This trial was aimed at investigating the effect of different doses of progesterone (P4) and types of vaginal bolts on new follicular wave induction, oestrus synchronization and superovulation in Holstein heifers. Results showed that the interval after prostaglandin F2α (PGF2α) application and oestrus occurrence between the sponge blot (SP) group (48.3 h) and the progestin release device (PRD) group (39 h) was significantly different (P<0.05). The average number of embryos obtained in the SP group (13.2) was significantly higher than that (8.7) in the PRD group (P<0.05). The same trend was observed for the number of usable embryos (5.7 and 3.3 for SP and PRD groups, respectively). The injection of 50 and 100 mg of P4 efficiently induced a new follicular wave, whereas no significant difference (P>0.05) was recorded on oestrus synchronization and superovulation between the two groups.
