JR Newcombe, AR Peters
The use of the GnRH analogue buserelin administered in mid dioestrus has been shown to increase early pregnancy rate, reduce embryo loss and increase litter size in several species. The mechanism by which this works is unclear and may differ between species. In cattle it is believed that buserelin, by reducing oestrogen concentrations, causes the development of endometrial oxytocin receptors to be delayed. Luteal oxytocin binds with these receptors to promote the formation and secretion of prostaglandin. Any delay in the release of PGF2α would allow an underdeveloped embryo extra time to produce and release the maternal recognition of pregnancy signal, which might otherwise be lost following luteolysis. This may also be the mechanism in sheep and pigs although an increase in progesterone concentrations could also be important. However in the mare, following buserelin administration at 9 or 10 days after the detection of ovulation, pregnancy rates are already increased by 12-13 days compared with untreated controls, i.e. before the time of luteolysis, without any change in progesterone concentrations. In addition to an increased pregnancy rate, the twin embryo rate is also increased and subsequent embryo loss rate reduced. The possible mechanisms by which this may result in the mare are discussed.
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