The reasons for a reduced oestrous response in heifers following a 9-day treatment with implants of progesterone were determined. Progesterone implants were inserted into Hereford cross heifers for 9 days at different stages of the oestrous cycle. Oestradiol benzoate (5 mg) was also given at the time of insertion of implants. The oestrous response obtained was low in animals receiving implants on Days 3 or 17 but was high in those receiving implants between Days 5 to 15. Injecting 50 mg progesterone with the 5 mg oestrogen increased the oestrous response in animals receiving implants on Day 17 but the onset of oestrus was more variable following this treatment in animals receiving implants on Day 3 or between Days 5 and 15. Removing the implants after 12 days instead of 9 days, however, gave a precise onset of oestrus. The conception rate to A.I. was not different in heifers synchronized with a 9- or 12-day treatment from that of the controls but it was lower in heifers synchronized with implants of progesterone removed after 18 or 21 days. The high conception rates obtained at slaughter 30 to 40 days after the last insemination were substantiated by allowing a group of heifers to calve after synchronization with a 9-day progesterone treatment.
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