Comparison of two doses of oestradiol benzoate administered at a resynchronised oestrus on reproductive performance of dairy cows

OBJECTIVE: To compare the effects of two doses of oestradiol benzoate (ODB) administered as part of a treatment designed to resynchronise returns to oestrus on the reproductive performance of cows in a controlled breeding program. DESIGN: Cohort study. PROCEDURE: Lactating dairy cows on two farms were treated to synchronise three successive oestrous cycles. An intravaginal progesterone releasing insert (IVP4) was used to synchronise the first oestrous cycle. The cows were then treated 15 days after the first treatment by reinsertion of an IVP4 that had been used to synchronise the first oestrus and administration of 1.0 mg of oestradiol benzoate (ODB) i.m. at device insertion. The IVP4 device was removed 8 days later and either 0.5 (n = 421) or 1.0 mg of ODB (n = 446) was administered 24 h later. Injections of (ODB) with or without the use of an IVP4 were used to synchronise the third oestrous cycle. Different synchronisation treatments were used to synchronise first and third oestrous cycles but differences were included in statistical models to account for variation in the data. This enabled examination of effects due to differences in the dose of ODB used to synchronise the second synchronised oestrus. RESULTS: The dose of ODB (0.5 or 1.0 mg) administered just before the second synchronised oestrus did not significantly (P > 0.10) affect the cumulative pregnancy rates over three successive rounds of artificial insemination, the mating start date to the conception intervals, the conception rates to the first or second insemination, the proportion of cows submitted for insemination at the second synchronised oestrus or the proportion of cows that were not pregnant yet failed to show signs of oestrous (phantom cows) identified within each herd. CONCLUSION: There was no difference in reproductive performance between cows receiving either 0.5 mg or 1.0 mg ODB after removal of used IVP4 devices that had been inserted to resynchronise them for a second insemination.