315 IVF-DERIVED CROSSBRED EMBRYOS PRODUCED WITH SEXED SEMEN AND TRANSFERRED IN PAIRS TO BOS TAURUS × BOS INDICUS COWS AND HEIFERS

Abstract

Biotechnology has continued to evolve rapidly, allowing the development of techniques to increase reproductive efficiency and contribute to the genetic improvement of cattle. Some of these techniques include the in vitro maturation (IVM) and IVF of oocytes, sperm sexing and embryo transfer (ET) to recipient females to obtain pregnancies and offspring. These modern assisted-reproduction techniques (ART) can help produce twin pregnancies and calving of a pre-determined sex. The aim of this study was to produce a high proportion of female bovine embryos in vitro using X-chromosome-selected sexed semen and to transfer them in pairs to recipient females, in order to evaluate the efficiency of transferring two female embryos in both cows and heifers. Cebu-cross ovaries were obtained from a local slaughterhouse and transported to a nearby laboratory in Chiapas, Mexico, to obtain cumulus-oocyte complexes by follicular aspiration and culture in maturation medium for 24 h. For IVF, frozen X-sorted semen (Milking Gyr and Holstein breeds, 90% purity, Sexing Technologies, Navasota, TX, USA) was used. Gametes were co-incubated for 22 h, then moved to embryo development medium and cultured for 7 days. Recipient Cebu-cross commercial cows (n = 98) and heifers (n = 50) were synchronized, using intravaginal devices impregnated with progesterone, administering eCG and prostaglandin at withdrawal. Seven days after heat, 88 recipients were subjected to non-surgical ET (59 cows and 29 heifers). Embryo transfers were performed in Tamaulipas and Veracruz, Mexico, and were divided into 2 groups: A) cows, and B) heifers. Only grade-1 embryos were selected for ET. Two embryos were loaded in a single 0.25 mL French straw and transferred to the uterine horn ipsilateral to the ovary with a corpus luteum. Pregnancy diagnosis was performed by ultrasound or rectal palpation 60 days after ET. A Fisher's exact test (SPSS v. 16.0, SPSS Inc., Chicago, IL, USA) was used to determine statistical differences (α = 0.05). Of IVF oocytes, 176/180 (98%) and 242/300 (81%) were fertilized, producing 96/180 (53.3%) Milking Gyr (semen)-Cebu (oocytes) and 92/300 (30.7%) Holstein (semen)-Cebu (oocytes) grade-1 embryos, respectively. Of the 88 recipients, 33 were pregnant (37.5%), however, it was not possible (at that time) to determine the number and sex of fetuses developing in utero. Overall, 8 of the 29 heifers were pregnant (27.6%), compared to 25 pregnancies in 59 cows (42.4%). For heifers, the pregnancy results for transferring Milking Gyr-Cebu embryos (4/11) versus Holstein-Cebu embryos (4/18) were not significantly different (P = 0.433, two-tailed Fisher's exact test). A similar comparison could not be made for cows given smaller sample sizes due to the extra variable of having taken place on several different ranches (n = 11). The concept herein is that smaller twin female crossbred calves may reduce issues associated with freemartinism and dystocia while still maintaining the vigor of crossbred offspring. Results from this research can help contribute to the study and development of ART for increasing cattle production efficiency.