Abstract
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.
Highlights
Field studies in several domestic species have consistently shown that the administration of a single treatment with buserelin or other GnRH analogues in mid to late dioestrus, results in increases in pregnancy rate and/or litter size
This review explores and compares the relationships between progesterone, luteolysis and GnRH in the mare and the cow and suggests other possible avenues for future investigation
There is much clinical evidence that the administration of a single subcutaneous injection of 40 or 20μg of aqueous buserelin to mares, 9 to 10 days after the detection of ovulation produces a highly significant improvement in pregnancy rate [77], including an increase in embryo numbers in multiple ovulating mares. This is evident by the time of early ultrasonic detection of the embryonic vesicle at 12 to 13 days after ovulation, before the time of luteolysis and only 2 to 4 days after treatment
Summary
Field studies in several domestic species have consistently shown that the administration of a single treatment with buserelin or other GnRH analogues in mid to late dioestrus, results in increases in pregnancy rate and/or litter size. Temporary support of luteostasis during the time of maternal recognition is believed to be the mechanism for this increase in cows. Treatment of dioestrous mares with buserelin has shown improvements in pregnancy rate of up to 10%. Because pregnancy diagnosis using ultrasound can be made in mares before the time of luteolysis, temporary luteostasis cannot be the mechanism in this species. This review explores and compares the relationships between progesterone, luteolysis and GnRH (buserelin) in the mare and the cow and suggests other possible avenues for future investigation
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