=249), and 5-d CO-Synch + CIDR (n=252) protocols were 61.2, 53.4, and 50.8% respectively; the 14-d 50 PG and 5-d CO-Synch + CIDR differed (P < 0.025, chi-squared). Cycling status at 2 locations (n=243; n=391) and was determined from blood collected on d -7 and d 0; progesterone levels >1 ng/mL at either bleed were considered cycling. Averaged over the 2 locations, conception rates by cycling status did not interact with treatments (P >0.05) (14-d 50 PG: cycling-79/117 (67.5%), not cycling-58/111 (52.3%); 14-d 6h PG: cycling-59/102 (57.8%), not cycling-54/99 (54.6%); 5-d CO-Synch: cycling-53/99 (53.5%), not cycling-58/106 (54.72%). Overall conception rates were higher for 14-d 50 PG group than the 5-d COSynch protocol.