EFFECTS OF ESTROUS-CYCLE CONTROL ON REPRODUCTIVE EFFICIENCY

Original methods for controlling the oestrous cycle of cattle were bas ed on extending or reducing the luteal phase to produce a synchronised decline in concentrations of progesterone or a synthetic progestagen. Modifications with GnRH or oestradiol at treatment initiation can alt er patterns of ovarian follicle development to synchronise the emergen ce of a new dominant follicle. Otherwise, dominance can persist and th e synchronised oestrus may be associated with reduced fertility. Avoid ing this effect may increase variation in the duration of pro-oestrus and complicate inseminating animals at a prescribed post-treatment int erval (timed insemination). Using oestradiol, GnRH or hCG during pro-o estrus can reduce variation in the interval to ovulation, Timing of th ese pro-oestrous treatments is important if the effects of inducing pr emature ovulations are to be avoided. Oestradiol has also been shown t o be more effective than eCG as a pro-oestrous treatment with anoestro us dairy cows, allowing them to be included in synchrony programmes in itially using progesterone supplementation. Current trials are investi gating techniques for processing semen for use with inseminations made during pro-oestrus. Finally, concepts of oestrous cycle control to im prove reproductive efficiency should be extended to include re-synchro nisation of returns to service and the further development of treatmen ts which increase the probability of conceptus recognition. Effective oestrous cycle control to improve reproductive efficiency may require modifications to ovarian follicle patterns when the treatment is initi ated, as well as during pro-oestrus and at a specified period in the c ycle following insemination.