The Effect of the Initial GnRH and Dose of PGF2α on Pregnancy Rate to TAI in Beef Heifers Submitted to the 5-d CO-Synch + CIDR Program
The objective was to determine the effects of GnRH (Factrel®) at CIDR insertion and number of doses of PGF2α (PGF; Lutalyse®) at CIDR removal in a 2x2 factorial on pregnancy rates to timed AI (TAI) in beef heifers enrolled in a 5-d CO-Synch + CIDR program. Yearling beef heifers (n = 1105) in 7 locations (Ohio [2 locations; n = 149 and n = 116; Angus x Simmental], Utah [n = 274; Angus x Hereford], Wyoming [n = 156; Angus] and Minnesota [3 locations; n = 150, n = 150 and n = 110; Crossbred]) were enrolled in the 5-d CO-Synch + CIDR protocol and randomly assigned to either receive 100 μg GnRH (G+, n = 547) or not to receive GnRH (G-, n = 558) at CIDR insertion (d 0 of the experiment). At CIDR removal (d 5), heifers within G+ and G- groups were randomly assigned to receive either a single 25 mg PGF (PGF1) or two 25 mg PGF 6 ± 2 h apart (PGF2), resulting in 4 distinct treatments (G+PGF1, n = 272; G+PGF2, n = 275; G-PGF1, n = 277; G-PGF2, n = 281). All heifers received either tail paint or Estrotect®patches at CIDR removal to determine estrus response and were inseminated by TAI concomitant with 100 μg GnRH at 60 h after CIDR removal. Pregnancy diagnosis was performed between 32 and 36 d after TAI. Estrus response, as determined by estrous detection aids, did not differ among treatments. Pregnancy rate to TAI averaged 55.5% and was similar among treatments (G+PGF1, 53.3%; G+PGF2, 57.4%; G-PGF1, 55.2%; G-PGF2, 55.9%). Heifers classified as having been in estrus before TAI had a greater (P < 0.05) pregnancy rate to TAI (64.6%; 277/429) than either heifers with minimal (49.8%; 150/301) or no (48.7%; 172/353) evidence of estrus before TAI. In conclusion, omission of the initial GnRH treatment in the 5-d CO-Synch + CIDR program did not influence TAI pregnancy rate in yearling beef heifers. Moreover, an additional dose of PGF at CIDR removal did not improved fertility in these yearling beef heifers, regardless of whether or not the initial GnRH treatment was given.
Keywords: GnRH, PGF, Yearling Beef Heifers