The efficacy of
GnRH treatments are reviewed in relation to prevention of embryo mortality, control of follicle development in synchronization programmes using PG as luteolysin, induction of ovulation in post-partum anoestrus and in bovine cystic
ovarian disease. It is suggested that in cattle that
GnRH is effective in increasing pregnancy rates when given either at the time of insemination (first or repeat) or between days 11 and 14 after insemination. Evidence is also presented for positive effects on pregnancy rates in sheep, mares and sows. Use of
GnRH as an integral part of synchronising regimens where it is given 7 days before PG and then again 48-60 h after PG appears to be effective in increasing the synchrony of ovulation in controlled breeding programmes. The main synchronizing effect seems to reside in the second
GnRH injection whereas the importance of the first is in prolonging the luteal phase in those cows treated late in the cycle. The published work on the potential use of
GnRH to induce ovulation in anovulatory cattle is reviewed. Neither bolus dose
injections, pulsatile, continuous infusion, nor
controlled release formulations of
GnRH, have yet proved effective in inducing fertile ovulations in a predictable or consistent manner. It is suggested that this is due to the variability of follicular status when treatment is initiated.
GnRH is commonly used in the treatment of bovine cystic
ovarian disease. However, although stimulating ovulation/luteinisation of a new follicle and luteinisation of the
cyst, fertility of treated cattle remains very poor and it is suggested that a better understanding of the disease is needed before more effective treatments can be developed.