Breeding heifers by appointment with PGF2a and GnRH

The purpose of this study was to determine the feasibility of inseminating a group of heifers without estrous detection at a predetermined time after injections of prostaglandin Fga and GnRH. Sixty-eight cycling Hereford heifers were divided randomly into five groups. Groups 1 through 4 were given 2 injections (IM) 11 days apart of 30 mg of PGF2α-THAM salt (PGF2α). Groups 1 and 3 were injected with 100 ug IM of GnRH 60 hr after the second PGF2α injection. Groups 1 and 2 were inseminated at 72 and 96 hr after the. second PGF injection and groups 3 and 4 were inseminated once at 80 hr. Group 5 was a control, and they were bred at 12 hr after estrus. Forty of the 56 PGF2α treated heifers (71%) expressed estrus after the second injection. The average interval from the first and second PGF2α injection to estrus was 53 and 51 hr, respectively. Conception at the synchronized breeding in the 72 and 96 hr and 80 hr groups was 43 and 14 % (P=.04), respectively, for all heifers bred and 63 and 19 % (P=.01), respectively, for those that were observed in estrus. The heifers that were inseminated twice at 72 and 96 hr all conceived to the 72 hr breeding except one which conceived to the 96 hr breeding. The first service conception rates for all heifers bred and those showing estrus were 21 and 32 % for the GnRH groups and 36 and 48 % for the no GnRH groups, respectively. There were no significant differences in conception rates between the treated heifers bred for 25 days by AI (63%) and the control heifers (66%) bred for 38 days- by AI. The conception rate