An asymmetric distribution of the sexes within the left and right uterine horns has been described in multiple species. A series of experiments were conducted to evaluate the sex ratio (% male) of calves gestated in the left and right uterine horns, as well as the sex ratio of embryos originating from the left and right ovaries of cattle. The sex ratio of calves gestated in the right uterine horn of naturally mated cows was significantly higher compared with the sex ratio of calves gestated in the left uterine horn. In addition, the sex ratio of the left and right uterine horns differed significantly from parity. The sex ratio of embryo transfer calves born following transfer to the left and right uterine horns was not significantly different. Additionally, the proportion of male embryos collected from the right uterine horns was significantly greater than from the left uterine horns of superovulated cows. The sex ratio of embryos collected from the left and right uterine horns of unilaterally ovariectomized cows was not significantly different. However, more female than male embryos were produced when left ovary oocytes fertilized in vitro. In conclusion, the results of these experiments demonstrate that a significantly greater proportion of males are gestated in the right uterine horn of cattle and a greater proportion of females in the left. Additionally, the data indicate that sex-specific selection pressure may be applied to embryos by ovarian factors rather than by the uterine environment.

Abstract. reproductive potential in genetically valuable animals (BEAL et al., 1992). Now that repeatable oocyte retrieval methods are being fine-tuned, it is likely these procedures will become routinely used to obtain oocytes for further gamete and embryo research and also by seedstock producers for in vitro embryo production from farm animals in the commercial sector. The use of transvaginal ultrasound-guided oocyte aspiration and IVF procedure does offer an alternative to cattle producers who have genetically valuable cows that for some reason are unable to produce viable embryos through standard embryo collection procedures. This technology can be used on oocytes harvested from older ovulating or nonovulating cows, females with physical injuries (e.g., fractured leg) and problem cows having an abnormal cervix. Good success has been reported using IVF procedures on oocytes obtained from supplemental follicles of cows with cystic ovarian disease. With IVF the potential exists for more embryos to be produced in a shorter period of time, since the procedure can be repeated on the same cow 3 to 4 times or more a month. At this station, we are harvesting oocytes from early postpartum (< 40 days) beef and dairy cattle, before the female begins cyclic activity. The approach allows the opportunity to produce one or more extra calves from the cow before she is mated for a natural pregnancy. Currently, transvaginal ultrasound-guided oocyte aspiration is now being used to harvest valuable oocytes from minor farm animal breeds, from domestic females representing rare bloodlines, clinically infertile females and reproductively senescent cows. Research continues to find applications for this technology, including harvesting oocytes from young prepubertal heifers and early postpartum beef cows for in vitro embryo production. The use of ultrasound-guided oocyte aspiration should not be overlooked to obtain oocytes for in vitro embryo production and to aid in germplasm preservation of endangered exotic species.