The effect of an intramuscular (im) injection of oestradiol benzoate (ODB) at insertion of an intravaginal progesterone-releasing device (CIDRTM, InterAg, NZ) during hormonal treatment for postpartum anoestrum in dairy cattle was investigated. Inclusion of injected ODB at CIDRTM insertion in synchronisation programmes for cycling cows can increase fertility, possibly by initiating an emergence of a new ovarian follicle wave with resultant improved oocyte competence. A total of 882 cows, of mixed age and breed from 18 herds, were examined and diagnosed as anoestrous during the week preceding each herd’s planned start of mating. Cows were allocated randomly as controls or to receive 0.5 or 1 mg ODB (Oestradiol Benzoate Inj., Intervet, Netherlands) by im injection at CIDRTM insertion. The device was removed after 7 days, and any cow which was not observed in spontaneous oestrus within 48 h received 1 mg ODB im (CIDIROLTM Injection, InterAg, NZ). Cows with an observed heat within 12 days of CIDRTM removal were defined as responders, and returns to service in the subsequent 30 days were recorded. There were no significant differences between the control, 0.5 and 1 mg ODB treatments for either the proportion responding (89.5%, 89.4% and 90.9%, respectively) or returning (52.3%, 50.3% and 55.4%, respectively); however the proportions of cows observed in spontaneous oestrus varied by treatment (P<0.01; 28.5%, 22.9% and 17.2% for control, 0.5 and 1 mg ODB treatments, respectively). Conception rates were not significantly affected by treatment (35.9%, 42.5% and 43.2% for control, 0.5 and 1 mg ODB treatments, respectively). Of cows which did not become pregnant to the induced oestrus, 21.7%, 20.7% and 29.9% for control, 0.5 and 1 mg ODB treatments respectively, returned to service with an interval of more than 24 days. These observations suggest that an injection of ODB at CIDRTM insertion may alter follicle wave patterns in cows with PPA, but that this may not result in increased conception rate, and might be detrimental to ovarian activity in a proportion of cows.
Proceedings of the New Zealand Society of Animal Production, Volume 58, , 82-84, 1998
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