Bovine Embryo Transfer: Are Efficiencies Improving?

Embryo transfer (ET) in cattle has been commercially practiced in a number of countries for nearly four decades. Numerous technologies have been introduced and adopted by the industry during this period, including non-surgical embryo recovery and transfer, closely timed hormonal synchronization of donor and recipient estrous cycles, reliable cryopreservation of embryos and direct transfer following thawing, embryo splitting, embryo sexing, production of in vitroderived embryos, and cloning. However, actual success rates within the ET industry, as measured by the mean number of embryos recovered per donor or by conception rates following embryo transfer, have changed very little over the years. Nevertheless, there are fundamental principles related to donor and recipient management and also to ET procedures that do profoundly influence the level of success. Introduction Commercial bovine ET is a well established, mature industry in its fourth decade of existence in many countries. The history of the development of the industry has been previously described (Betteridge, 1981; Betteridge, 2000). As used henceforth, ET will refer individually or collectively to the collection, handling and transfer of embryos. Research in the field of ET is very expensive and many of the basic procedures were established some years ago. Consequently, little academic research currently is being conducted that is likely to immediately benefit the commercial industry. Because success rates in well-managed cattle herds are generally quite high, most recent improvements involve rather small, albeit important, increments. A valuable approach to understanding how various factors in ET influence success is to analyze well kept, accurate commercial records. A good share of the data presented in this review involves just such an approach. Commercial ET programs vary widely in terms of factors that may be beyond the control of practitioners. For example, selection of specific donor cows and service sires are ultimately chosen by the owner. Long term weather problems or storms during the superovulation/recipient synchronization process are beyond the control of anyone and can wreck havoc with ET success. Travel problems sometimes means traveling to a farm a day or two late, which mandates working with older embryos than planned. Probably the single most important variable affecting success in ET is the level of donor and recipient management. Many factors go into cattle management and there are sometimes opportunities for ET practitioners to make a meaningful contribution to improving or changing certain management programs. In some cases, however, change either is not welcome or not possible and then the practitioner must make the best of the