Sex ratio and conception rates of fresh/vitrified embryos at different developmental stages by ovum pick up in Hanwoo cows

Abstract

Embryo transfer plays a crucial role in enhancing the breeding value of livestock; it has been applied in Hanwoo cattle, which is a popular breed for beef production in Korea. Both in vivo-derived (IVD) and in vitro-produced (IVP) embryos are used for this purpose; however, IVP embryos have been preferred recently owing to advancements in ovum pick-up (OPU) technology and genomic selection. Despite technological advancements, comprehensive data on large-scale OPU/IVEP/embryo transfer in Hanwoo cows are lacking. In this study, 16 elite Hanwoo donor cows were selected on the basis of specific criteria. Oocytes were retrieved from 241 cows using OPU. The collected cumulus-oocyte complexes (COCs) were matured, fertilized, and cultured in vitro to produce transferable embryos. Embryos were classified according to their developmental stage and then transferred to 675 recipient cows. A total of 3,317 COCs were collected, with an average of 13.76 COCs per cow. The number of transferable embryos produced per cow was 3.7. Hanwoo OPU-derived IVP embryos exhibited a higher production yield than the global average, indicating a stable IVEP environment. Both fresh and frozen IVP embryos yielded similar conception rates; hence, the use of vitrified-thawed embryos in transfer plans feasible. However, frozen-thawed embryos at Stage 7 had a lower conception rate than those at earlier stages. There was no significant difference between the conception rates of sexually mature heifers and postpartum cows used as recipients. The male-to-female offspring ratio increased as the developmental stage progressed. Seasonal effects on conception rates were not observed; however, higher abortion rates and a higher proportion of male offspring were observed during winter. This study provides valuable data for the Korean embryo transfer industry, enabling more strategic growth of the domestic Hanwoo embryo industry.

Keywords: Conception rate; Embryo transfer; Frozen- thawed embryos; In vitro-produced (IVP) embryos; Ovum pick-up (OPU); Sex ratio.

Fig. 1.. Estrus synchronization schedule.

Estrus synchronization schedule of recipient cow for embryo transfer. PMSG, pregnant mare’s serum gonadotropin; EB, estradiol benzoate; GnRH, gonadotropin hormone-releasing hormone; ET, embryo transfer.

Fig. 2.. Conception rate and delivery rate by stage.

(A) The conception rate of embryo transfer by stage of fresh and frozen/thawed embryos in Hanwoo cows. P = 0.651216. Statistical significance established at p < 0.05. (B) The delivery rate of embryo transfer by stage of fresh and frozen/thawed embryos in Hanwoo cows. P= 0.554952. Statistical significance established at p < 0.05.

Fig. 3.. IVP embryo status (fresh vs. frozen/thawed) and sex ratio by stage.

(A) Sex ratio of calves using fresh embryos. (B) Sex ratio of calves using frozen/thawed embryos. (C) Result of sex ratio of total calves. IVP, in vitro-produced.

Fig. 4.. The sex ratio of Hanwoo calves by season.