ABSTRACT
Embryo transfer studies in the farm mammals have clearly shown that the survival of embryos depends on their stage of development corresponding closely with the developmental stage of the recipient's uterus. In the early days of surgical embryo transfer, recipient cattle were almost invariably selected on the basis of a naturally occurring estrus. Workers in Ireland, where high pregnancy rates were obtained by surgical transfers, reported a higher proportion of viable embryos in exactly synchronized recipients than in those ±1 day out-of-phase. The aim in most cattle superovulation treatments is to administer an appropriate follicle stimulating hormone-type preparation several days in advance of estrus and ovulation in doses sufficient to grow 10 to 20 or more follicles to maturity. An alternative to relying on natural or prostaglandin-induced luteolysis occurring a few days after pregnant mare serum gonadotrophin administration in cattle superovulation treatments is to time the gonadotrophin injection to precede the withdrawal of an appropriate progestagen treatment.
